ArticlePDF Available

High genetic diversity in the clonal aquatic weed, Alternanthera philoxeroides in the United States

Authors:

Abstract and Figures

The distribution of genetic diversity in invasive plant populations can have important management implications. Alligator weed ( Alternanthera philoxeroides (Mart.) Griseb.) was introduced into the United States around 1900 and has since spread throughout much of the southern U.S. and California. A successful biological control program was initiated in the late 1960s which reduced alligator weed in the southern U.S., although control has varied geographically. The degree to which variation among genotypes may be responsible for variation in control efficacy has not been well studied due to a lack of genetic data. We sampled 373 plants from 90 sites across the U.S. and genotyped all samples at three chloroplast regions to help inform future management efforts. Consistent with clonal spread, there was high differentiation between sites, yet we found six haplotypes and high haplotype diversity (mean h = 0.48) across states, suggesting this plant has been introduced multiple times. Two of the haplotypes correspond to previously described biotypes that differ in their susceptibility to herbicides and herbivory. The geographic distribution of the three common haplotypes varied by latitude and longitude while the other haplotypes were widespread or localized to one or a few sites. All the haplotypes we screened are hexaploid (6n = 102) which may enhance biological control. Future studies can use these genetic data to determine if genotypes differ in their invasiveness or respond differently to control measures. Some states for instance, have mainly a single haplotype and so may respond more uniformly to a single control strategy compared to other states which may require a variety of control strategies. These data will also provide the basis for identifying the source regions in South America, which may lead to the discovery of new biological control agents more closely matched to particular genotypes.
Content may be subject to copyright.
Invasive Plant Science and
Management
www.cambridge.org/inp
Research Article
Cite this article: Williams DA, Harms NE,
Knight IA, Grewell BJ, Futrell CJ, and Pratt PD
(2020) High genetic diversity in the clonal
aquatic weed Alternanthera philoxeroides in the
United States. Invasive Plant Sci. Manag 13:
217225. doi: 10.1017/inp.2020.32
Received: 28 May 2020
Revised: 21 September 2020
Accepted: 13 October 2020
First published online: 4 November 2020
Associate Editor:
Marie Jasieniuk, University of California, Davis
Keywords:
Chloroplast DNA; invasive species; ploidy;
population genetics
Author for correspondence:
Dean A. Williams, Department of Biology, Texas
Christian University, Fort Worth, TX 76129.
(Email: dean.williams@tcu.edu)
© The Author(s), 2020. Published by Cambridge
University Press on behalf of the Weed Science
Society of America.
High genetic diversity in the clonal aquatic weed
Alternanthera philoxeroides in the United States
Dean A. Williams1, Nathan E. Harms2, Ian A. Knight3, Brenda J. Grewell4,
Caryn Joy Futrell5and Paul D. Pratt6
1Professor, Department of Biology, Texas Christian University, Fort Worth, TX, USA; 2Research Biologist, Aquatic
Ecology and Invasive Species Branch, Environmental Laboratory, U.S. Army Engineer Research and
Development Center, Vicksburg, MS, USA; 3Postdoctoral Research Participant, Oak Ridge Institute for Science
and Education, Oak Ridge, TN, USA; 4Research Ecologist, U.S. Department of Agriculture, Agricultural Research
Service, Invasive Species and Pollinator Health Research Unit, Davis, CA, USA; 5Biological Sciences Technician,
U.S. Department of Agriculture, Agricultural Research Service, Invasive Species and Pollinator Health Research
Unit, Davis, CA 95616, USA and 6Research Leader and Entomologist, U.S. Department of Agriculture, Agricultural
Research Service, Invasive Species and Pollinator Health Research Unit, Albany, CA, USA
Abstract
The distribution of genetic diversity in invasive plant populations can have important manage-
ment implications. Alligatorweed [Alternanthera philoxeroides (Mart.) Griseb.] was introduced
into the United States around 1900 and has since spread throughout much of the southern
United States and California. A successful biological control program was initiated in the late
1960s that reduced A. philoxeroides in the southern United States, although control has varied
geographically. The degree to which variation among genotypes may be responsible for varia-
tion in control efficacy has not been well studied due to a lack of genetic data. We sampled 373
plants from 90 sites across the United States and genotyped all samples at three chloroplast
regions to help inform future management efforts. Consistent with clonal spread, there was
high differentiation between sites, yet we found six haplotypes and high haplotype diversity
(mean h=0.48) across states, suggesting this plant has been introduced multiple times.
Two of the haplotypes correspond to previously described biotypes that differ in their suscep-
tibility to herbicides and herbivory. The geographic distribution of the three common haplo-
types varied by latitude and longitude, while the other haplotypes were widespread or localized
to one or a few sites. All the haplotypes we screened are hexaploid (6n=102), which may
enhance biological control. Future studies can use these genetic data to determine whether gen-
otypes differ in their invasiveness or respond differently to control measures. Some states, for
instance, have mainly a single haplotype that may respond more uniformly to a single control
strategy, whereas other states may require a variety of control strategies. These data will also
provide the basis for identifying the source regions in South America, which may lead to the
discovery of new biological control agents more closely matched to particular genotypes.
Introduction
The distribution of genetic diversity in invasive plants can have important management impli-
cations. Different genotypes of the same species and inter- or intraspecific hybrids can require
different control methods due to differing growth potential or resistance to particular agents or
herbicides (Bultemeier et al. 2009; LaRue et al. 2013; Manrique et al. 2008; Michel et al. 2004;
Thum et al. 2012; Williams et al. 2014). Investigations into the distribution of genetic diversity
across the invasive range can potentially shed light on whether management efforts will need to
be tailored to specific regions or whether an overall broad strategy can be employed (Benoit and
Les 2013; Thum et al. 2020). For example, the aquatic invader hydrilla (Hydrilla verticillata L. f.
Royle) is widespread in the United States, but management must be tailored to the infested area,
because there are multiple introduced biotypes with different geographic distributions (Jacono
et al. 2020); introduced biological control agents differ in their effectiveness between biotypes
(Grodowitz et al. 2010); and herbicide-tolerant strains have emerged in some areas, requiring
alternative herbicides for effective control (Netherland and Jones 2015). Understanding and
comparing the distribution of genetic diversity in native and invasive ranges can also shed light
on the number of introductions and evolutionary processes acting during invasion (founder
effects, bottlenecks, etc.) and identify source regions in the native range to facilitate the explo-
ration for biological control agents and the design of appropriate host-specificity plant tests
(Croxton et al. 2011; Gaskin et al. 2011; Ward et al. 2008; Williams et al. 2005,2018).
Alligatorweed [Alternanthera philoxeroides (Mart.) Griseb.] is native to the Parana and
Paraguay River regions in Argentina, Paraguay, Uruguay, and southeastern Brazil (Sosa
et al. 2004). It was introduced into the United States around 1900, most likely through
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
contaminated ship ballast, and quickly spread throughout water-
ways in the southern United States and California (Zeiger 1967).
The species has also been introduced to Asia, Europe, Australia,
Central America, the Caribbean, and other regions in South
America. The plant can grow in both terrestrial and aquatic hab-
itats and is a serious threat to waterways, agriculture, and natural
habitats due to its rapid growth rate and ability to outcompete
native species (Buckingham 2002).
Phenotypic plasticity for growth form and sex in A. philoxer-
oides is common and has been documented for aquatic versus ter-
restrial habitats and between different soil types (Geng et al. 2007;
Kay and Haller 1982; Liu et al. 2011). In the United States, two bio-
types have been recognized, one with a broad stem and long, nar-
row leaves (BSA) and one with a narrow stem and short, broad
leaves (NSA), which are differentiated at allozyme loci and are
not a phenotypic response to different environmental conditions
(Kay and Haller 1982; Wain et al. 1984). These biotypes may cor-
respond to two forms described in South America, A. philoxeroides
philoxeroides and A. philoxeroides angustifolia (Sosa et al. 2004,
2008). A study utilizing nuclear inter-simple sequence repeats
(ISSRs) found that populations in the United States have much
higher genetic diversity compared with populations in China
and genetic diversity similar to samples from the plants native
range in Argentina (Geng et al. 2016). Geng et al. (2016) suggest
a single clonal lineage was introduced into China, while multiple
lineages were introduced into the United States.
Alternanthera philoxeroides ploidy is difficult to study due to a
large number of small chromosomes, but the presence of aneuploidy
and low pollen viability in a number of samples in the native range
suggests A. philoxeroides exists as a complex of hybrids (Sosa et al.
2008; Telesnicki et al. 2011). In Argentina, the species exists as tetra-
ploid (4n=ca. 68) or hexaploid (6n=ca. 102), while in China, indi-
viduals are hexaploid (6n=102) (Cai et al. 2009; Krug and Sosa 2019;
Sosa et al. 2008). Based on estimates of genome size, A. philoxeroides
may be pentaploid in the United States (Chen et al. 2015). The ploidy
of A. philoxeroides may be important from a control standpoint,
because increased ploidy has been suggested to contribute to invasive-
ness for a number of taxa by increasing adaptive potential or plasticity
to environmental conditions and management actions (Pandit et al.
2014;teBeestetal2012). However, for A. philoxeroides,femalealli-
gator weed flea beetles (Agasicles hygrophila Selman and Vogt,
Coleoptera: Chrysomelidae) prefer to oviposit, and larvae have better
performance on hexaploids than tetraploids in the native range (Krug
and Sosa 2019). Therefore, the role of polyploidy in A. philoxeroides
invasion success and management remains unclear.
