Morphological variations in southern African populations of Myriophyllum spicatum: Phenotypic plasticity or local adaptation?

Abstract

Variability in aquatic plant morphology is usually driven by phenotypic plasticity and local adaptations to environmental conditions experienced. This study aimed to elucidate which of these drivers is responsible for the morphological variation exhibited by three populations of Myriophyllum spicatum L. (Haloragaceae), a submerged aquatic plant whose status as native or exotic within southern Africa is uncertain. Individuals from three populations on the Vaal River (Northern Cape), Klipplaat River (Eastern Cape) and Lake Sibaya (KwaZulu-Natal) were grown under two nutrient treatments (high: 30mgN/kg sediment and low: sediment only), while all other variables were kept the same. Morphological characteristics were measured at the start of the experiment to obtain a baseline morphology, and again eight weeks later. By the end of the experiment, the individuals from each population had responded to the different growing conditions. In most cases, the individuals from each population were significantly larger under the high nutrient treatment (Stem diameter: F(5,86) =18.435, P< 0.001, Internode length: F(5,86) =5.0747, P< 0.001, Leaf length: F(5,86) =19.692, P< 0.001). Despite these differences in nutrient treatments, the growth pattern of each population remained true to the original starting point indicated by the lack of overlap between populations in the PCA groupings. This suggests that local adaptations are responsible for the differences in morphology between populations of M. spicatum, but shows that phenotypic plasticity does play a role as evidenced by individual responses to the different nutrient conditions. The development of these local adaptations within southern Africa suggests that the populations have had a long evolutionary history in the region and are relatively isolated with little reproductive mixing.

Full text

Morphological variations in southern African populations of
Myriophyllum spicatum: Phenotypic plasticity or local adaptation?
P.S.R. Weyl ⁎, J.A. Coetzee
Department of Zoology and Entomology, Rhodes University, P.O. Box 94, Grahamstown 6139, South Africa
abstractarticle info
Article history:
Received 5 June 2015
Received in revised form 24 July 2015
Accepted 28 July 2015
Available online xxxx
Edited by JS Boatwright
Keywords:
Genotypes
Gene flow
Geographic barriers
Haloragaceae
Isolated populations
Variability in aquatic plant morphology is usually driven by phenotypic plasticity and local adaptations to
environmental conditions experienced. This study aimed to elucidate which of these drivers is responsible for
the morphological variation exhibited by three populations of Myriophyllum spicatum L. (Haloragaceae), a
submerged aquatic plant whose status as native or exotic within southern Africa is uncertain. Individuals
from three populations on the Vaal River (Northern Cape), Klipplaat River (Eastern Cape) and Lake Sibaya
(KwaZulu-Natal) were grown under two nutrient treatments (high: 30 mg N/kg sediment and low: sediment
only), while all other variables were kept the same. Morphological characteristics were measured at the start
of the experiment to obtain a baseline morphology, and again eight weeks later. By the end of the experiment,
the individuals from eachpopulation had responded to the different growingconditions. In most cases, the indi-
viduals from each population were significantly larger under the high nutrient treatment (Stem diameter:
F
(5,86)
=18.435,Pb0.001, Internode length: F
(5,86)
=5.0747,Pb0.001, Leaf length: F
(5,86)
=19.692,Pb
0.001). Despite these differences in nutrient treatments, the growth pattern of each population remained true
to the original starting point indicated by the lack of overlap between populations in the PCA groupings. This
suggests that local adaptations are responsible for the differences in morphology between populations of
M. spicatum, but shows that phenotypic plasticity does play a role as evidenced by individual responses to the
different nutrient conditions. The development of these local adaptations within southern Africa suggests
that the populations have had a long evolutionary history in the region and are relatively isolated with little
reproductive mixing.
© 2015 SAAB. Published by Elsevier B.V. All rights reserved.
