Introduction history and population genetics of the invasive grass Bromus tectorum (Poaceae) in Canada

The invasive annual Bromus tectorum (cheatgrass) is distributed in Canada primarily south of 52° N latitude in two diffuse ranges separated by the extensive coniferous forest in western Ontario. The grass was likely introduced independently to eastern and western Canada post-1880. We detected regional variation in the grass's genetic diversity using starch gel electrophoresis to analyze genetic diversity at 25 allozyme loci in 60 populations collected across Canada. The Pgm-1a & Pgm-2a multilocus genotype, which occurs in the grass's native range in Eastern Europe, is prevalent in eastern Canada but occurs at low frequency in western Canada. In contrast, the Got-4c multilocus genotype, found in the native range in Central Europe, is widespread in populations from western Canada. Overall genetic diversity of B. tectorum is much higher in eastern Canada than in the eastern U.S., while the genetic diversity in populations in western North America is similar between Canada and the U.S. The distribution of genetic diversity across Canada strongly suggests multiple introduction events. Heterozygous individuals, which are exceedingly rare in B. tectorum, were detected in three Canadian populations. Formation of novel genotypes through occasional outcrossing events could spark adaptive evolution and further range expansion across Canada of this exceedingly damaging grass.     

Ecological genetics of vernalization response in Bromus tectorum L. (Poaceae)

BACKGROUND AND AIMS: Bromus tectorum (cheatgrass or downy brome) is an exotic annual grass that is dominant over large areas of former shrubland in western North America. To flower in time for seed production in early summer, B. tectorum plants generally require vernalization at winter temperatures, either as imbibed seeds or as established seedlings. METHODS: Variation in response to increasing periods of vernalization as seeds or seedlings for progeny of ten full-sib families from each of four B. tectorum populations from contrasting habitats was studied. KEY RESULTS: As vernalization was increased from 0 to 10 weeks, the proportion of plants flowering within 20 weeks increased, weeks to initiation of flowering decreased, and seed yield per plant increased, regardless of whether plants were vernalized as seeds or seedlings. Most of the variation was accounted for by differences among populations. Plants of the warm desert population flowered promptly even without vernalization, while those of the cold desert, foothill and montane populations showed incremental changes in response variables as a function of vernalization period. Populations differed in among-family variance, with the warm desert population generally showing the least variance and the cold desert population the most. Variation among populations and among families within populations decreased as vernalization period increased, whereas the non-genetic component of variance showed no such pattern. CONCLUSIONS: Variation in vernalization response was found to be adaptively significant and apparently represents the result of contrasting selection regimes on a range of founder genotypes.     

Local population differentiation in <Emphasis Type="Italic">Bromus tectorum</Emphasis> L. in relation to habitat-specific selection regimes

A central question of invasion biology is how an exotic species invades new habitats following its initial establishment. Three hypotheses to explain this expansion are: (1) the existence of ‘general purpose’ genotypes, (2) the in situ evolution of novel genotypes, and (3) the dispersal of existing specialized genotypes into habitats for which they are pre-adapted. Bromus tectorum is a selfing exotic winter annual grass that has achieved widespread dominance in semiarid western North America and that is actively invading salt desert habitats. We examined mechanisms driving this invasion in three complementary studies. In reciprocal seeding experiments with ten populations from saline playa, salt desert shrubland, and upland sagebrush communities along a salinity gradient in western Utah, we found that seeds from the playa population were able to establish better than those of most other populations across all habitats, including two highly saline sites. Seeds of one of the two seed sources from upland big sagebrush communities established best on the site of origin, while the seeds of the other source established well across a range of non-saline sites. In a greenhouse experiment with playa and upland seed sources, we found evidence of adaptation to high salinity in the playa source. Playa plants were larger than upland plants in control treatments and invested less in root. They maintained vigor and seed production at high salinity better than upland plants. Molecular genetic analysis showed that the playa population was dominated by a single SSR genotype absent from upland big sagebrush populations, which were comprised of SSR genotypes generally rare or absent in populations from salt desert shrubland sites. Our results demonstrate local population differentiation in B. tectorum, resulting at least in part from differential selection on pre-adapted genotypes with characteristic marker fingerprints. We found little evidence for selection favoring novel genotypes.     