Three biological control agents were introduced into the United
States in 1964 to 1971 to control A. philoxeroides (Spencer and
Coulson 1976). These agents, especially A. hygrophila, have signifi-
cantly reduced infestations in the southern United States
(Cofrancesco 1988; Spencer and Coulson 1976). Nevertheless,
there is still widespread variability in control efficacy that may have
a climatic or genetic basis. Because A. hygrophila cannot survive
colder winter temperatures in the northern part of A. philoxeroides
range in the United States, it must migrate annually from overwin-
tering locations in the South (Coulson 1977). This leads to annual
variation in the timing of A. hygrophila activity in more northern
A. philoxeroides populations and contributes to the observed vari-
ability in control in those regions (Harms and Cronin 2020).
Additionally, the two A. philoxeroides biotypes (BSA and NSA)
in the United States appear to differ in their susceptibility to her-
bivory by A. hygrophila (Kay and Haller 1982; Pan et al. 2013) and
they respond differently to some herbicides (Kay 1992). To what
extent observed variability in control may be due to presence of
these different genotypes or others has not been well studied.
Here we report on the largest A. philoxeroides genetic study to
date, conducted across the entire invaded range of the United
States to help inform future management of this species. We char-
acterized the genetic structure of A. philoxeroides in the United
States using chloroplast (cpDNA) markers, to better understand
the invasion history of A. philoxeroides and explore whether this
genetic diversity can be linked to geographic patterns, climate,
morphology, and ploidy, which could have important manage-
ment implications. Chloroplast markers are particularly well suited
for phylogeographic studies, and so these data will also provide the
basis for identifying the specific source regions in South America,
which may lead to the discovery of new biological control agents
more closely matched to particular genotypes (e.g., Cuda et al.
2012; Schaal et al. 2003; Williams et al. 2005,2018). The response
of plants to management such as biological control agents and her-
bicides is expected to be related to nuclear DNA variation rather
than cpDNA haplotypes per se. In clonally reproducing popula-
tions, however, cpDNA variation will be linked to nuclear DNA,
and cpDNA haplotypes may therefore provide a good marker
for distinguishing biotypes, as they are for invasive H. verticillata
in the United States (Madeira et al. 2007). Alternanthera philoxer-
oides produces viable seed in the native range but is believed to
spread only through fragmentation and vegetative growth in the
invasive range (Geng et al. 2007; Sainty et al. 1998; Thayer and
Pfingsten 2020; Ye et al. 2003). Seedlings have never been found
in the invasive range, and although seeds are produced, they do
not appear viable, so cpDNA haplotypes could potentially be useful
as markers to indicate which management efforts will be most
effective for a given area.
We specifically ask: (1) Is cpDNA diversity consistent with high
genetic diversity and the presence of multiple introductions into
the United States? (2) Does genetic diversity in A. philoxeroides
reflect geographic and climatic patterns? (3) Do genotypes corre-
spond to the previously described NSA and BSA biotypes? (4) Do
genotypes correspond to previously described ploidy levels in
A. philoxeroides? These data will provide a better understanding
Management Implications
We found high genetic diversity of Alternanthera philoxeroides
(alligatorweed) across the United States that is structured geographi-
cally. Two of the chloroplast haplotypes (genotypes) we found corre-
spond to two previously described narrow-stem (NSA) and broad-
stem (BSA) biotypes that are reported to respond differently to both
biological control and herbicides. The other genotypes we found may
also differ in their invasiveness (e.g., varying growth rates) or may
respond differently to control measures. The control of these different
haplotypes should be tested with currently available herbicides and
biological control agents. These studies may encourage foreign explo-
ration for additional agents that are better adapted to specific haplo-
types and climates. Managers may need to consider that different
areas will need different management strategies. Some states have
mainly a single invasive haplotype that may respond more uniformly
to a single control strategy, while other states and river basins have a
variety of haplotypes and therefore may require a variety of control
strategies depending on population genetics and specific locality.
The markers we developed in this study could be used to quickly
determine which haplotypes are present in a given area.
218 Williams et al.: A. philoxeroides genetic diversity
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
of the genetic variability across the invasive range in the United
States that can be used in future studies to determine whether
genotype differences may be correlated with growth rates and
effectiveness of different control methods.
Materials and Methods
Sampling
Alternanthera philoxeroides tissues (n=375 samples) were col-
lected during 2017 to 2020 at locations in the United States.
Most samples were collected by the authors (66 sites), but in several
cases (33 sites), we received material from collaborators. Sampling
locations were identified through a variety of means, including
chance encounters while driving, directed searching from online
databases, museum herbarium records, or local knowledge.
Plants within sites were chosen for sampling at random, and col-
lections were spaced apart by several meters to avoid sampling the
same plant. At each site, one to eight plants were sampled. The
uppermost two leaf-pairs were removed and placed in a sealable
plastic bag with silica gel desiccant. Once dry, samples were frozen
until they could be processed.
Genetics
We extracted DNA from all samples using the IBI Scientific MINI
Genomic DNA kit (Plants) (Dubuque, IA) as per the manufacturers
instructions. We amplified and sequenced three chloroplast (cpDNA)
regions (rpL16,trnS-G,trnL intron, and trnL-F spacer), using primers
reported in Shaw et al. (2005) for 24 samples. Polymerase chain reac-
tions (PCR) (10 μl) contained 10 to 50 ng of DNA, 0.5 μMofeach
primer, and 1X AccuStartTM II PCR SuperMix (2X) (Quanta
Biosciences, Gaithersburg, MD). Reactions were cycled in an ABI
2720 thermal cycler (Thermo Fisher Scientific, Waltham, MA).
The cycling parameters were one cycle at 95 C for 10 min, followed
by 30 cycles of 30 s at 94 C, 30 s at 55 C, 60 s at 72 C, and then a final
extension at 72 C for 5 min. Products were sequenced bidirectionally
with the PCR primers using BrightDye®Terminator Cycle
Sequencing Kit (MCLAB, South San Francisco, CA). The only poly-
morphisms between samples in these cpDNA regions were
differences in the size of mononucleotide repeats. Unique sequences
have been deposited in GenBank (NCBI accession numbers
MW015925MW015933). We therefore designed primers around
these repeat regions (5 030)rpL16FTGGAATCATAGTGGA
TTGTCAAA, rpL16RCAATTCATTGGGAAGGATGG; trnS-GF
AAGTAACAAAGATTCAACGAATTCAA, trnS-GRAGGCCGT
GGGAATACTCCTT; trnFfFCTTCTCTCGCATCATCTTCTCA,
trnFfRGTCCCTCTATCCCCAAAAGC. Each forward primer was
labeled with 6-FAM. All three cpDNA regions were amplified in a
single multiplex reaction. PCRs (10 μl)contained10to50ngof
DNA, 0.1 μM of each primer, and 1X AccuStartTM II PCR
SuperMix (2X). The cycling parameters were one cycle at 95 C for
10 min, followed by 30 cycles of 30 s at 94 C, 30 s at 60 C, 30 s at
72C,andthenafinalextensionat60Cfor30minonanABI
2720 thermal cycler. The resulting multiplexes were diluted 20X with
dH
2
O. For each sample, 1.0 μl of diluted product was loaded in 10 μl
of HIDI formamide with 0.1 μl of LIZ-500 size standard (Thermo
Fisher Scientific) and electrophoresed on an ABI 3130XL Genetic
Analyzer. Genotypes were then scored and binned using
GENEMAPPER v. 5.0 (Thermo Fisher Scientific). Positive controls
for each haplotype were run in all amplification batches to control
for slight size shifts (~ ±0.5 bp) that can occur between runs.
Reamplification of 10% of all samples, including the less common
genotypes, gave identical results.
Leaf Morphology and Plant Architecture
Plants were collected from field locations during 2017 to 2018 and
then clonally propagated three to four times in a common garden
before morphological measurements. Plants from each population
were cultured separately in 20-L plastic buckets with municipal-
delivered tap water supplemented with slow-release Osmocote®
fertilizer (15-9-2; Scotts Miracle-Gro, Marysville, OH). For mor-
phology/plant architecture measurements, plants were recultured
in a standard nutrient solution. Genotyping was conducted using
the original plant material collected from the field.
To assess differences in leaf shape, stem diameter, and stem
branching, plants were grown hydroponically in a temperature-
controlled greenhouse at the U.S. Army Engineer Research and
Development Center, Vicksburg, MS. Six replicate clones from 39
populations (Table 1) were placed into net pots (12.7-mm diameter)
filled with washed expanded clay rocks (8- to 16-mm diameter). Net
pots were placed individually within white 4-L polyethylene food
containers, and charcoal-filtered tap water was added to each con-
tainer. After 1 wk, the water was replaced with 1.5 L of half-strength
Hoaglands nutrient solution. Six weeks after adding nutrients, mea-
surements were taken on all plants. We collected four leaves from
each plant (n=24 leaves per population), imaged them on a flatbed
scanner, and then measured their dimensions (length and width)
using ImageJ image analysis software (National Institutes of
Health, Bethesda, MD). Leaf shape was calculated as the length-
to-width ratio. Outer stem diameter of each plant was measured
in millimeters at the widest location on the stem. Branching was
measured as the total number of side branches off the main stem.