1. Introduction
It is widely accepted that aquatic plants are plastic in their responses
to environmental variables, and their morphology can be extremely
variable between populations and/or between seasons (Barko and
Smart, 1981; Barko and Smart, 1986; Koch and Seeliger, 1988; Barrett
et al., 1993; Idestam-Almquist and Kautsky, 1995; Santamaria, 2002).
Changes in plant morphology and physiology between populations of
the same species are often linked to both physiological stresses, such
as limited resources (De Kroon and Hutchings, 1995; Hutchings and
John, 2004), and to physical/mechanical stresses such as wave action
or current (Strand and Weisner, 2001; Boeger and Poulsan, 2003;
Arshid and Wani, 2013). These species responses are usually driven by
adaptive mechanisms, such as phenotypic plasticity (Grace, 1993;
Barrett et al., 1993; Hofstra et al., 1995) or local adaptations (Sultan,
2000; Kawecki and Ebert, 2004; Ward et al., 2008) that allow them to
adapt to the different climatic and environmental stresses to which
they are exposed.
Local adaptation is a genetic change, primarily driven by natural
selection on a local scale, where specific characters of a plant that en-
hance its fitness are selected for in a novel environment (Kawecki and
Ebert, 2004), while phenotypic plasticity is the ability of a single geno-
type to respond with changes in phenotypic characters that will better
suit the population to the prevailing habitat conditions (Bradshaw,
1965). The wide distributional range of many aquatic species is often
coupled with relatively low genetic variation of individuals within
populations, but high variation between populations, probably linked
to clonal or vegetative reproduction (Grace, 1993; Barrett et al., 1993).
In many cases, aquatic plants are thought to have a “general purpose
genotype”usually characterised by low levels of genetic variability but
capable of adapting to a diverse range of environmental conditions
through phenotypic plasticity (Baker, 1965; Barrett et al., 1993). There
are two forms of phenotypicplasticity that can be classed as either phys-
iological plasticity, where the responses have a physiological end point,
such as changes in photosynthetic capabilities; or as morphological
plasticity where the responses are manifested as a change in
South African Journal of Botany xxx (2015) xxx–xxx
⁎Corresponding author.
E-mail address: philipweyl@gmail.com (P.S.R. Weyl).
SAJB-01385; No of Pages 6
http://dx.doi.org/10.1016/j.sajb.2015.07.016
0254-6299/© 2015 SAAB. Published by Elsevier B.V. All rights reserved.
Contents lists available at ScienceDirect
South African Journal of Botany
journal homepage: www.elsevier.com/locate/sajb
Please cite this article as: Weyl, P.S.R., Coetzee, J.A., Morphological variations in southern African populations of Myriophyllum spicatum:
Phenotypic plasticity or local adaptation?, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.07.016

morphology (Bradshaw, 1965). These plastic responses, both physiolog-
ical and morphological, are important for the survival of a species in a
multitude of different environments over the wide geographical ranges
in which they are found (Bradshaw, 1965; Barrett et al., 1993).
Understanding the mechanisms that drive changes in thephenotype
of aquatic plants can prove useful in gaining insights into the genetic di-
versity and evolutionary history of the species in a region. Morphologi-
cal differences in introduced populations of aquatic plants are thought
to be primarily driven by phenotypic plasticity because of the relatively
low levels of genetic diversity and short time spent in the region (Riis
et al., 2010). In a study of three invasive submerged macrophytes in
New Zealand, Riis et al. (2010) concluded that the primary adaptive
strategy of all three species was phenotypic plasticity due to the low
levels of genetic diversity, coupled with the relatively short time period
since the firstintroduction of any of the species. The oldest introduction
was Elodea canadensis Mitch. (Hydrocharitaceae), and at just over 100
years, is considered too young for the development of local adaptations,
especially given the lack of genetic diversity within and between popu-
lations in NewZealand (Riis et al., 2010). Local adaptations are driven by
the process of natural selection, which result in different genotypes
adapted to local conditions and are likely to express differences in mor-
phological characters over much longer time scales (Kawecki and Ebert,
2004). A prerequisite for local adaptations to take place between popu-
lations is a relatively diverse gene pool within populations for natural
selection to act upon (Ward et al., 2008). In the case of introduced spe-
cies, this can be achieved through multiple introductions from different
source populations, and local adaptation can be considered an impor-
tant adaptive mechanism for the successful invasion of a species
(Parker et al., 2003).