Bromus tectorum (Poaceae) in midcontinental United States: Population genetic analysis of an ongoing invasion

Biological invasions can be substantially influenced by the genetic sampling associated with a species' introduction. As a result, we assessed the genetic and evolutionary consequences of the entry and spread of the invasive grass Bromus tectorum (cheatgrass) across the United States midcontinent through an analysis of 54 populations, using enzyme electrophoresis. On average, these populations display 1.04 alleles per locus (A), 4.1% percent polymorphic loci per population (%P) and an expected mean heterozygosity (H(exp)) value of 0.009. Heterozygotes, which have been rarely reported for B. tectorum in North America, occur in three populations in the midcontinent and are likely novel multilocus genotypes that arose postimmigration. The midcontinent distribution of multilocus genotypes suggests that plant immigrants came directly from either the native range or the eastern United States, or both. Continued dispersal of preadapted genotypes and the assembly of populations that are genetic admixtures may enhance this invasion by increasing both the genetic diversity within populations and the selection of novel genotypes arising from occasional outcrossing. The potential for postimmigration evolution in most species points to the largely unrecognized need to block the introduction of new, potentially aggressive genotypes of an alien species already in the United States.     

Fine-scale genetic structure enhances biparental inbreeding by promoting mating events between more related individuals in wild soybean (Glycine soja; Fabaceae) populations

Outcrossing between genetically distant individuals in a plant population enhances allelic heterozygosity-an important source for genetic diversity and adaptive evolution. Fine-scale spatial genetic structure (FSGS) can interfere with outcrossing by promoting mating between more related individuals. To test the influence of FSGS on outcrossing, FSGS and outcrossing rates were analyzed with four wild soybean (Glycine soja) populations from different habitats, using simple sequence repeat (SSR) fingerprints. Spatial autocorrelation analysis indicated variable FSGS (15.44-25.87 m) in all four populations. Multilocus mixed-mating analysis of 1605 progeny indicated substantial variation in single-locus outcrossing (T(s) = 6.3-12.6%) although the total outcrossing rates as estimated by multilocus outcrossing (T(m) = 12.8-17%) did not vary significantly among populations. The comparison between FSGS and outcrossing rates demonstrated that strong FSGS with large genetic patch size can enhance biparental inbreeding by promoting mating between more related individuals in a population. The results suggest that patch size management can aid in situ conservation by avoiding formation of strong FSGS and encouraging true outcrossing among individuals.     

Strong genetic differentiation in the invasive annual grass <Emphasis Type="Italic">Bromus tectorum</Emphasis> across the Mojave–Great Basin ecological transition zone

Bromus tectorum, an inbreeding annual grass, is a dominant invader in sagebrush steppe habitat in North America. It is also common in warm and salt deserts, displaying a larger environmental tolerance than most native species. We tested the hypothesis that a suite of habitat-specific B. tectorum lineages dominates warm desert habitats. We sampled 30 B. tectorum Mojave Desert and desert fringe populations and genotyped 10–26 individuals per population using 69 single nucleotide polymorphic (SNP) markers. We compared these populations to 11 Great Basin steppe and salt desert populations. Populations from warm desert habitats were dominated by members of two haplogroups (87 % of individuals) that were distinct from haplogroups common in Great Basin habitats. We conducted common garden studies comparing adaptive traits and field performance among haplogroups typically found in different habitats. In contrast to the haplogroup abundant in sagebrush steppe, warm desert haplogroups generally lacked a vernalization requirement for flowering. The most widespread warm desert haplogroup (Warm Desert 1) also had larger seeds and a higher root:shoot ratio than other haplogroups. In the field, performance of warm desert haplogroups was dramatically lower than the sagebrush steppe haplogroup at one steppe site, but one warm desert haplogroup performed as well as the steppe haplogroup under drought conditions at the other site. Our results suggest that B. tectorum succeeds in widely disparate environments through ecotypic variation displayed by distinct lineages of plants. Accounting for this ecotypic variation is essential in modeling its future distribution in response to climate change.     