Ploidy
Shoot cuttings from five haplotypes (Ap 1, 2, 3, 4, and 6) were cul-
tured hydroponically in flasks within an incubator until new roots
emerged for collection. Root tips were excised from actively grow-
ing adventitious roots, fixed for 2 h in 3:1 ethanol: acetic acid, and
stored in 70% ethanol until analysis. Fixed root tips were stained
with Schiff reagent using the Feulgen reaction. Five evaluations
were performed for each of the population samples using meri-
stematic cells from preserved root tip tissue that were squashed
between cover slips and microscope slides in 45% acetic acid. Slides
were examined directly under phase-contrast optics at 1,000×(oil
immersion) using a Zeiss Axiostar Plus microscope (Zeiss
Microscopy, Jena, Germany). Images were photographed with
an Infinity 2-1 digital camera (Lumenera, Ottawa, Canada) using
Image-Pro Insight v. 9.0 software (Media Cybernetics, Bethesda,
MD). Composite images, including 10 to 30 images in sequence,
from 6 to 8 cells per squash were then evaluated for chromosome
counts. Ploidy level was assigned based on chromosome numbers.
Statistical Analyses
We merged sampling sites that were within the same river system
or lake to increase sample sizes within sites. This resulted in 90 sites
with 3 to 13 samples (mean 4.1 ±0.18 SE samples per site).
Haplotype diversity (h) and analysis of molecular variance
(AMOVA) were calculated in GenAlEx v. 6.5 (Peakall and
Smouse 2006,2012). Haplotype diversity is the probability that
two randomly selected samples are different haplotypes. A TCS
haplotype network (Clement et al. 2000; Templeton et al. 1992)
Invasive Plant Science and Management 219
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
was constructed using POPART (Leigh and Bryant 2015) to visu-
alize similarities among haplotypes.
To explore the spatial distribution of common A. philoxeroides
haplotypes in the United States, we used an information theoretic
approach. We excluded rare haplotypes (Ap2, 4, 5) from this explo-
ration, because they only occurred in a few locations, suggesting
they are recent introductions or post-introduction mutations
and have not yet spread to their potential extent. We extracted cli-
mate data for A. philoxeroides collection locations from the first
five principal components (PCs) of the 35 bioclimatic variables
in the CliMond 1975H data set (Bio36, Bio37, Bio38, Bio39,
Bio40) (Kriticos et al. 2014). The PCs are described in Kriticos
et al. (2014) and explain more than 90% of the variation in the
original CliMond data set. PCs were used to obtain climate infor-
mation for each survey location and as potential explanatory var-
iables for distribution of haplotypes, assessed through model
selection.
We used the Akaike information criterion adjusted for small
sample size (AICc) to select the most informative multinomial
logistic regression model from the full set of candidate models
(Burnham and Anderson 2003). For the full model, haplotype
was the nominal response variable, and latitude, longitude, and
each climate PC were included as covariates. None of the PCs were
strongly correlated with latitude or longitude (i.e., r >0.90), so all
were included in the full model. We used multinomial logistic
regression to test whether haplotype identity was related to lati-
tude, longitude, or any of the five climate PCs. Top models (i.e.,
those with substantial support; Burnham and Anderson 2003)
were those with AICc values within 2 of the model with the lowest
AICc value. AICc weights are also reported, which represent the
relative strength of support that a model is the best given the data
and other candidate models. Spearman rank correlation was used
to test for associations between the relative abundance of haplo-
types and latitude.
The effects of haplotype on leaf shape, stem diameter, and
branching were modeled in R (R Core Team 2013) using general-
ized linear mixed models. Source population was treated as a ran-
dom effects variable. Branching count data were not normally
distributed and thus were modeled using a Poisson error distribu-
tion. Post hoc means separations were conducted using the
Benjamini-Hochberg adjustment for multiple comparisons
(Benjamini and Hochberg 1995). All results displayed are untrans-
formed means with standard error.
Results and Discussion
Genetic Diversity
Six haplotypes were detected across the United States in 375 samples
(Table 2;Figures1and 2). The majority of individuals had haplotypes
Ap1, 3, and 6 (Figure 2). Most sites (74.4% of 90 sites) contained only a
single haplotype, 24.4% contained two haplotypes, and 1.1% had three
haplotypes. There was significant genetic structuring in the United
States, as expected for a clonally spreading plant; AMOVA revealed
that 20% of the variance was found within sites, 53% was found
among sites (PhiPT =0.80), and 27% was found among states
(P =0.001 in all cases). Haplotype diversity of A. philoxeroides within
states was not correlated with the number of sampling sites (ρ
s
=0.09,
P=0.79). Haplotype diversity was lowest in South Carolina (h=0.11)
and highest in Georgia (h=0.79) and averaged 0.48 ±0.06 (SE) across
states (Table 3). The lower Mississippi River and Arkansas River
drainagestogetherhavehighA. philoxeroides diversity, with five of
the six haplotypes present (h=0.64). This high number of haplotypes
and diversity across different statessuggeststhisplantwasintroduced
multiple times. Other invasive clonally spreading aquatic weeds in the
United States have fewer haplotypes. Hydrilla verticillata has three
haplotypes in the United States and was introduced on three separate
occasions (Madeira et al. 2007; Tippery et al. 2020)andhygrophila
[Hygrophila polysperma (Roxb.) T. Anderson] has a single haplotype
Table 1. Alternanthera philoxeroides locations used for morphological leaf
measurements.
Location cpDNAHap
Latitude
°N
Longitude
°W
333 Cove, AR Ap1 35.320 93.214
Aberdeen Lake, MS Ap1 33.825 90.500
Blind River, LA Ap1 30.095 90.779
Choctaw Boat Ramp, LA Ap1 29.850 90.679
Coosa River, AL Ap1 33.055 86.527
Cooters Pond, AL Ap1 32.431 86.400
Longbranch, MS Ap1 33.769 90.144
Lake Monroe, FL Ap1 28.835 81.322
Lake Merrisach, AR Ap1 34.033 91.266
Navidad River 1, TX Ap1 29.036 96.563
Navidad River 2, TX Ap1 29.038 96.571
Newnans Lake, FL Ap1 29.618 82.253
TennTom Waterway, MS Ap1 33.661 88.487
Lake Waco, TX Ap1 31.610 97.305
Montezuma Slough, CA Ap2 38.097 121.894
Baldwin College, GA Ap3 31.485 83.533
CVS Pond, SC Ap3 32.213 80.701
Lake Marion, SC Ap3 33.535 80.331
Lake Wallace, SC Ap3 34.630 79.680
Suwanee, FL Ap3 30.301 82.932
Valley Park, MS Ap3 32.635 90.863
NorCo, CA Ap4 33.924 117.598
333 Cove, AR Ap6 35.320 93.214
Aberdeen Lake, MS Ap6 33.840 88.508
Anguilla, MS Ap6 33.023 90.848
Boardman Landing, NC Ap6 34.443 78.690
Beards Lake, AR Ap6 33.697 93.942
Coal Creek, OK Ap6 35.898 95.398
Coosa River, AL Ap6 33.055 86.527
Ditch, GA Ap6 31.839 81.414
Emerald Valley Lake, AL Ap6 33.759 86.607
Germantown Greenway,
TN
Ap6 35.117 89.820
Lansbrook Lake, OK Ap6 35.561 97.623
Lake Martin, AL Ap6 32.798 85.820
Lake Micosukee, FL Ap6 30.529 83.980
Nickajack, AL Ap6 34.832 87.322
Poverty Point Reservoir,
LA
Ap6 32.530 91.490
Schad ditch, TX Ap6 29.499 98.577
Lake Waccamaw, NC Ap6 34.300 78.552
Table 2. Six chloroplast haplotypes of Alternanthera philoxeroides in the United
States.a
Haplotype rpL16 trnS-G trnFf
Ap1 163 (A
10
) 173 (A
11
) 155 (A
9
/A
11
)
Ap2 163 (A
10
) 172 (A
10
) 155(A
10
/A
10
)
Ap3 163 (A
10
) 172 (A
10
) 156 (A
9
/A
12
)
Ap4 163 (A
10
) 173 (A
11
) 156 (A
9
/A
12
)
Ap5 163 (A
10
) 174 (A
12
) 156 (A
9
/A
12
)
Ap6 164 (A
11
) 172 (A
10
) 157 (A
9
/A
13
)
aHaplotypes are composed of three chloroplast regions. Their respective size in base pairs
and their corresponding mononucleotide repeats are given. The trnFf region encompasses
two separate mononucleotide repeats.
220 Williams et al.: A. philoxeroides genetic diversity
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
in the United States and appears to belong to a single clonal lineage
(Mukherjee et al. 2016).
Some A. philoxeroides haplotypes are widespread (Ap1, 3, 6) and
some appear to be relatively localized (Ap2, 4, 5), which is also con-
sistent with a pattern of multiple introductions. California is also rel-
atively distinct from the southeastern United States, although the
two haplotypes in California were also found at one site each in
the southern United States, suggesting they may have been moved
between these areas. Haplotype Ap4 was found in historic invasion
sites in southern California and in one sample from Arkansas, and
Ap5 was found at three sites in Louisiana (Figure 1). Haplotype Ap2
was most common at recently invaded sites in northern California,
suggesting a new introduction rather than dispersal from Ap4 pop-
ulations in southern California, and Ap2 was also found in four
Figure 1. Sampling localities and chloroplast haplotypes for Alternanthera philoxeroides in the United States. The colored pie diagrams indicate the relative abundance of
haplotypes (Ap16) in each state, and the red dots are sampling localities.
Figure 2. Haplotype TCS network for Alternanthera philoxeroides. Size of circles is related to relative abundance of each haplotype. Cross hatches on lines indicates one mono-
nucleotide base difference between haplotypes.