Myriophyllum spicatum L. (Haloragaceae) is considered an invasive
species in southern Africa (Weyl and Coetzee, 2014), however, there
are questions as to how long this species has been present in the region.
Understanding the drivers of the morphological differences between
populations can infer how long this species has been here. There are
three distinct varieties or growth forms of M. spicatum which are
found in different regions that have very different climatic conditions
(Jacout Guillarmod, 1979). The differences in the morphology are so
great that it was initially thought that there were at least two species
of Myriophyllum in southern Africa, however, pollen analysis confirmed
a single species (Jacout Guillarmod, 1979). The first variety is
characterised as large and robust with large leaves and relatively thick
stems (Fig. 1A and D) and is found in the Vaal River, Northern Cape,
South Africa. This is the only population to be recorded as problematic
in South Africa. The second variety is characterised as delicate small
plants, with small leaves and highly branched, thin stems (Fig. 1B and
E). It is found growing in the subtropical environment in Lake Sibaya,
KwaZulu-Natal, South Africa. The third variety is large plants, and simi-
lar to the first variety in growth form, but the internode length is very
short so the leaves become tightly packed leading to a bottlebrush
type appearance (Fig. 1C and F), and is found in the high altitude re-
gions, including the Amathola Mountains, Eastern Cape and the
KwaZulu-Natal Midlands, South Africa. These varieties in southern
Africa can be identified in the earliest herbarium specimens from the re-
gions where they originate, for example, the Vaal River variety was first
collected in 1897, the Lake Sibaya variety in 1966 and the high altitude
variety collected in the Mooi River in 1894 (Fig. 1). These morphological
characteristics are still present in the populationsfound in these biogeo-
graphic regions today (Fig. 1).
A
F
ED
CB
Fig. 1. The three morphological variations of Myriophyllum spicatum found in southern Africa as recorded by herbarium specimens (A–C) and present day photographic representations of
living specimens (D–F). The first A & D: robust large leaf form collected from the Vaal River, Northern Cape, the second B & E: delicate branched form collected in Lake Sibaya, KwaZulu-Natal
and thethird C & F: the large growth formwith very shortinternodelengthsgiving it a bottlebrush appearance,the herbarium specimen collectedin the Mooi River, KwaZulu-Natal Midlands
and the living specimen collected in Hogsback, Eastern Cape.
2P.S.R. Weyl, J.A. Coetzee / South African Journal of Botany xxx (2015) xxx–xxx
Please cite this article as: Weyl, P.S.R., Coetzee, J.A., Morphological variations in southern African populations of Myriophyllum spicatum:
Phenotypic plasticity or local adaptation?, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.07.016

The aim of this study was to determine whether the morphological
differences between three populations of M. spicatum in southern
Africa are driven by phenotypic plasticity or local adaptations through
underlying genetic variation. If the morphological differences between
the populations are driven primarily by plastic responses to environ-
mental conditions, then plants grown under the same conditions
would respond in a similar way and their morphologies would converge
(Santamaria, 2002; Riis et al., 2010). However, if the differences are local
adaptations within the species, then the morphology of the plants from
the different populations would not converge and the varieties would
remain distinct fromeach other. If the driver of these morphological dif-
ferences is local adaptations, then it would suggest that the populations
have been isolated with limited gene flow for a considerable time
within the different biogeographic regions.
