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ABSTRACT: Studies of invasive species often demonstrate that exotic or invasive plants have reduced levels of predation when first introduced
into novel geographic areas. In some systems, native predators may subsequently be introduced and the plant-predator association
is re-established. In this study, we analyzed a recent reassociation between the introduced tree, Albizia julibrissin, and its native seed predator, Bruchidius terrenus, which has also been introduced, in the vicinity of Athens, GA, USA. We documented a rapid increase in the levels of bruchid
beetle predation on seeds of A. julibrissin from 2001 to 2002 and substantial predation in the following 5years. From a six-year survey, we found that relative susceptibility
to beetle predation among individual trees remained consistent over time. Furthermore, we found that phenology had significant
effects on beetle predation on fruits and seeds, with fruits developed from early flowers having higher levels of beetle attacks
than ones developed from later flowers in the season. Finally, seeds and fruits attacked by beetles were more likely to be
attacked by a Fusarium fungus, probably due to coinfestation of the two predators. Host-predator reassociation that causes substantial loss of fertility
or shows a relationship with reproductive phenology may have population level demographic effects or evolutionary consequences
and deserve further research attention.
Keywords
Albizia julibrissin
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Bruchidius terrenus
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Fusarium
–Fruit phenology–Mimosa–Seed predation
Biological Invasions 05/2012; 13(4):1043-1053. · 2.90 Impact Factor
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ABSTRACT: Scattered patches of Polylepis forest that occur within the 3,000–4,500m altitudinal belt of the Andean Cordillera from Venezuela to Argentina have been
hypothesized to be remnants of once continuous forests whose range became fragmented through anthropogenic activities that
probably preceded the Spanish conquest. Allozyme variation of Polylepis pauta from 12 forest populations in three adjacent watersheds in Northeastern Ecuador was investigated to assess whether observed
patterns of gene diversity were consistent with a more continuous historical range of the species and to evaluate the populations’
potential for restoration. Genetic diversity and polymorphism in P. pauta populations were higher than mean values for most wind pollinated and dispersed temperate and tropical tree species with
regional distributions. Genetic differentiation among watersheds was lower than among populations within each watershed. Isolation
by distance was not evident and several populations from different watersheds were more genetically similar than populations
from the same watershed. Larger forest patches with broader altitudinal ranges had more alleles. Forest patches on steeper
slopes and at higher elevations supported populations with less genetic diversity; this might have resulted from the predominance
of vegetative reproduction in these landslide prone areas. The amount of genetic diversity maintained by P. pauta, coupled with low genetic differentiation among populations within and among watersheds, is consistent with a more continuous
historical range of the species in Northeastern Ecuador and point to the Oyacachi basin as having the highest levels of genetic
diversity.
Keywords
Polylepis pauta
–Allozyme variation–Isolation by distance–Historical range–Landscape genetics–Forest restoration
Conservation Genetics 04/2012; 12(3):607-618. · 1.61 Impact Factor
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ABSTRACT: American ginseng, Panax quinquefolius L., is one of the most heavily traded medicinal plants in North America. The effect of harvest on genetic diversity in ginseng was measured with a single generation culling simulation program. Culling scenarios included random harvest at varying levels, legal limit random harvest and legal limit mature plant harvest. The legal limit was determined by the proportion of legally harvestable plants per population (% mature plants per population). Random harvest at varying levels resulted in significant loss of genetic diversity, especially allelic richness. Relative to initial levels, average within-population genetic diversity (H
e) was significantly lower when plants were culled randomly at the legal limit (Mann–Whitney U=430, p<0.001) or when only mature plants were culled (Mann–Whitney U=394, p<0.01). Within-population genetic diversity was significantly higher with legal limit mature plant harvest (H
e=0.068) than when plants were culled randomly at the legal limit (H
e=0.064; U=202, p<0.01). Based on these simulations of harvest over one generation, we recommend that harvesting fewer than the proportion of mature plants could reduce the negative genetic effects of harvest on ginseng populations.