Invasive Plant Science and Management 221
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
samples from a single site in Georgia (Figure 1). The phylogeo-
graphic structure of A. philoxeroides in the entire native range will
need to be described to better evaluate the introduction history and
determine possible source regions.
Geographic and Climatic Patterns
Latitude, longitude, and Bio40 were influential in explaining varia-
tion in the three most common haplotypes (Ap1, 3, 6) (Table 4;
Figure 3). Three equally plausible models were identified by
AICc (Table 4), and latitude was included in all three. The top
model had only latitude and longitude (AICc weight =0.27) and
had twice the support of the next model, which had latitude and
Bio40 (AICc weight =0.12), and nearly three times the support
of the third-best model, which had latitude only (AICc weight
=0.10). Latitude was common to all three models and was due
to the relative abundance of haplotype Ap1 being negatively cor-
related to latitude (ρ
s
=0.88, P =0.0008) and haplotype Ap6
being positively correlated with latitude (ρ
s
=0.96, P =0.0005).
Ap3 was widespread from Texas to Florida and did not covary with
latitude. Latitudinal patterns in haplotypes may be related to intro-
duction histories, historical control efforts, or environmental
differences and deserve further study. Longitude may be related
to differences across the southeastern United States in haplotype
composition that may represent different introduction histories.
Bio40, which appeared in one of the models, represents radiation
during the warmest quarter (Bio26) and maximum weekly radia-
tion (Bio21) (Kriticos et al. 2014). Maximum radiation levels
appear to be relatively high in the southeastern United States
but lower in Florida and the East Coast (Kriticos et al. 2014).
Further studies will be necessary to determine whether A. philox-
eroides haplotypes differ in their growth patterns under high and
low radiation levels.
Leaf Morphology and Plant Architecture
Alternanthera philoxeroides haplotypes Ap1 and Ap6 were clearly
distinguished by all three morphological measurements in the
study, with haplotypes Ap2, Ap3, and Ap4 displaying somewhat
intermediate characteristics. The leaves of Ap1, Ap3, and Ap4
plants were more lanceolate in appearance, with significantly
greater length-to-width ratios (3.6 ±0.1, 3.5 ±0.2, and 4.3 ±0.5
cm, respectively) compared with the more ovate leaves of Ap6
(2.7 ±0.1 cm) (Wald X2=30.78; df =4; P <0.001) (Figure 4;
Table 5). Stem diameters of Ap1 were approximately twice as large
as those of Ap6, consistent with descriptions of broad- and narrow-
stemmed biotypes (Wald X2=154.18; df =4; P <0.001).
Branching from the main stem was significantly less frequent in
Ap1 compared with Ap6; however, neither displayed branching
that differed significantly from that of the intermediate haplotypes
(Wald X2=12.60; df =4; P <0.013) (Table 5). Morphologically,
Ap1 and 6 also correspond to descriptions of A. p. angustifolia,
with a more northern distribution in South America, and A. p. phil-
oxeroides, with a more southern distribution (Sosa et al. 2004).
These native ranges are also consistent with Ap1 being more
common in more southern regions and Ap6 being more common
in more northern regions of the United States. Whether Ap1 and 6
actually correspond to the two forms in the native range needs to be
verified by genetically testing samples from the native range.
Ploidy
Invasiveness in plants is associated with both smaller genomes and
high ploidy levels (Pandit et al. 2014; Suda et al. 2015; te Beest et al.
Table 3. Alternanthera philoxeroides genetic diversity in states at chloroplast
loci.a
State LocN N N Hap Ap h
AL 9 41 2 1, 6 0.44
AR 6 34 4 1, 3, 4, 6 0.53
CA 8 37 2 2, 4 0.51
FL 19 79 3 1, 3, 6 0.45
GA 4 16 4 1, 2, 3, 6 0.79
LA 18 66 4 1, 3, 5, 6 0.54
MS 9 32 3 1, 3, 6 0.65
NC 3 11 2 3, 6 0.55
OK 2 8 1 6 0.00
SC 5 19 2 3, 6 0.11
TN 1 7 2 3, 6 0.48
TX 6 25 3 1, 3, 6 0.57
aAbbreviations: LocN, number of sampling sites; N, total sample size; NHap, number of
haplotypes in each state; Ap, numbers correspond to haplotypes; h, haplotype diversity.
Table 4. Top best-fit models explaining variation in Alternanthera philoxeroides
haplotype, based on Akaike information criterion adjusted for small sample size
(AICc) selection.
Response
variable
Candidate
model AICc ΔAICc Likelihood
AICc
weight
Haplotype Latitude,
Longitude
204.3 0 1 0.27
Latitude,
Bio40
205.89 1.59 0.45 0.12
Latitude 206.25 1.95 0.38 0.10
Table 5. Least squares means ±standard error for stem diameter, stem
branching (total number of side branches off of main stem), and leaf shape
(length:width ratio) for Alternanthera philoxeroides haplotypes.a
Haplotype Diameter No. of branches L:W ratio
mm
Ap1 11.06 ±0.32a 6.9 ±0.5b 3.6 ±0.1a
Ap2 8 ±1.19ab 7.2 ±1.7ab 3 ±0.5ab
Ap3 6.94 ±0.49b 8 ±0.7ab 3.5 ±0.2a
Ap4 8.67 ±1.19ab 4.3 ±1.7b 4.3 ±0.5a
Ap6 5.82 ±0.29b 8.3 ±0.4a 2.7 ±0.1b
aDifferent letters indicate significant differences between haplotypes for a given measured
variable.
Figure 3. Dot plot of Alternanthera philoxeroides haplotype distribution in relation to
latitude in the United States. Although all haplotypes are represented here, only
common haplotypes 1, 3, and 6 were used in model selection.
222 Williams et al.: A. philoxeroides genetic diversity
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
2012). All chromosome counts from populations with haplotypes
Ap 1, 2, 3, 4, and 6 were 6n=102, indicating the invasive A. phil-
oxeroides cytotype in the United States is hexaploid (Figure 5).
Invasive A. philoxeroides populations in China are also hexaploid,
consistent with invasive populations having higher ploidy levels.
Chen et al. (2015) used flow cytometry to quantify genome size
in four plants from Argentina, two from the United States (NSA
and BSA), and one from China. Alternanthera philoxeroides in
China had a genome size similar to that of a population in
Argentina that is also known to be hexaploid (Sosa et al. 2008).
Another sample from Argentina that was thought to be hexaploid
had a DNA content that was similar to that of known tetraploids in
Argentina, and the U.S. samples had genome sizes that suggested
they were pentaploid. These results suggest that genome size in this
species can vary within a ploidy level. The presence of hexaploids in
the United States and China may be an advantage for control with
A. hygrophila (Krug and Sosa 2019). Whether genome size per se
might also impact biological control agents or is related to invasive-
ness in this species is unknown and deserves further study.
Conclusions
Most studies of A. philoxeroides to date have focused on differences
between plants from the native and invaded range and explored
development and performance of the biocontrol agent or plant
defensive responses rather than biomass response (equivalent to
successful control) of the plants to feeding (Liu et al. 2018;Pan
et al. 2013; Zhang et al. 2019). It will therefore be valuable to deter-
mine how effective the currently available biological control agents
will be on the introduced haplotypes. Our data suggest that haplo-
types Ap1 and 6 correspond to the NSA and BSA biotypes, which
respond differently to both A. hygrophila herbivory and herbicides
(Kay 1992; Kay and Haller 1982). Anecdotal observations in the
native range also suggest that A. p. philoxeroides may be less com-
monly attacked by A. hygrophila than A. p. angustifolia (Sosa et al.
2004). In combination with climatic limitations in some parts of the
invaded range, genetic differences among haplotypes may lead to
reduced control and encourage foreign exploration for additional
agents that are better adapted to specific haplotypes and climates.
Figure 4. Narrow-stem (NSA)/Ap6 (A) and broad-stem (BSA)/Ap1 (B) Alternanthera philoxeroides biotype/haplotype.
Figure 5. Representative composite images of Alternanthera philoxeroides cells with stained chromosomes: (A)haplotype Ap1 (Coosa River,AL); (B) haplotype Ap2(Suisun Marsh,
Montezuma Slough, CA); (C) haplotype Ap3 (Lake Wallace, SC); (D) haplotype Ap4 (Visalia, CA); and (E) haplotype Ap6 (Lake Micosukee, FL).
Invasive Plant Science and Management 223
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
If future studies determine that genotypes differ in their inva-
siveness (e.g., varying growth rates) or respond differently to con-
trol measures, then it may be necessary to consider that different
areas will need different management strategies. Florida and South
Carolina, for instance, have mainly a single invasive haplotype, so
plants may respond more uniformly to a single control strategy. In
contrast, the Mississippi River drainage has a variety of haplotypes,
and therefore may require a variety of control strategies depending
on the population genetics and specific locality. The cpDNA gen-
otyping we developed in this study provides a quick method to
determine which haplotypes are present in a given area.
Acknowledgments. We thank the following people for assistance in sample
collection or processing: Jim Cronin, Julie Nachtrieb, Aaron Schad, Lynde
Dodd, Brett Hartis, Keith Thomas, Rachael Klopfenstein, Chris Beals, David
Webb, Curtis Tackett, Diana Rashash, Tim Harris, David Lattuca, Chelsea
Bohaty, Mariah McInnis, Ram Medrano, Al Cofrancesco, Rodrigo Diaz, Kurt
Getsinger, Stacey Springfield, Derek Medina, Mike Pitcairn, and Robin
Carter-Ervin. Daniella Biffi helped with laboratory work. Funding was provided
by the U.S. Army Engineer Research and Development Center, Aquatic Plant
Control Research Program. No conflicts of interest have been declared.