2. Materials and methods
2.1. Experimental design
The initial stock plants from the three populations of M. spicatum
used in this experiment were collected from wild populations within a
two week period during the beginning of April 2013. The three
populations collected were 1) ‘Vaal’, collected from Vaalharts Weir,
Vaal River, Northern Cape (28°06′55.5″S 24°56′19.1″E); 2) ‘Sibaya’,col-
lected from Lake Sibaya, KwaZulu-Natal (27°25′02.9″S 32°41′47.4″E)
and 3) ‘Hogsback’, collected from the Klipplaat River, Eastern Cape
(32°29′06.4″S 26°56′48.5″E). Specimens were collected from all three
localities on previous surveys and lodged in the Selmar Schonland
Herbarium (GRA) (Table 1). The plants were returned to a greenhouse
tunnel at Rhodes University, where they were acclimated to the green-
house conditions by floating sprigs freely in borehole water for a period
of at least four days prior to the commencement of the experiment. A
total of 40 sprigs from each population were cut to 10 cm growth tips
with no branches, and initial morphological measurements were
taken from each sprig. The morphological measurements that were
taken included the stem diameter, internode length, leaf length and
number of leaflet pairs on each leaf. All measurements were taken at
the 5th internode to standardise the position from where the measure-
ments were taken on each individual and between varieties.
In comparison to the experiments of Santamaría et al. (2003) and
Riis et al. (2010), the environmental conditions that may have played
a role in determining the morphological characters were all kept con-
stant viz. sediment type (the same sediment type used by Martin and
Coetzee, 2014), temperature, light and photoperiod. The sprigs were
then planted in two growth conditions; a low nutrient, pond sediment
only treatment, and a high nutrient pond sediment treatment fertilised
with 30 mg N/kg from Multicote® 15-8-12 N:P:K 8–9monthformula-
tion fertiliser (Haifa Group, Matam-Haifa, Israel). The sprigs were ran-
domly planted into seedling trays which held approximately 100 ml of
treatment sediment in each pot, and then placed into a plastic pond
(height 60 cm; width 90 cm and length 110 cm) which contained bore-
hole water to a depth of 50 cm. The seedling trays were arranged in a
random block design to rule out any possible location effects of light
and temperature on the plants in the pond. The same morphological
measurements were taken again after an eight week growth period
which is sufficient time to observe differences in growth responses
(Riis et al., 2010), and were compared between populations at both nu-
trient levels.
2.2. Statistical analyses
The morphological measurements, including stem diameter,
internode length, leaf length and number of leaflet pairs, between the
three populations of M. spicatum in each nutrient treatment at the be-
ginning and at the end of the experiment were compared using a GLM
Repeated-Measures ANOVA followed by a Tukey Post-Hoc test to
identify homogenous groups, in Statistica V.12 (StatSoft Inc., 2013). To
determine the similarity between the three populations of M. spicatum
at the start of the experiment and 8 weeks later grown under the
same conditions, a Principal Component Analysis (PCA) in Primer V.6
(Clarke and Gorley, 2006) was performed using the morphological
characters (stem diameter, internode length, leaf length and number
of leaflets) and plotted to visualise the results.
3. Results
By the end of the experiment, the plants from each population
showed a response to the different growing conditions in their mor-
phology when compared to the starting measurements within popula-
tions. The stem diameter (Fig. 2A) for all three populations remained
unchanged under low nutrients but were significantly larger under
high nutrient conditions (F
(5,86)
= 18.435, Pb0.001). The internode
length (Fig. 2B) showed a similar trend with no change at low nutrient
conditions but significantly longer for the Vaal and Sibaya population
under high nutrient conditions, while the Hogsback population signifi-
cantly decreased in internode length at high nutrient conditions
(F
(5,86)
= 5.0747, Pb0.001). Under low nutrient conditions the leaf
length (Fig. 2C) was significantly smaller for all three populations,
while under high nutrient conditions it remained unchanged
(F
(5,86)
= 19.692, Pb0.001). The number of leaflets (Fig. 2D) remained
unchanged for all three populations irrespective of nutrient level
(F
(5,86)
= 0.4126, P=0.838).