Biodiversity and Conservation 04/2012; 14(2):493-504. · 2.24 Impact Factor
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ABSTRACT: Self-fertilization is a common mating system in plants and is known to reduce genetic diversity, increase genetic structure and potentially put populations at greater risk of extinction. In this study, we measured the genetic diversity and structure of two cedar glade endemic species, Leavenworthia alabamica and L. crassa. These species have self-incompatible (SI) and self-compatible (SC) populations and are therefore ideal for understanding how the mating system affects genetic diversity and structure. We found that L. alabamica and L. crassa had high species-level genetic diversity (H(e)=0.229 and 0.183, respectively) and high genetic structure among their populations (F(ST)=0.45 and 0.36, respectively), but that mean genetic diversity was significantly lower in SC compared with SI populations (SC vs SI, H(e) for L. alabamica was 0.065 vs 0.206 and for L. crassa was 0.084 vs 0.189). We also found significant genetic structure using maximum-likelihood clustering methods. These data indicate that the loss of SI leads to the loss of genetic diversity within populations. In addition, we examined genetic distance relationships between SI and SC populations to analyze possible population history and origins of self-compatibility. We find there may have been multiple origins of self-compatibility in L. alabamica and L. crassa. However, further work is required to test this hypothesis. Finally, given their high genetic structure and that individual populations harbor unique alleles, conservation strategies seeking to maximize species-level genetic diversity for these or similar species should protect multiple populations.
Heredity 02/2011; 106(2):310-8. · 4.60 Impact Factor
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ABSTRACT: Several Agave species have played an important ethnobotanical role since prehistory in Mesoamerica and semiarid areas to the north, including central Arizona. We examined genetic variation in relict Agave parryi populations northeast of the Mogollon Rim in Arizona, remnants from anthropogenic manipulation over 600 years ago. We used both allozymes and microsatellites to compare genetic variability and structure in anthropogenically manipulated populations with putative wild populations, to assess whether they were actively cultivated or the result of inadvertent manipulation, and to determine probable source locations for anthropogenic populations. Wild populations were more genetically diverse than anthropogenic populations, with greater expected heterozygosity, polymorphic loci, effective number of alleles and allelic richness. Anthropogenic populations exhibited many traits indicative of past active cultivation: fixed heterozygosity for several loci in all populations (nonexistent in wild populations); fewer multilocus genotypes, which differed by fewer alleles; and greater differentiation among populations than was characteristic of wild populations. Furthermore, manipulated populations date from a period when changes in the cultural context may have favoured active cultivation near dwellings. Patterns of genetic similarity among populations suggest a complex anthropogenic history. Anthropogenic populations were not simply derived from the closest wild A. parryi stock; instead they evidently came from more distant, often more diverse, wild populations, perhaps obtained through trade networks in existence at the time of cultivation.
Molecular Ecology 03/2010; 19(8):1622-37. · 5.52 Impact Factor
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Kenneth D Whitney,
Eric J Baack, James L Hamrick,
Mary Jo W Godt,
Brian C Barringer,
Michael D Bennett,
Christopher G Eckert,
Carol Goodwillie,
Susan Kalisz,
Ilia J Leitch,
Jeffrey Ross-Ibarra
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ABSTRACT: Genome sizes vary widely among species, but comprehensive explanations for the emergence of this variation have not been validated. Lynch and Conery (2003) hypothesized that genome expansion is maladaptive, and that lineages with small effective population size (N(e)) evolve larger genomes than those with large N(e) as a consequence of the lowered efficacy of natural selection in small populations. In addition, mating systems likely affect genome size evolution via effects on both N(e) and the spread of transposable elements (TEs). We present a comparative analysis of the effects of N(e) and mating system on genome size evolution in seed plants. The dataset includes 205 species with monoploid genome size estimates (corrected for recent polyploidy) ranging from 2Cx = 0.3 to 65.9 pg. The raw data exhibited a strong positive relationship between outcrossing and genome size, a negative relationship between N(e) and genome size, but no detectable N(e)x outcrossing interaction. In contrast, phylogenetically independent contrast analyses found only a weak relationship between outcrossing and genome size and no relationship between N(e) and genome size. Thus, seed plants do not support the Lynch and Conery mechanism of genome size evolution. Further work is needed to disentangle contrasting effects of mating systems on the efficacy of selection and TE transmission.