References
Benjamini Y, Hochberg Y (1995) Controlling the false discovery rate: a practical
and powerful approach to multiple testing. J R Stat Soc Series B Stat
Methodol 57:289300
Benoit LK, Les DH (2013) Rapid identification and molecular characterization
of phytoene desaturase mutations in fluridone-resistant hydrilla (Hydrilla
verticillata). Weed Sci 61:3240
Buckingham GR (2002) 1 Alligatorweed. Pages 515 in Driesche RV, Blossey B,
Hoddle M, Lyon S, Reardon R, eds. Biological Control of Invasive Plants in
the Eastern United States. Morgantown, WV: USDA Forest Service
Publication FHTET-2002-04
Bultemeier BW, Netherland MD, Ferrell JA, Haller WT (2009) Differential her-
bicide response among three phenotypes of Cabomba caroliniana. Invasive
Plant Sci Manag 2:352359
Burnham, KP, Anderson DR (2003) Model Selection and Multimodal Inference:
A Practical Information-theoretic Approach. New York: Springer Science &
Business Media. 488 p
Cai H, Wei CL, Chen N (2009) Chromosome karyotype characters of biological
invasion in Alternanthera philoxeroides. Chin J Tropical Crops 30:530534
Chen Z, Xiong Z, Pan X, Shen S, Geng Y, Xu C, Chen J, Zhang W (2015)
Variation of genome size and the ribosomal DNA ITS region of
Alternanthera philoxeroides (Amaranthaceae) in Argentina, the USA, and
China. J Syst Evol 53:8287
Clement M, Posada D, Crandall KA (2000) TCS: a computer program to esti-
mate gene genealogies. Mol Ecol 9:16571659
Cofrancesco AF Jr (1988) Alligatorweed Survey of Ten Southern States.
Miscellaneous Paper A-88-3. Vicksburg, MS: U.S. Army Engineer
Waterways Experiment Station. 115 p
Coulson JR (1977) Biological Control of Alligatorweed, 19591972. A Review
and Evaluation. Washington, DC: USDA Technical Bulletin 1547. 98 p
Croxton MD, Andreu MA, Williams DA, Overholt WA, Smith JA (2011)
Geographic origins and genetic diversity of air-potato (Dioscorea bulbifera)
in Florida. Invasive Plant Sci Manag 4:2230
Cuda JP, Christ LR, Manrique V, Overholt WA, Wheeler GS, Williams DA
(2012) Role of molecular genetics in identifying fine tunednatural enemies
of the invasive Brazilian peppertree, Schinus terebinthifolius: a review.
Biocontrol 57:227233
Gaskin JF, Bon M-C, Cock MJW, Cristofaro M, De Biase A, De Clerck-Floate R,
Ellison CA, Hinz HL, Hufbauer RA, Julian MH, Sforza R (2011) Applying
molecular-based approaches to classical biological control of weeds. Biol
Control 58:121
Geng Y, Pan X, Xu C, Zhang W, Li B, Chen J, Lu B, Song Z (2007) Phenotypic
plasticity rather than locally adapted ecotypes allows the invasive alligator
weed to colonize a wide range of habitats. Biol Invasions 9:245256
Geng Y, van Klinken RD, Sosa A, Li B, Chen J, Xu C (2016) The relative impor-
tance of genetic diversity and phenotypic plasticity in determining invasion
success of a clonal weed in the USA and China. Front Plant Sci 7:213
Grodowitz M, Nachtrieb J, Harms N, Freedman J (2010) Suitability of Using
Introduced Hydrellia spp. for Management of Monoecious Hydrilla verticil-
lata (Lf) Royle. Vicksburg, MS: U.S. Army Engineer Research and
Development Center. 14 p
Harms NE, Cronin JT (2020) Biological control agent attack timing and pop-
ulation variability, but not density, best explain target weed density across an
environmental gradient. Sci Rep 10:11062
Jacono CC, Richerson MM, Howard Morgan V, Pfingsten IA (2020) Hydrilla
verticillata (L.f.) Royle. Gainesville, FL: U.S. Geological Survey,
Nonindigenous Aquatic Species Database. https://nas.er.usgs.gov/queries/
FactSheet.aspx?SpeciesID=6. Revision date: February 3, 2020, peer review
date: October 27, 2015
Kay SH (1992) Response of two alligatorweed biotypes to Quinclorac. J Aquat
Plant Manag 30:3540
Kay SH, Haller WT (1982) Evidence for the existence of distinct alligatorweed
biotypes. J Aquat Plant Manag 20:3741
Kriticos DJ, Jarošik V, Ota N (2014) Extending the suite of bioclim variables: a
proposed registry system and case study using principal components analy-
sis. Methods Ecol Evol 5:956960
Krug P, Sosa AJ (2019) Mother knows best: plant polyploidy affects feeding and
oviposition preference of the alligator weed biological control agent,
Agasicles hygrophila. Biocontrol 64:623632
LaRue EA, Grimm D, Thum RA (2013) Laboratory crosses and genetic analysis
of natural populations demonstrate sexual viability of invasive hybrid water-
milfoils (Myriophyllum spicatum x M. sibiricum). Aquat Bot 109:4953
Leigh JW, Bryant D (2015) POPART: full-feature software for haplotype net-
work construction. Methods Ecol Evol 6:11101116
Liu M, Zhou F, Pan X, Zhang Z, Traw MB, Li B (2018) Specificity of herbivore-
induced responses in an invasive species, Alternanthera philoxeroides (alli-
gator weed). Ecol Evol 8:5970
Liu W, Deng R, Liu W, Wang Z, Ye W, Wang L, Cao H, Shen H (2011)
Phenotypic differentiation is associated with gender plasticity and its respon-
sive delay to environmental changes in Alternanthera philoxeroides - pheno-
typic differentiation in alligator weed. PLoS ONE 6:e27238
Madeira PT, Coetzee JA, Center TD, White EE, Tipping PW (2007) The origin
of Hydrilla verticillata recently discovered at a South African dam. Aquat Bot
87:176180
Manrique V, Cuda JP, Overholt WA, Williams DA, Wheeler GS (2008) Effect of
host-plant genotypes on the performance of three candidate biological con-
trol agents of Schinus terebinthifolius in Florida. Biol Control 47:167171
Michel A, Arias RS, Scheffler BE, Duke SO, Netherland M, Dayan FE (2004)
Somatic mutation-mediated evolution of herbicide resistance in the nonindig-
enous invasive plant hydrilla (Hydrilla verticillata). Mol Ecol 13:32293237
Mukherjee A, Williams D, Gitzendanner MA, Overholt WA, Cuda JP (2016)
Microsatellite and chloroplast DNA diversity of the invasive aquatic weed
Hygrophila polysperma in native and invasive ranges. Aquat Bot 129:5561
Netherland MD, Jones D (2015) Fluridone-resistant hydrilla (Hydrilla verticil-
lata) is still dominant in the Kissimmee Chain of Lakes, FL. Invasive Plant Sci
Manag 8:212218
Pan XY, Jia X, Fu DJ, Li B (2013) Geographical diversification of growth-defense
strategies in an invasive plant. J Syst Evol 51:308317
Pandit MK, White SM, Pocock MJO (2014) The contrasting effects of genome
size, chromosome number and ploidy level on plant invasiveness: a global
analysis. New Phytol 203:697703
Peakall R, Smouse PE (2006) GENALEX 6: genetic analysis in Excel. Population
genetic software for teaching and research. Mol Ecol Notes 6:288295
Peakall R, Smouse PE (2012) GenAlEx 6.5: genetic analysis in Excel. Population
genetic software for teaching and research-an update. Bioinformatics
28:25372539
R Core Team (2013) R: A Language and Environment for Statistical Computing.
Vienna, Austria: R Foundation for Statistical Computing. http://www.R-
project.org
Sainty G, McCorkelle G, Julien M (1998) Control and spread of alligator weed
Alternanthera philoxeroides (Mart.) Griseb. in Australia: lessons for other
regions. Wetl Ecol Manag 5:195201
224 Williams et al.: A. philoxeroides genetic diversity
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
Schaal BA, Gaskin JF, Caicedo AL (2003) Phylogeography, haplotype trees and
invasive plant species. J Hered 94:197204
Shaw J, Lickey EB, Beck JT, Farmer SB, Liu W, Miller J, Siripun KC, Winder CT,
Schilling EE, Small RL (2005) The tortoise and the hare II: relative utility of
21 noncoding chloroplast DNA sequences for phylogenetic analysis. Am J
Bot 92:142166.
Sosa AJ, Greizerstein E, Cardo MV, Telesnick MC, Julien MH (2008) The evolu-
tionary history of an invasive species: alligator weed, Alternanthera philox-
eroide. Pages 435442 in Julien MH, Sforza R, Bon MC, Evans HC, Hatcher
PE, eds. Proceedings of the XII International Symposium on Biological
Control of Weeds. Wallingford, UK: CAB International
Sosa AJ, Julien MH, Cordo HA (2004) New research on alligator weed
(Alternanthera philoxeroides) in its South American native range. Pages
180185 in Cullen JM, Briese DT, Kriticos DJ, Lonsdale WM, Morin L,
Scott JK, eds. Proceedings of the XI International Symposium on
Biological Control of Weeds. Canberra, Australia: CSIRO Entomology
Spencer NR, Coulson JR (1976) The biological control of alligatorweed
Alternanthera philoxeroides in the USA. Aquat Bot 2:177190
Suda J, Meyerson LA, Leitch IJ, Pysek P (2015) The hidden side of plant inva-
sions: the role of genome size. New Phytol 205:9941007
te Beest M, Le Roux JJ, Richardson DM, Brysting AK, Suda J, Kubesova M,
Pysek P (2012) The more the better? The role of polyploidy in facilitating
plant invasions. Ann Bot 109:1945
Telesnicki MC, Sosa AJ, Greizerstein E, Julien MH (2011) Cytogenetic effect of
Alternanthera philoxeroides (alligatorweed) on Agasicles hygrophila
(Coleoptera: Chrysomelidae) in its native range. Biol Control 57:138142
Templeton AR, Crandall KA, Sing CF (1992) A cladistic-analysis of phenotypic
associations with haplotypes inferred from restriction endonuclease mapping
and DNA sequence data. 3. Cladogram estimation. Genetics 132:619633
Thayer DD, Pfingsten IA (2020). Alternanthera philoxeroides (Mart.) Griseb.