The growth pattern of each population relative to the other popula-
tions, however, did not change based on nutrient condition despite dif-
ferences between nutrient treatments. The stem diameter under both
nutrient treatments was always larger for the Vaal and Hogsback popu-
lations compared to the Sibaya population (F
(5,86)
= 18.435, Pb0.001)
(Fig. 2A). But there was a significant increase in the stem diameter for
the Vaal (0.15 ± 0.008 cm to 0.23 ± 0.006 cm) and Hogsback
(0.16 ± 0.005 cm to 0.22 ± 0.008 cm) population at high nutrient con-
ditions, while the Sibaya population remained unchanged (0.11 ±
0.003 cm to 0.13 ± 0.004 cm) (F
(5,86)
= 18.435, Pb0.001) (Fig. 2A).
While there was no difference between the internode length for both
the Vaal and Sibaya population under both nutrient levels (ranging
from 1.05 cm to 1.49 depending on nutrient condition), the Hogsback
population was always significantly smaller ranging between 0.5 and
0.57 cm depending on nutrient treatment (F
(5,86)
=5.0747,Pb0.001)
(Fig. 2B). Only the Vaal and Sibaya populations showed an increase in
internode length under high nutrient conditions (F
(5,86)
= 5.0747, Pb
0.001) (Fig. 2B). Irrespective of nutrient level, the Sibaya population
had significantly smaller leaf lengths (1.21 ± 0.027 cm high nutrients
and 0.9 ± 0.028 cm low nutrients) than both the Vaal and Hogsback
Table 1
The most recent herbarium specimens of Myriophyllum spicatum lodged from the three localities where the test plants were collected for the current study.
Country Collector Collector # Date Locality Lat. Long. Herbarium
South Africa P. Weyl PW 1 2012 Lake Sibaya −27.2796 32.68426 GRA
P. Weyl PW 2 2012 Lake Sibaya −27.2796 32.68426 GRA
P. Weyl PW 20 2012 Vaalhaarts Weir −28.0922 24.9686 GRA
P. Weyl PW 24 2012 Klipplaat River −32.4938 26.9496 GRA
3P.S.R. Weyl, J.A. Coetzee / South African Journal of Botany xxx (2015) xxx–xxx
Please cite this article as: Weyl, P.S.R., Coetzee, J.A., Morphological variations in southern African populations of Myriophyllum spicatum:
Phenotypic plasticity or local adaptation?, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.07.016

populations which ranged between 2.22 cm and 2.76 cm depending on
nutrient level (F
(5,86)
= 19.692, Pb0.001) (Fig. 2C). There was no differ-
ence between the leaf lengths of each variety, except for the Sibayapop-
ulation which had a significantly longer leaf under high nutrient
conditions (Fig. 2C). The Vaal population always had significantly
fewer leaflet pairs (11.75 ± 0.437 for high and 11.38 ± 0.317 for low
nutrient conditions) than both the Sibaya and Hogsback populations
which ranged between 12.47 and 13.71 leaflet pairs (F
(5,86)
= 0.4126,
P=0.838) (Fig. 2D). The leaflet pairsremained unchanged between nu-
trient treatments (F
(5,86)
= 0.4126, P=0.838) (Fig. 2D).
The PCA shows nooverlap in the groupings between the populations
at both the start of the experiment when the plants were first collected
from the field populations and after the eight week experimental peri-
od, grown under the two nutrient treatments (Fig. 3). The PC1 accounts
for 59.7% of the variation (Eigenvalue = 1.75 × 10
−2
) and the PC2 ac-
counts for 27.3% of the variation (Eigenvalue = 8.02 × 10
−3
).