Evolution 02/2010; 64(7):2097-109. · 5.15 Impact Factor
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ABSTRACT: Question: How do studies of the distribution of genetic diversity of species with different life forms contribute to the development of conservation strategies?Location: Old-growth forests of the southeastern United States.Methods: Reviews of the plant allozyme literature are used to identify differences in genetic diversity and structure among species with different life forms, distributions and breeding systems. The general results are illustrated by case studies of four plant species characteristic of two widespread old-growth forest communities of the southeastern United States: the Pinus palustris – Aristida stricta (Longleaf pine – wiregrass) savanna of the Coastal Plain and the Quercus – Carya – Pinus (Oak-hickory-pine) forest of the Piedmont. Genetic variation patterns of single-gene and quantitative traits are also reviewed.Results: Dominant forest trees, represented by Pinus palustris(longleaf pine) and Quercus rubra (Northern red oak), maintain most of their genetic diversity within their populations whereas a higher proportion of the genetic diversity of herbaceous understorey species such as Sarracenia leucophylla and Trillium reliquum is distributed among their populations. The herbaceous species also tend to have more population-to-population variation in genetic diversity. Higher genetic differentiation among populations is seen for quantitative traits than for allozyme traits, indicating that interpopulation variation in quantitative traits is influenced by natural selection.Conclusion: Developing effective conservation strategies for one or a few species may not prove adequate for species with other combinations of traits. Given suitable empirical studies, it should be possible to design efficient conservation programs that maintain natural levels of genetic diversity within species of conservation interest.
Applied Vegetation Science 02/2009; 9(1):51 - 58. · 1.68 Impact Factor
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ABSTRACT: Genetic and genotypic diversity found within populations of threatened plant species can have important implications for their conservation and management. In this study we describe genetic and genotypic diversity found within 10 populations of the endemic shrub Elliottiaracemosa (Ericaceae), the Georgia plume. E. racemosa is a threatened species known from fewer than 50 locations, all within the state of Georgia, USA. Seedset is limited to nonexistent in some E. racemosa populations and sexual recruitment has not been documented. However, the species is known to spread vegetatively via root-sprouts. Twenty-one allozyme loci were resolved for E. racemosa, nine of which were polymorphic. Compared with other woody taxa, E. racemosa has low genetic (i.e. allelic) diversity within populations (Hep = 0.063) and at the species level (Hes = 0.091). Most of the genetic variation (82%) was found within populations, and genetic identities between populations were high (mean I = 0.96). However, genotypic diversity (i.e. the number of multilocus genotypes) differed markedly among populations. Two of the 10 populations consisted almost entirely of single multilocus genotypes, whereas more than 20 multilocus genotypes (in samples of 48 stems) were detected at three sites. Sites in which few multilocus genotypes were detected have low seedset, suggesting that the lack of clonal diversity limits reproduction in some populations of this reportedly self-incompatible species.
Molecular Ecology 10/2008; 8(1):75 - 82. · 5.52 Impact Factor
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ABSTRACT: The balance between clonal and sexual reproduction can vary widely among plant populations, and the extent of clonality may be influenced by the combined effects of historical land use and variation in environmental conditions. We investigated patterns of clonal spread in five Trillium cuneatum populations, two in the Appalachian Mountains characterized by mesic, cooler conditions, and three at lower elevations experiencing warmer, drier conditions and greater disturbance. Using a new measure of the genet effective number and innovative orthogonal contrast methods, we quantified genet structure, contrasting clonal growth in the mountains with that in the Piedmont. Asexual propagation was more common in the Piedmont, where 25% of the sampled ramets were clonally derived, but was much less frequent in the mountains (7% clonal replicates). Hierarchical partitioning of variation in genet diversity showed that the majority (75.8%) of the variation resulted from more vegetative replication in the Piedmont. Most of the remaining variation (21.6%) was attributable to differences between urban and rural Piedmont populations, and a small, statistically nonsignificant fraction of the variation (2.6%) was due to interpopulation differences within the mountains. Higher frequency of cloning may enhance both genetic and demographic population viability in fragmented Piedmont habitats.
American Journal of Botany 10/2008; 95(10):1254-61. · 2.66 Impact Factor
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ABSTRACT: The importance of genotype-specific selection between host and symbiont, founder effect, and clonal reproduction in Rhizobia leguminosarum biovar trifolii populations is relatively unknown. A field experiment was conducted to sample 1268 isolates of R. l. bv. trifolii from four genotypically distinct Trifolium pratense plants for allozyme variation at nine loci. Genetic and genotypic variation, population genetic substructure, and linkage disequilibrium were estimated. Of the 1268 isolates 188 genotypically distinct strains (electrophoretic types or ETs) were identified with an average of 11.04 different ETs per plant. Total genetic diversity in the plot was 0.346 and most of the variation was found within plants (= 80%). Our data suggests that genotype-specific selection between the rhizobia and the four host-plant genotypes tested does not influence local population structure, but evidence of founder effect was present. Significant linkage disequilibrium was observed and is most likely due to the clonal reproduction of R. l. bv. trifolii.