Gainesville, FL: U.S. Geological Survey, Nonindigenous Aquatic Species
Database. https://nas.er.usgs.gov/queries/FactSheet.aspx?SpeciesID=227.
Accessed: May 20, 2020
Thum RA, Chorak GM, Newman RM, Eltawely JA, Latimore J, Elgin E, Parks S
(2020) Genetic diversity and differentiation in populations of invasive
Eurasian (Myriophyllyum spicatum) and hybrid (M. spicatum ×M. sibiri-
cum) watermilfoil. Invasive Plant Sci Manag 13:5967
Thum RA, Wcisel DJ, Zuellig MP, Heilman M, Hausler P, Tyning P, Huberty
L, Netherland MD (2012) Field documentation of decreased herbicide
response by a hybrid watermilfoil population. J Aquat Plant Manag
50:141146
Tippery NP, Bugbee GJ, Stebbins SE (2020) Evidence for a genetically distinct
strain of introduced Hydrilla verticillata (Hydrocharitaceae) in North
America. J Aquat Plant Manag 58:16
Wain RP, Haller WT, Martin DF (1984) Genetic relationship among two forms
of alligatorweed. J Aquat Plant Manag 22:104105
Ward SM, Gaskin JF, Wilson LM (2008) Ecological genetics of plant invasion:
what do we know? Invasive Plant Sci Manag 1:98109
Williams DA, Harms NE, Grodowitz MJ, Purcell M (2018) Genetic structure of
Hydrilla verticillata L.f. Royle in eastern China and the Republic of Korea:
implications for surveys of biological control agents for the invasive monoe-
cious biotype. Aquat Bot 149:1727
Williams DA, Overholt WA, Cuda JP, Hughes CR (2005) Chloroplast
and microsatellite DNA diversities reveal the introduction history of
Brazilian peppertree (Schinus terebinthifolius) in Florida. Mol Ecol
14:36433656
Williams WI, Friedman JM, Gaskin JF, Norton AP (2014) Hybridization of an
invasive shrub affects tolerance and resistance to defoliation by a biological
control agent. Evol Appl 7:381393
Ye WH, Li J, Cao HL, Ge XJ (2003) Genetic uniformity of Alternanthera phil-
oxeroides in south China. Weed Res 43:297302
Zeiger CF (1967) Biological control of alligatorweed with Agasicles n. sp. in
Florida. Hyacinth Control Journal 6:3134
Zhang Z, Zhou F, Pan X, van Kleunen M, Liu M, Li B (2019) Evolution of
increased intraspecific competitive ability following introduction: the impor-
tance of relatedness among genotypes. J Ecol 107:387395
Invasive Plant Science and Management 225
https://www.cambridge.org/core/terms. https://doi.org/10.1017/inp.2020.32
Downloaded from https://www.cambridge.org/core. Texas Christian University, on 25 Jan 2021 at 19:45:43, subject to the Cambridge Core terms of use, available at
... For this study, six populations of A. andersoni were sourced from locations across the distribution in 2020-2021 (Table 1) and maintained in rearing culture for 12-18 months before experimentation. Rearing containers consisted of 4L opaque plastic containers filled with 1L of half-strength Hoagland's nutrient solution, then provisioned with 16 shoots of alligatorweed haplotype Ap1 (Williams et al., 2020). Thirty-two adult thrips were added to each container at a stocking rate of two Table 1. ...
... Each petri dish contained a damp cotton round in the lid, which was used to maintain humidity within the dish, and seal the dish against thrips escape. Each dish was also provisioned with a single leaf of alligatorweed (genotype Ap1; Williams et al., 2020). At the start of each experiment, petri dishes were added to chambers set to their designated temperatures, then removed following their assigned exposure duration. ...
... In samples collected from its native range in Argentina, both tetraploid and hexaploid individuals have been identified, capable of reproducing through sexual and asexual means (Sosa et al. 2004;Chen et al. 2015). In the introduced range, invasive populations in the United States exhibit a high level of genetic diversity (Williams et al. 2020). Conversely, studies have shown that populations in China possess a uniform genetic background, with limited sexual reproduction, exemplifying a clonal invasive plant (Xu et al. 2003;Wang et al. 2005;Pan et al. 2007;Geng et al. 2016). ...
... However, some studies indicate that in other invaded areas, alligator weed may exhibit higher genetic diversity. Williams et al. (2020) collected samples from 373 plants across 90 locations throughout the United States and conducted sequencing on three chloroplast (cpDNA) regions. Their analysis revealed six haplotypes, characterized by significant haplotype diversity across various states, suggesting a high level of differentiation between sites. ...
Article
Full-text available
Alligator weed (Alternanthera philoxeroides) is a highly invasive species that has successfully established in numerous tropical and subtropical regions worldwide. Previous literature suggests that alligator weed was introduced to China in the 1930s as fodder for military horses by Japanese, while its presence in Japan only became apparent in the 1990s. Consequently, the introduction and genetic relationship between alligator weed populations in China and Japan remain uncertain, and the native source population is still unidentified. This study aimed to characterize the genetic diversity and structure of populations within the introduced range of China and Japan, as well as the native range of Argentina, using amplified fragment length polymorphism (AFLP) markers. Nine primer pairs were employed, resulting in a total of 573 distinct amplified bands for the China and Japan populations. However, none of these bands displayed polymorphism, indicating a uniform genetic background across all sampled populations in China and Japan. In contrast, the Argentine populations yielded 251 identifiable amplified bands using four well-performing primer pairs, of which 209 (80.69%) were found to be polymorphic. Genetic relationship and population structure analyses based on AFLP data revealed that the population from Jujuy, Argentina, exhibited the closest genetic affinity to the invasive populations in China and Japan, as indicated by Nei’s genetic identity value of 0.9281. Additionally, using methylation-sensitive amplified polymorphism (MSAP), we identified 258 epigenetic variation sites using five primer pairs in the Chinese and Japanese populations. Principal coordinate analysis (PCoA) based on the MSAP data revealed a geographic epigenetic structure within the alligator weed populations of China and Japan, with DNA methylation variation patterns exhibiting correlation with geographic distribution, thus implying their potential involvement in environmental adaptation. This research enhances our understanding of the invasion mechanisms of alligator weed and provides valuable insights into the roles of epigenetic factors in its successful spread.
... In the invasive range in Australia and China the plant presents only hexaploids with low genetic variation (Sosa et al., 2008;Chen et al., 2015), where its phenotypical plasticity could be explained by epigenetic variation (Gao et al., 2010). However, only hexaploids with high levels of genetic variation were recently reported in USA (Williams et al., 2020). ...
... These associations were also observed in other studies that considered similar variables such as radiation during the warmest quarter and maximum weekly radiation (Kriticos et al., 2014). Additionally, Williams et al. (2020) suggest that such distribution patterns may be associated with genetic variation of alligator weed in the USA. Regarding the flea beetles, in particular A. hygrophila, different field and laboratory experiments suggest that its performance and distributions are limited by both high and low temperatures. ...
Article
Ecological niche models of species occurrence have gained interest in biological control programs to improve efficiency, reduce risks, and to inform when and how control agents may be released and/or surveyed. Alligator weed, Alternanthera philoxeroides (Mart.) Griseb. (Amaranthaceae), is an amphibious aquatic plant native to southern South America that has invaded several countries around the world. In this study, our aim was to quantify the current and to model the future alligator weed geographical distributions in its native range in South America, and in its introduced range (USA, the country where biological control of the weed was first implemented). Additionally, we modelled the current and potential distribution of its biocontrol agent Agasicles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae), released in the USA in 1964, and other potential biocontrol agents, including the flea beetles Disonycha argentinensis Jacoby and Systena sp. In total, 19438 occurrence records of alligator weed, 253 of A. hygrophila, 48 of D. argentinesis, and 19 of Systena sp. were compiled. Niche models predicted expanded distributions of alligator weed, particularly in northern regions of South America. However, the models of the flea beetles A. hygrophila, D. argentinensis, and Systena sp. did not predict geographic range expansion and the future scenarios are similar to current distributions. Our study predicts an increase in the suitable areas for alligator weed in South America in future scenarios of global warming, whereas these new areas would not be as favourable for the biocontrol agents historically used.
... Agasicles hygrophila colonies were maintained in quarantine facilities at 25 °C and L14:D10 photoperiod. Populations were fed A. philoxeroides haplotype Ap2 (sensu Williams et al., 2020), a common haplotype found in California. Alternanthera philoxeroides cultured for feeding the beetle colonies was grown hydroponically in full strength Hoagland's nutrient solution (Hoagland & Arnon, 1950) (200 mg L −1 N) in a greenhouse in Albany, CA, USA. ...