4. Discussion
Despite the same growth conditions, nutrient treatments, sediment
type, water depth and light, the three different populations of
M. spicatum did not converge in their morphologies. This suggests that
the differences in the morphological varieties are driven by local adap-
tations and southern Africa has different genotypes of M. spicatum that
have adapted to their current environmental conditions over a long
period of time. This is fairly common in terrestrial plants with wide dis-
tributions, which are often characterised by large genetic variation
(Bradshaw, 1965) and often specialised to local environmental condi-
tions which allows them to cover such wide geographic and climatic
regions (Van Tienderen, 1990). On the other hand, aquatic plants are
often characterised by low levels of genetic variation which makes
phenotypic plasticity extremely important, and while the development
of a “general purpose genotype”capable of surviving in a wide range of
environments is common (Baker, 1965; Barrett et al., 1993), this is not
always the case. Several studies on aquatic plants have shown that
0
0.1
0.2
0.3
Stem diameter (cm)
Low nutrients High nutrients
1,a
3,c
4,ab
3,c
1,a
2,b
1
2
1
1
2
1
A
0
0.4
0.8
1.2
1.6
Internode length (cm)
Low nutrients High nutrients
1,a
4,b
3,c
3,c
2,b
1,a
2
1
1
2
1
1
B
0
1
2
3
Leaf length (cm)
Low nutrients High nutrients
3,a
1,c
2,b
1,c
3,a
4,b
1
2
1
1
2
1
C
0
4
8
12
16
Vaal Sibaya Hogsback Vaal Sibaya Hogsback
Vaal Sibaya Hogsback Vaal Sibaya Hogsback
Vaal Sibaya Hogsback Vaal Sibaya Hogsback
Vaal Sibaya Hogsback Vaal Sibaya Hogsback
Number of leaflets
Low nutrients High nutrients
1,a
2,b
2,b
1,a
2,ab
2,b
2
2
1
2
2
1
D
Fig. 2. Thestart (white bars)and end (grey bars)mean measurementsfor A: stem diameter, B: internodelength, C: leaflength and D: numberof leaflets of eachpopulation of Myriophyllum
spicatum grown under the two nutrient conditions, high nutrients (30 mg N/kg pond sediment) and low nutrients (pond sediment only).The numbers above the means indicate signif-
icant differences between the start and end values, while the letters indicate significant differences between treatments at the end of the experiment (Pb0.001) and error bars indicate
standard error of the mean.
-0.3 -0.2 -0.1 0 0.1 0.2 0.3
PC1
-0.2
-0.1
0
0.1
0.2
0.3
PC2
VV
V
V
V
V
V
V
V
V
V
V
VV
V
V
V
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
S
S
SS
S
SSS
S
SS
S
S
SS
S
Vl
Vl
Vl
Vl
Vl
Vl
Vl
Vl
Vl
Vl
Vl
Vl
Vl
Vl Vl
Vl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Hl
Sl
Sl
Sl
Sl
Sl
Sl
Sl
Sl
Sl
Sl
Sl
Sl
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh
Vh Vh
Hh
Hh
Hh
Hh Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
Hh
HhHh
Sh
Sh
Sh Sh
Sh
Sh Sh
Sh
Sh
Sh
Sh
Sh
Sh
Sh Sh
Sh
Stem diameter
Internode length
Leaf length
# leaflets
Fig. 3. Similarity between the populations of Myriophyllum spicatum at the startof the ex-
periment and 8 weeks later, basedon the four morphological characters measured (stem
diameter, internode length, leaf length and number of leaflets). PC1 accounts for 59.7% of
the variation (Eigenvalue = 1.75 × 10
−2
) while PC2 accounts for 27.3% of the variation
(Eigenvalue = 8.02 × 10
−3
). The letterscorrespond to the sourcepopulation and growing
condition for an 8 week period; V = Vaal population at start, Vl = Vaal low nutrients,
Vh = Vaal high nutrients, S = Sibaya population at st art, Sl = Sibaya low nutrients,
Sh = Sibaya high nutrients, H = Hogsback population at start, Hl = Hogsback low nutri-
ents, Hh = Hogsback high nutrients.
4P.S.R. Weyl, J.A. Coetzee / South African Journal of Botany xxx (2015) xxx–xxx
Please cite this article as: Weyl, P.S.R., Coetzee, J.A., Morphological variations in southern African populations of Myriophyllum spicatum:
Phenotypic plasticity or local adaptation?, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.07.016

although phenotypic plasticity is important, local adaptations are possi-
ble and do play a role in the survival and fitness of some species across a
multitude of environmental conditions (Barrett et al., 1993; Santamaría
et al., 2003).