Molecular Ecology 06/2008; 5(6):707 - 714. · 5.52 Impact Factor
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ABSTRACT: Comparative analyses of spatial genetic structure (SGS) among species, populations, or cohorts give insight into the genetic consequences of seed dispersal in plants. We analysed SGS of a weedy tree in populations with known and unknown recruitment histories to first establish patterns in populations with single vs. multiple founders, and then to infer possible recruitment scenarios in populations with unknown histories. We analysed SGS in six populations of the colonizing tree Albizia julibrissin Durazz. (Fabaceae) in Athens, Georgia. Study sites included two large populations with multiple, known founders, two small populations with a single, known founder, and two large populations with unknown recruitment histories. Eleven allozyme loci were used to genotype 1385 individuals. Insights about the effects of colonization history from the SGS analyses were obtained from correlograms and Sp statistics. Distinct differences in patterns of SGS were identified between populations with multiple founders vs. a single founder. We observed significant, positive SGS, which decayed with increasing distance in the populations with multiple colonists, but little to no SGS in populations founded by one colonist. Because relatedness among individuals is estimated relative to a local reference population, which usually consists of those individuals sampled in the study population, SGS in populations with high background relatedness, such as those with a single founder, may be obscured. We performed additional analyses using a regional reference population and, in populations with a single founder, detected significant, positive SGS at all distances, indicating that these populations consist of highly related descendants and receive little seed immigration. Subsequent analyses of SGS in size cohorts in the four large study populations showed significant SGS in both juveniles and adults, probably because of a relative lack of intraspecific demographic thinning. SGS in populations of this colonizing tree is pronounced and persistent and is determined by the number and relatedness of founding individuals and adjacent seed sources. Patterns of SGS in populations with known histories may be used to indirectly infer possible colonization scenarios for populations where it is unknown.
Molecular Ecology 06/2008; 17(12):2865-79. · 5.52 Impact Factor
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ABSTRACT: Understanding the spatial distribution of genetic diversity (i.e., spatial genetic structure [SGS]) within plant populations can elucidate mechanisms of seed dispersal and patterns of recruitment that may play an important role in shaping the demography and spatial distribution of individuals in subsequent generations. Here we investigate the SGS of allozyme diversity in 2 populations of the southeastern North American endemic shrub, Ceratiola ericoides. The data suggest that the 2 populations have similar patterns of SGS at distances of 0-45 m that likely reflect the isolation by distance (IBD) model of seed dispersal. However, at distances >or=50 m, the pattern of SGS differs substantially between the 2 populations. Whereas one population continues to reflect the classical IBD pattern, the second population shows a marked increase in autocorrelation coefficient (r) values at 50-75 m. Furthermore, r values at these distances are as much as 33% higher than at 0-5 m where the highest r value would be predicted by IBD. A likely explanation is the differing frequencies of 2 fruit morphologies in these populations and the greater role that birds play in seed dispersal in the second population.
The Journal of heredity 05/2008; 99(6):604-9. · 2.05 Impact Factor
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ABSTRACT: We isolated nine microsatellite loci from the Guanacaste tree (Enterolobium cyclocarpum) and optimized them for future research on breeding populations of this species. Loci were screened across 53 individuals from one population and were shown to be variable with the number of alleles per locus ranging from five to 15. Polymorphic information content ranged from 0.420 to 0.900 and observed heterozygosity ranged from 0.547 to 0.906.