... An effort was made to collect from as broad a geographic range and latitudinal gradient as possible (Table 1). Cultures were maintained at 23 °C and L14:D10 photoperiod, and fed A. philoxeroides haplotype Ap1, the dominant haplotype found in the southeastern United States (Williams et al., 2020). Plants for beetle colonies were grown hydroponically in half strength Hoagland's solution (100 mg L −1 ). ...
Article
Full-text available
Lack of successful biological control of alligatorweed, Alternanthera philoxeroides (Mart.) Griseb. (Amaranthaceae), by Agasicles hygrophila Selman & Vogt (Coleoptera: Chrysomelidae) in the USA is an emerging problem as A. philoxeroides expands into temperate climates due to poor cold tolerance of the beetles. Sourcing climatically suited biotypes of biological control agents is traditionally done through foreign exploration into their native range; however, surveying introduced populations may provide a cost‐effective and regulatory‐friendly alternative. To compare foreign biotypes to those established in the USA, two populations of A. hygrophila were collected from the coolest extent of their range in South America, and two populations were collected from stable populations in the southeastern USA. For comparing cold tolerance among introduced populations, eight populations were collected from across a climatic gradient. For all populations, cold tolerance was measured as a function of supercooling point, critical thermal minimum (CTmin), chill coma, and temperature‐dependent survival and development of eggs and larvae. Results of cold‐tolerance measures for North American populations were subjected to principal component analysis (PCA) to reduce dimensionality and detect trends in cold tolerance among populations. For foreign and locally collected A. hygrophila , South American populations were significantly more cold tolerant as measured by CTmin (Δ1.98 °C), chill coma (Δ1.95 °C), larval survival (Δ35.0%), and development time of eggs (24.7% faster) and larvae (17.4% faster) at 15 °C. Comparisons among North American populations found significant differences in cold tolerance across populations; however, results were less consistent than those observed for foreign populations. The first principal component (PC) explained 43.0% of variation in cold‐tolerance metrics and was strongly related to mean temperature of the coldest quarter. The use of a multivariate approach to evaluate and compare cold‐tolerance phenotypes provides strong evidence of local adaptation in introduced populations of A. hygrophila . However, South American populations may still provide the best chance for control of A. philoxeroides in northern infestations.
... Alligator weed (Alternanthera philoxeroides) is native to South America and has emerged as a noxious aquatic plant in the states of Florida, Mississippi, Alabama, Georgia, and Louisiana in the US [18,19]. Alligator weed (Figure 2) thrives vigorously in hot and humid conditions and can dominate in wetlands, slow-moving rivers, and the standing waters of ponds or lakes. ...
Chapter
Full-text available
In southern states of the United States (US), particularly in Louisiana, noxious broadleaf weeds have become undesired flora that tends to dominate crop plants in all types of farming systems. By genotypic superiority-driven robust growth habits and unprecedented reproductive potential, weed species acquire more growth resources (moisture, nutrients, solar radiation, etc.) than crop plants. Weed species can also survive periods of suboptimal growth conditions (salinity, drought, heat, chilling, heavy metal toxicity, water logging, soil erosion, heavy grazing and trampling by livestock, etc.). Considering changing climate scenarios and environmental pollution associated with the extensive use of herbicides, researchers have realized the need to explore and understand the remarkable agro-botanical superiority of weeds. Formulating and re-optimizing weed management approaches has become essential for improving farming practices. To attain these objectives, this study has been tailored to synthesize fundamental knowledge on a few prevalent weeds (e.g., pigweed, alligator weed, Chinese tallow, and parthenium weed) of Louisiana. Moreover, the prevalence of invasive weed species in the region has been objectively analyzed, and an economically viable chemical engineering-based weed management strategy (hydrothermal carbonization) for converting weed biomass into organic soil amendment (hydrochar) has been proposed. Such an approach holds the potential to keep weeds below the threshold level and reduce the use of herbicides, along with impart�ing sustainability to common Louisiana farming systems.
... Griseb., commonly known as alligator weed, is native to South America. A. philoxeroides has become a problematic invader in many regions worldwide, including parts of North America, Asia, and Australia (Tanveer et al., 2018;Williams et al., 2020). This clonal, amphibious plant is capable of rapid growth and vegetative reproduction, forming dense mats that outcompete native vegetation in both terrestrial and aquatic habitats (Eckert et al., 2016). ...
Preprint
Full-text available
Invasive plants have caused huge damages in ecosystems. Arbuscular mycorrhizal fungi (AMF) play important roles in plant growth. However, the importance of AMF in pathogenic stress on invasive plants were rarely studied. The effects of AMF ( Glomus etunicatum ) on the resistance to pathogenic fungus Rhizoctonia solani of an invasive plant Alternanthera philoxeroides were examined in this study. Our results showed that AMF significantly promoted stem length, spacer length, and leaf area of A. philoxeroides . The pathogen R. solani negatively impacted plant growth, including above-ground biomass and root characteristics. However, AMF inoculation mitigated these negative effects. Notably, AMF colonization rates increased significantly in the presence of pathogen. AMF significantly promoted the above-ground growth and decreased the root/shoot ratio to help resist pathogen. These findings indicate that AMF can enhance A. philoxeroides resistance to pathogenic stress, potentially contributing to its invasive success. This study provides insights into the complex interactions between invasive plants, beneficial fungi, and pathogens, which may have implications for understanding and managing plant invasions.
... The genetic diversity at the haplotype level in the ITS regions and trnL-trnF of E. densa, as reported in this study for four Brazilian reservoirs, is comparatively low when compared with the corresponding indices documented for other aquatic macrophytes (Fehrmann et al. 2012;Williams et al. 2020;Scorsim et al. 2023). Few haplotypes were found to colonize different ecosystems, indicating minimal genetic differentiation among the studied reservoir populations. ...
Article
Full-text available
Macrophytes harbor numerous potentially invasive species that pose a threat to biodiversity and ecosystem services in freshwater environments. Egeria densa (Hydrocharitaceae) stands out as a prominent invasive species, recognized as a significant global invader across various ecosystems. In this study, we conducted an assessment of the genetic variability of this species in four Brazilian reservoirs, which are part of the species’ native range, employing the internal transcribed spacer (ITS) from the nuclear marker and the intergenic chloroplast marker trnL-trnF. The obtained sequences were then compared with those available in GenBank (NCBI). The results of our investigation revealed a low genetic differentiation among the sampled populations. For ITS and trnL-trnF, we identified four and three distinct haplotypes, respectively, with a predominant single haplotype shared by most specimens. Notably, we did not observe a discernible phylogeographic structure. The data we obtained represents the first sequences of E. densa within its native habitat, providing valuable insights on its genetic diversity and helping to understand invasive processes.
... The genetic diversity (at the haplotype level) measured for the ITS regions (h and π = zero) and trnL-trnF (h = 0.4136, π = 0.00355) reported for E. najas from the Upper Paraná River basin in this study is low when compared to these same indices presented for other aquatic macrophytes (Fehrmann et al., 2012;Williams et al., 2020;Scorsim et al., 2023), with few haplotypes colonizing different ecosystems. The prevalence of a single haplotype found across all environments, along with the presence of single haplotypes in various environments along a cascade of dams, and the absence of genetic variability in the nuclear marker (ITS), suggest that the series of dams do not have a significant impact on the dispersal of E. najas propagules, and that the spatial dynamics of subpopulations adhere to metapopulation dynamics. ...
Article
Egeria najas is a submerged aquatic macrophyte native to South America, with high propagation in reservoirs and natural lakes, whose reproductive strategy is little known. Understanding the genetic diversity of macrophyte populations can provide important information about this species' dispersion and colonization strategies, and support management actions. We aimed to genetically characterize populations of E. najas that colonize reservoirs and natural aquatic habitats (in a floodplain) in the Upper Paraná River basin, using the molecular markers ITS and trnL-trnF. The results showed the absence of genetic variation for the nuclear marker ITS and 13 distinct haplotypes for trnL-trnF. One of these haplotypes occurred in all habitats and 11 are unique haplotypes, of which 5 occurred in the Itaipu Reservoir and 6 in the floodplain. The null genetic diversity for the nuclear marker and the genetic homogeneity of the studied populations indicates that the reproduction of E. najas is mostly vegetative. The source of chloroplast marker haplotype variability may be somatic mutations. The connectivity among aquatic environments associated with river flow favors the transport of aquatic macrophyte propagules to different habitats. In the case of E. najas, whose vegetative propagules regenerate easily, the frequency of migrations supports the low genetic variability observed in populations of the Upper Paraná. In addition, the ability to occupy new habitats and recolonize disturbed ones strongly indicates that E. najas populations follow the metapopulation dynamics.
Article
Full-text available
Crested floating heart [ Nymphoides cristata (Roxb.) Kuntze] is an invasive aquatic plant in the southeastern United States. For clonal plants like N. cristata , clonal diversity may influence response to control tactics and/or evolutionary potential. However, little is known about the diversity of introduced N. cristata . In this study, we used genotyping-by-sequencing to quantify N. cristata diversity in the southeastern U.S. and determine how that diversity is distributed across the invaded range. Our results show that at least three distinct genetic lineages of N. cristata are present in the southeastern U.S. Geographic distribution of the lineages varied, with one widespread lineage identified across several states and others only found in a single waterbody. There is also evidence of extensive asexual reproduction, with invaded waterbodies often host to a single genetic lineage. The genetic diversity reported in this study likely results from multiple introductions of N. cristata to the southeastern U.S. and should be considered by managers when assessing control tactics such as screening for biocontrol agents or herbicide testing. The extent and distribution of genetic diversity should also be considered by researchers studying the potential for invasive spread of N. cristata within the U.S. or hybridization with native Nymphoides species.