The different M. spicatum populations that were grown under the
two nutrient treatments did show some degree of plasticity and a re-
sponse to the growing conditions. All three genotypes that were
grown under the lower nutrient condition (pond sediment only)
adopted a significantly smaller size in most of the morphological char-
acters measured. This was not surprising as several aquatic plant species
respond to limiting nutrient conditions. For example, Riis et al. (2010)
reported that Lagarosiphon major (Ridl.) Moss and E. canadensis (both
Hydrocharitaceae) had reduced shoot diameter and leaf width under
low nitrogen and phosphorous conditions, while individuals of Ranun-
culus peltatus Schrank (Ranunculaceae) were smaller when grown
under low nutrient conditions, than individuals grown under high nu-
trient conditions, the latter tending to have long branching shoots
(Garbey et al., 2004). Barko (1983) suggested that not only nutrient
composition, but also the interaction with nutrients and sediment
type, play a role in the growth form of M. spicatum, while Barko and
Smart (1981) showed that lightplays a role in the morphology of aquat-
ic macrophytes. Strand and Weisner (2001) also indicated that light and
water depth play a role in the morphological characteristics of
M. spicatum where plants that are light limited (periphyton growth or
depth) are usually longer and more branched. The drivers of the subtle
morphological changes in the current study indicate that nutrient level
was the most important, as this varied across treatments, but the poten-
tial effect of light and water depth on the morphology of M. spicatum
was not tested here.
The findings from the present study are in contrast to the situation in
North America (Aiken et al., 1979) where M. spicatum was introduced
in the 1940s (Couch and Nelson, 1985). Plants from different regions
in Canada and the USA grown under the same conditions, side by side
in a greenhouse, responded by converging in their morphologies,
which Aiken et al. (1979) attributed to a common clonal origin of the
North American material. This is despite the recent discovery of two ge-
notypes of M. spicatum in North America (Zuellig and Thum, 2012). It is
possible that during the study by Aiken et al. (1979), plants from the
same genotype were inadvertently selected. Riis et al. (2010) had simi-
lar findings when three introduced submerged aquatic plants, L. major,
Egeria densa Planch. (Hydrocharitaceae) and E. canadensis were grown
under similar conditions. Initially the plants had different growth
forms or morphologies, and physiologies (photosynthetic rates) that
were presumably adapted to the conditions to which they were ex-
posed in the wild populations. The morphologies and photosynthetic
rates of all the species reverted to a similar point or growth form after
a seven-week growing period (Riis et al., 2010). In addition to this,
they tested the genetic diversity between the populations using ampli-
fied fragment length polymorphisms (AFLPs), which resulted in very lit-
tle difference between the populations, suggesting that each species
originated from a single introduction (Riis et al., 2010). This suggests
that for introduced species that lack genetic diversity, phenotypic
plasticity may be the most important factor driving the differences
between populations of the same species growing in different climatic
or environmental conditions (Riis et al., 2010).
The three genotypes of M. spicatum identified in southern Africa are
similar to populations from the European range, where in a study by
Aiken et al. (1979), populations from England and the Netherlands
showed slightly different morphologies when grown under the same
conditions. This suggests that these populations also exhibit subtle
local adaptations. The different populations of M. spicatum in southern
Africa are presumably locally adapted to the conditions where they
are found, however, this does not rule out the relative importance of
phenotypic plasticity for these populations to adapt to changing condi-
tions. In a transplanting experiment, Santamaría et al. (2003)
transplanted Stuckenia pectinata (L.) (Syn. Potamogeton pectinatus)
(Potamogetonaceae) populations from different regions of Europe.