Molecular Ecology Resources 01/2008; 8(1):129-31. · 3.06 Impact Factor
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ABSTRACT: Pre-Columbian farmers cultivated several species of agave in central Arizona from ca. A.D. 600-1350. Because of the longevity and primarily asexual reproduction of these species, relict agave clones remain in the landscape and provide insights into pre-Columbian agricultural practices. We analyzed variation in allozyme allele frequencies to infer genetic effects of prehistoric cultivation on Agave murpheyi and A. delamateri, specifically to estimate genetic diversity and structure, to determine whether cultivated populations descended from a single clone, and to examine regional-scale genetic variation. Agave murpheyi maintained more genetic diversity at the species and population levels than A. delamateri, and A. murpheyi populations typically included more multilocus genotypes. Relict plants from prehistoric fields reflect a more complex history than descent from a single clone; A. murpheyi populations may have included more diversity initially because bulbils (produced routinely in A. murpheyi but not A. delamateri) and possibly seed would have facilitated transport of genetically diverse planting stock. Genetic variation in both cultigens was lower than in most contemporary commercial crops but similar to that observed in modern traditional agricultural systems.
American Journal of Botany 09/2007; 94(9):1479-90. · 2.66 Impact Factor
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ABSTRACT: The floral architecture and phenology of the tree species Albizia julibrissin (Fabaceae) offer the potential for flowers within inflorescences to share common pollen donors. Patterns of paternity within individual tree crowns may differ among isolated individuals and those in populations due to differences in pollinator foraging behavior. To determine how genetic diversity is partitioned within individual seed pools and whether these patterns differ among isolated and population trees, we obtained all fruits from three inflorescences from four clusters from three isolated trees and from three population trees in Athens, Georgia. We assayed 14 polymorphic allozymes to genotype all progeny within singly sired fruits to determine the multilocus genotype of each fruit's pollen donor. Inflorescences had multiple pollen donors, but simulation analyses revealed that redundancy of pollen donors tended to be more likely within inflorescences than randomly across the crown. Analysis of genetic and genotypic diversity indicated that individual maternal trees received pollen from many donors in uneven frequencies. Results suggest that isolated trees receive pollen from slightly fewer pollen donors and experience more within-plant pollinator movement than trees in populations. However, isolated trees receive qualitatively similar pollen from many sources, suggesting that these trees are not effectively isolated and that pollen moves long distances in this species.
American Journal of Botany 01/2007; 94(1):111-8. · 2.66 Impact Factor
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ABSTRACT: Pollen movement plays a critical role in the distribution of genetic variation within and among plant populations. Direct measures of pollen movement in the large, continuous populations that characterize many herbaceous plant species are often technically difficult and biologically unreliable. Here, we studied contemporary pollen movement in four large populations of Trillium cuneatum. Three populations, located in the Georgia Piedmont, are exposed to strong anthropogenic disturbances, while the fourth population, located in the Southern Appalachian Mountains, is relatively undisturbed. Using the recently developed TwoGener analysis, we extracted estimates of the effective number of pollen donors (N(ep)), effective mating neighbourhood size (A(ep)) and the average distance of pollen movement (delta) for each population. We extended the TwoGener method by developing inference on the paternal gametic contribution to the embryo in situations where offspring genotypes are inferred from seeds and elaiosomes of species with bisporic megagametogenesis. Our estimates indicate that maternal plants do not sample pollen randomly from a global pool; rather, pollen movement in all four populations is highly restricted. Although the effective number of pollen donors per maternal plant is low (1.22-1.66) and pollen movement is highly localized in all populations, N(ep) in the disturbed Piedmont populations is higher and there is more pollen movement than in the mountains. The distance pollen moves is greater in disturbed sites and fragmented populations, possibly due to edge effects in Trillium habitats.
Molecular Ecology 08/2006; 15(8):2047-58. · 5.52 Impact Factor
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ABSTRACT: Pollinator behavior plays a central role in determining patterns of pollen-mediated gene movement in zoophilous angiosperms. A species' floral display can strongly influence the behavior of its pollinators and thereby affect its evolutionary pathway. We used paternity analysis to directly measure and describe mating patterns within 15 populations of the epiphytic orchid, Laelia rubescens, in Costa Rican dry forest. Strict correlated mating by orchids allows inference of the precise multilocus diploid genotype of the pollen parents. Our data show that mean effective population sizes were small (11.2 in 1999 and 11.8 in 2000) relative to the number of flowering genets (63 and 56, respectively). Fewer genets were reproductively successful as females than males. The relationship between reproductive success (RS) and floral display within three cluster size classes was consistent between years, with large (>30 inflorescences) and small (≤10 inflorescences) clusters often having significantly lower RS than expected, while the RS of medium-sized clusters (11-30 inflorescences) often significantly exceeded expectations. Paternity analysis allowed us to take advantage of the pollination biology of L. rubescens to provide unusually detailed insights into mating patterns, pollen-mediated gene movement and RS for populations of this epiphytic orchid, an herbaceous perennial, distributed in three-dimensional space.