Article
Full-text available
The invasive aquatic weed hydrilla [Hydrilla verticillata (L.f.) Royle] exists in North America as two genetically and morphologically distinct strains, with the dioecious strain mostly found in the southern United States and the monoecious strain being more northern, including previously known sites in Connecticut. In 2016 an additional hydrilla population was located in a portion of the Connecticut River in Hartford County, Connecticut, with unusual morphological features relative to other Connect-icut populations. Hydrilla plants from this population were subjected to genetic testing, and their molecular sequences for one chloroplast (trnL-F) and two nuclear gene regions (internal transcribed spacer and phytoene desaturase) were compared against published data. The Connecticut River hydrilla plants are distinct from all known North American plants, representing a novel introduction, likely from northern Eurasia. The genetic novelty of this recent introduction may present additional ecological and management challenges beyond what has been encountered for hydrilla to date.
Article
Full-text available
Spatial variation in plant–herbivore interactions can be important in pest systems, particularly when insect herbivores are used as biological control agents to manage invasive plants. The geographic ranges of the invasive plant alligatorweed (Alternanthera philoxeroides) and its biological control agent the alligatorweed flea beetle (Agasicles hygrophila) do not completely overlap in the southeastern USA, producing spatial heterogeneity in interaction strength that may be related to latitude-correlated environmental gradients. We studied this system near the range margin of the alligatorweed flea beetle to test whether spatial variation in alligatorweed density was best explained by agent mean or maximum density, variability in agent density, agent attack timing, or a combination of biological control and environmental (i.e., weather) variables. The pattern that emerged was that mean agent and host densities were negatively and positively associated with latitude, respectively. Variability in agent density increased with latitude and was positively correlated with host density. We further discovered that agent first attack timing was negatively correlated with winter and spring temperatures and spring and summer precipitation, and positively correlated with seasonal temperature extremes, which was then directly influential on agent density and variability in density, and indirectly on host density. This study demonstrates that, contrary to common wisdom, weather-related timing of agent activity and population variability, but not agent mean density, contribute to the spatial heterogeneity observed in alligatorweed populations.
Article
Full-text available
A long‐standing explanation for invasion success is that invasive plants could evolve to be more competitive following introduction. This evolution of increased competitive ability (EICA) hypothesis, however, has seldom been tested with regard to intraspecific competition. Given that plants can display different responses to related and unrelated conspecifics, the evolution of intraspecific competitive ability might be specific to genotypes of different relatedness. Here, we grew five native (South American) and five introduced (North American) genotypes of the clonal herbaceous invasive plant Alternanthera philoxeroides alone, with above‐ground competition from kin (the same genotype) or from one of two types of strangers (another genotype from the same range or another genotype from the other range). When grown alone, introduced and native genotypes produced similar total biomass and storage‐root biomass. However, in response to intraspecific competition, introduced genotypes showed increases in total biomass and stem length, and a decrease in specific stem length, whereas native genotypes showed the opposite pattern. When grown with kin instead of strangers, introduced genotypes showed an increase in branch number, whereas native genotypes showed the opposite. Synthesis. Our study provided evidence for evolution of increased intraspecific competitive ability in an invasive plant. We also found, for the first time, that the interactions among kin were likely to shift from competition towards facilitation following introduction.
Article
Full-text available
Herbivory-induced responses in plants can both negatively affect subsequently colonizing herbivores and mitigate the effect of herbivory on the host. However, it is still less known whether plants exhibit specific responses to specialist and generalist her-bivores in non-secondary metabolite traits and how specificity to specialists and gen-eralists differs between invasive and native plant populations. We exposed an invasive plant, Alternanthera philoxeroides, to Agasicles hygrophila (Coleoptera, Chrysomelidae; specialist), Spodoptera litura (Lepidoptera, Noctuidae; generalist), manual clipping, or application of exogenous jasmonic acid and examined both the specificity of elicitation in traits of fitness (e.g., aboveground biomass), morphology (e.g., root:shoot ratio), and chemistry (e.g., C/N ratio and lignin), and specificity of effect on the subsequent performance of A. hygrophila and S. litura. Then, we assessed variation of the specificity between invasive and native populations (USA and Argentina, respectively). The results showed S. litura induced higher branching intensity and specific leaf area but lower C/N ratio than A. hygrophila, whereas A. hygrophila induced higher trichome density than S. litura. The negative effect of induction on subsequent larval growth was greater for S. litura than for A. hygrophila. Invasive populations had a weaker response to S. litura than to A. hygrophila in triterpenoid saponins and C/N ratio, while native populations responded similarly to these two herbivores. The specific effect on the two herbivores feeding on induced plants did not vary between invasive and native populations. Overall, we demonstrate specificity of elicitation to specialist and generalist herbivores in non-secondary metabolite traits, and that the generalist is more susceptible to induction than the specialist. Furthermore, chemical responses specific to specialist and generalist herbivores only exist in the invasive populations, consistent with an evolutionary change in specificity in the invasive populations. K E Y W O R D S biotic stimuli, coevolution, diet breath, plant invasion, plant-herbivore interactions
Article
Population genetic studies of within- and among-population genetic variability are still lacking for managed submerged aquatic plant species, and such studies could provide important information for managers. For example, the extent of within-population genetic variation may influence the potential for managed populations to locally adapt to environmental conditions and control tactics. Similarly, among-population variation may influence whether specific control tactics work equally effectively in different locations. In the case of invasive Eurasian watermilfoil ( Myriophyllum spicatum ), including interspecific hybrids with native northern watermilfoil ( M. sibiricum ), managers recognize that there is genetic variation for growth and herbicide response. However, it is unclear how much overall genetic variation there is, and how it is structured within and among populations. Here, we studied patterns of within- and among-lake genetic variation in 41 lakes in Michigan and 62 lakes in Minnesota using microsatellite markers. We found that within-lake genetic diversity was generally low, and among-lake genetic diversity was relatively high. However, some lakes were genetically diverse, and some genotypes were shared across multiple lakes. For genetically diverse lakes, managers should explicitly recognize the potential for genotypes to differ in control response, and should account for this in monitoring and efficacy evaluation, and for pre-treatment herbicide screens to predict efficacy. Similarly, managers should consider differences in genetic composition among lakes as a source of variation in the growth and herbicide response of lakes with similar control tactics. Finally, laboratory or field information on control efficacy from one lake may be applied to other lakes where genotypes are shared among lakes.
Article
Alligator weed (Alternanthera philoxe-roides (Martius) Grisebach) is an amphibious invasive plant native to South America. It is an allopolyploid that, in Argentina, possess two cytotypes, tetraploids and hexaploids. In the exotic range, the plant is biologically controlled with flea beetle Agasicles hygrophila Selman and Vogt but with different levels of success. The genotype of host plant is an important factor that needs to be considered in biological control programs. We studied how alligator weed ploidy level affects the oviposition preferences of A. hygrophila and its relation with female feeding preference, egg survival and larval performance in its native range. Females recognized the different ploidy levels of alligator weed and preferred to lay eggs on hexaploids than tetraploids. This choice positively affected larval performance and may be explained by the preference-performance hypothesis in this case for different plant cytotypes. Polyploidy should be considered in evaluating and prioritising biological control agents.
Article
The common approach to the multiplicity problem calls for controlling the familywise error rate (FWER). This approach, though, has faults, and we point out a few. A different approach to problems of multiple significance testing is presented. It calls for controlling the expected proportion of falsely rejected hypotheses — the false discovery rate. This error rate is equivalent to the FWER when all hypotheses are true but is smaller otherwise. Therefore, in problems where the control of the false discovery rate rather than that of the FWER is desired, there is potential for a gain in power. A simple sequential Bonferronitype procedure is proved to control the false discovery rate for independent test statistics, and a simulation study shows that the gain in power is substantial. The use of the new procedure and the appropriateness of the criterion are illustrated with examples.
Article
Monoecious and dioecious biotypes of Hydrilla verticillata were introduced from Asia into the United States (U.S.). Although biological control agent development has been ongoing for many years to combat this invasive aquatic weed, the focus has now shifted towards the monoecious biotype because of apparent incompatibilities between previously introduced agents and this lineage. To facilitate collection of natural enemies, we surveyed eastern areas of China and all of South Korea (592 samples from 129 sites) to locate geographic source areas with the introduced monoecious biotype. We used both chloroplast and nuclear microsatellite markers to identify genotypes. Eastern China had high genetic diversity and significant genetic structure across river basins, including three previously described chloroplast clades (B, C, D), one of which (B) includes both biotypes that were introduced into the U.S. South Korea had the monoecious biotype from clade B and clade C. South Korea had a subset of the genetic diversity in China, consistent with China being the ancestral region for hydrilla. U.S. introduced monoecious hydrilla had significantly lower diversity than this genotype in China and South Korea. U.S. monoecious microsatellite profiles cluster with samples from both China and South Korea, failing to resolve a clear region of origin. Reproductive strategies for clade B are more variable than in the introduced range with both monoecious and dioecious individuals sharing the same chloroplast haplotypes and microsatellite clusters. The introduced monoecious biotype of hydrilla is becoming a major problem in the U.S., but in the native range it is rare, patchily distributed, and often mixed with individuals from clade C. Current exploration for biological control agents will need to determine the genetic identity of the plants from which potential biological control agents are collected.