Their results suggest that there were strong local adaptations and the
performance of transplanted individuals was much lower in the novel
environment than when grown at the source location. However, despite
the local adaptations, the different populations of S. pectinata also
showed a certain degree of phenotypic plasticity (Santamaría et al.,
2003), suggesting that local adaptation and phenotypic plasticity may
work synergistically. The study by Santamaría et al. (2003) was within
the native range of S. pectinata which suggests a long evolutionary
history in the region and local adaptations are not surprising due to
the relatively high genetic diversity in native populations compared to
introduced populations (Ward et al., 2008). In many introduced
aquatic species, including Eichhornia crassipes (Mart.) Solms-Laub.
(Pontederiaceae), E. densa and Alternanthera philoxeroides (Mart.)
Griseb. (Amaranthaceae), genetic variation is low between populations,
likely linked to their clonal reproduction (Ward et al., 2008) and the
adaptive mechanisms are probably linked to phenotypic plasticity rath-
er than local adaptations (Parker et al., 2003; Geng et al., 2007).
The evolution of locally adapted populations requires an interaction
between divergent selection and other evolutionary forces such as nat-
ural selection and gene flow (Kawecki and Ebert, 2004). The develop-
ment of locally adapted populations of M. spicatum in southern Africa
suggests that the populations are sufficiently isolated that there is little
or no gene flow between them. This isolation could be geographic as
there are significant distances, over major catchments, between the
populations (Weyl and Coetzee, 2014), or it could be reproductive, as
sexual reproduction is not considered very important for M. spicatum
(Patten, 1956; Aiken, 1981), or a combination of both, which would fur-
ther isolate the populations. This compares to North America, where
M. spicatum is characterised by two genotypes with overlapping distri-
butions, however, there is little evidence of sexual reproduction be-
tween them in the field populations (Zuellig and Thum, 2012). The
development of these locally adapted genotypes also suggests that
there could be a relatively high genetic diversity of the populations in
southern Africa. What is unclear is whether this diversity has resulted
from multiple introductions (Ward et al., 2008; Lavergne et al., 2010)
or a long enough evolutionary history in the region for genetic muta-
tions to occur. It is possible for genetic differentiation to occur quite rap-
idly, for example, despite the low levels of genetic variation, E. densa in
New Zealand is showing signs of genetic differentiation between popu-
lations in less than 100 years since it was first introduced (Lambertini
et al., 2010). This could suggest that the evolution of locally adapted
populations in E. densa could occur quite rapidly in New Zealand,
under the right conditions (Kawecki and Ebert, 2004; Ward et al.,
2008) despite the low genetic variation inherent in so many species
(Barrett et al., 1993; 2008; Lambertini et al., 2010).
The results from the present study point to local adaptation and not
phenotypic plasticity as the more likely driver of the different morpho-
logical variations of the M. spicatum populations from southern Africa
that were tested. This does not rule out the importance of phenotypic
plasticity in shaping the morphology of these species in the environ-
ments where they occur, and probably explains why they exist in a
wide variety of habitats in southern Africa (Weyl and Coetzee, 2014).
The presence of local adaptations in southern Africa suggests that the
populations have been and still are isolated and it is unlikely that
there is much genetic mixing between the systems where these popula-
tions are found. This is not surprising as the populations are separated
by major catchments, geographical barriers such as mountains and
climatic zones, all of which make dispersal between the populationsex-
tremely difficult. Future in depth genetic studies of the populations of
M. spicatum within southern Africa could shed light on this.
Acknowledgements
The Working for Water Programme of the Natural Resource Man-
agement Programmes (Department of Environmental Affairs), South
5P.S.R. Weyl, J.A. Coetzee / South African Journal of Botany xxx (2015) xxx–xxx
Please cite this article as: Weyl, P.S.R., Coetzee, J.A., Morphological variations in southern African populations of Myriophyllum spicatum:
Phenotypic plasticity or local adaptation?, South African Journal of Botany (2015), http://dx.doi.org/10.1016/j.sajb.2015.07.016

Africa, are thanked for their financial support of this project. We would
like to thank the anonymous reviewers for their constructive comments
that have improved the original manuscript.
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