American Journal of Botany 07/2006; 93(7):1010-8. · 2.66 Impact Factor
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ABSTRACT: Many factors interact to determine genetic structure within populations including adult density, the mating system, colonization history, natural selection, and the mechanism and spatial patterns of gene dispersal. We examined spatial genetic structure within colonizing populations of Quercus rubra seedlings and Pinus strobus juveniles and adults in an aspen-white pine forest in northern Michigan, USA. A 20-year spatially explicit demographic study of the forest enables us to interpret the results in light of recent colonization of the site for both species. We assayed 217 Q. rubra seedlings and 171 P. strobus individuals at 11 polymorphic loci using nine allozyme systems. Plant genotypes and locations were used in an analysis of spatial genetic structure. Q. rubra and P. strobus showed similar observed levels of heterozygosity, but Q. rubra seedlings have less heterozygosity than expected. Q. rubra seedlings show spatial genetic clumping of individuals on a scale to 25 m and levels of genetic relatedness expected from the clumped dispersion of half-siblings. In contrast, P. strobus has low levels of genetic relatedness at the smallest distance class and positive spatial genetic structure at scales < 10 m within the plot. The low density of adult Q. rubra outside the study plot and limited, spatially clumped rodent dispersal of acorns is likely responsible for the observed pattern of spatial genetic structure and the observed heterozygote deficit (i.e. a Wahlund effect). We attribute weaker patterns observed in P. strobus to the longer dispersal distance of seeds and the historical overlap of seed shadows from adults outside of the plot coupled with the overlap of seed shadows from younger, more recently established reproductive adults. The study demonstrates the utility of long-term demographic data in interpreting mechanisms responsible for generating contemporary patterns of genetic structure within populations.
Molecular Ecology 04/2006; 15(3):851-61. · 5.52 Impact Factor
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ABSTRACT: Epiphytes comprise ca. 10% of all vascular plant species and therefore contribute substantially to plant diversity, particularly in tropical and subtropical regions. Little is known, however, about the specificity of the relationship between epiphytes and their phorophytes. Phorophyte specificity is assessed for the neotropical epiphytic orchid, Laelia rubescens Lindley, in the tropical dry forest of Costa Rica. A rating system was used to account for local abundance of tree species supporting L. rubescens as well as for the geographic distribution of the orchid-phorophyte association. A nonspecific relationship was ob-served with L. rubescens growing on 33 tree species and, at one site, limestone rock. In every case where the orchid was locally abundant on a phorophyte, the association also was geographically widespread; however, 70% of the orchid-phorophyte associations were locally sparse. Of these, 35% were widespread, and 65% were restricted.
Selbyana. 01/2006; 27:60-64.
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ABSTRACT: We assessed genetic diversity and its distribution in the rare southeastern US forest understory species, Trillium reliquum. In all, 21 loci were polymorphic (PS=95.5%) and the mean number of alleles per polymorphic locus was 3.05. However, genetic diversity was relatively low (Hes=0.120) considering the level of polymorphism observed for this outcrossing species. A relatively high portion of the genetic diversity (29.7%) was distributed among populations. There was no relationship between population size and genetic diversity, and we did not detect significant inbreeding. These results are best explained by the apparent self-incompatibility of this species, its longevity and clonal reproduction. To address questions regarding the history of T. reliquum's rarity, we compared results for T. reliquum with that of its more common and partially sympatric congener, T. cuneatum. Despite shared life history traits and history of land use, we observed significant genetic differences between the two species. Although T. cuneatum contains slightly lower polymorphism (Ps=85%), we detected significantly higher genetic diversity (Hes=0.217); most of its genetic diversity is contained within its populations (GST=0.092). Our results suggest that not only is there little gene flow among extant T. reliquum populations, but that rarity and population isolation in this species is of ancient origins, rather than due to more recent anthropogenic fragmentation following European colonization. The Chattahoochee River was identified as a major barrier to gene exchange.
Heredity 11/2005; 95(4):306-14. · 4.60 Impact Factor