Adaptive phenotypic plasticity in a clonal invader

Organisms featuring wide trait variability and occurring in a wide range of habitats, such as the ovoviviparous New Zealand freshwater snail Potamopyrgus antipodarum, are ideal models to study adaptation. Since the mid‐19th century, P. antipodarum, characterized by extremely variable shell morphology, has successfully invaded aquatic areas on four continents. Because these obligately and wholly asexual invasive populations harbor low genetic diversity compared to mixed sexual/asexual populations in the native range, we hypothesized that (1) this phenotypic variation in the invasive range might be adaptive with respect to colonization of novel habitats, and (2) that at least some of the variation might be caused by phenotypic plasticity. We surveyed 425 snails from 21 localities across northwest Europe to attempt to disentangle genetic and environmental effects on shell morphology. We analyzed brood size as proxy for fitness and shell geometric morphometrics, while controlling for genetic background. Our survey revealed 10 SNP genotypes nested into two mtDNA haplotypes and indicated that mainly lineage drove variation in shell shape but not size. Physicochemical parameters affected both shell shape and size and the interaction of these traits with brood size. In particular, stronger stream flow rates were associated with larger shells. Our measurements of brood size suggested that relatively larger slender snails with relatively large apertures were better adapted to strong flow than counterparts with broader shells and relatively small apertures. In conclusion, the apparent potential to modify shell morphology plays likely a key role in the invasive success of P. antipodarum; the two main components of shell morphology, namely shape and size, being differentially controlled, the former mainly genetically and the latter predominantly by phenotypic plasticity.     

Adaptive plasticity and niche expansion in an invasive thistle

Phenotypic differentiation in size and fecundity between native and invasive populations of a species has been suggested as a causal driver of invasion in plants. Local adaptation to novel environmental conditions through a micro‐evolutionary response to natural selection may lead to phenotypic differentiation and fitness advantages in the invaded range. Local adaptation may occur along a stress tolerance trade‐off, favoring individuals that, in benign conditions, shift resource allocation from stress tolerance to increased vigor and fecundity and, therefore, invasiveness. Alternately, the typically disturbed invaded range may select for a plastic, generalist strategy, making phenotypic plasticity the main driver of invasion success. To distinguish between these hypotheses, we performed a field common garden and tested for genetically based phenotypic differentiation, resource allocation shifts in response to water limitation, and local adaptation to the environmental gradient which describes the source locations for native and invasive populations of diffuse knapweed (Centaurea diffusa). Plants were grown in an experimental field in France (naturalized range) under water addition and limitation conditions. After accounting for phenotypic variation arising from environmental differences among collection locations, we found evidence of genetic variation between the invasive and native populations for most morphological and life‐history traits under study. Invasive C. diffusa populations produced larger, later maturing, and therefore potentially fitter individuals than native populations. Evidence for local adaptation along a resource allocation trade‐off for water limitation tolerance is equivocal. However, native populations do show evidence of local adaptation to an environmental gradient, a relationship which is typically not observed in the invaded range. Broader analysis of the climatic niche inhabited by the species in both ranges suggests that the physiological tolerances of C. diffusa may have expanded in the invaded range. This observation could be due to selection for plastic, “general‐purpose” genotypes with broad environmental tolerances.     

Widespread generalist clones are associated with range and niche expansion in allopolyploids of Pacific Northwest Hawthorns (Crataegus L.)

Range and niche expansion are commonly associated with transitions to asexuality, polyploidy and hybridity (allopolyploidy) in plants. The ability of asexual polyploids to colonize novel habitats may be due to widespread generalist clones, multiple ecologically specialized clones, or may be a neutral by‐product of multiple, independent origins of asexual polyploids throughout the range. We have quantified niche size and divergence for hawthorns of the Pacific Northwest using data from herbarium vouchers with known cytotypes. We find that all polyploid niches diverge from that of the diploid range, and allopolyploids have the broadest niches. Allotetraploids have the largest niche and the widest geographic distribution. We then assessed the genetic mechanism of range expansion by surveying the ecological and geographic distribution of genotypes within each cytotype from sites in which fine‐scale habitat assessments were completed. We find no isolation by either geographic or ecological distance in allopolyploids, suggesting high dispersal and colonization ability. In contrast, autotriploids and diploids show patterns of isolation by geographic distance. We also compared the geographic and ecological distributions of clonal genotypes with those of randomly drawn sites of the most widespread cytotype. We found that most clones are geographically widespread and occur in a variety of habitats. We interpret these findings to suggest that patterns of range and niche expansion in Pacific Northwest Hawthorns may stem from these widespread, ecologically generalist clones of hybrid origin.     

Genomic footprints of a biological invasion: Introduction from Asia and dispersal in Europe of the topmouth gudgeon (Pseudorasbora parva)

Facilitated by the intensification of global trading, the introduction and dispersal of species to areas in which they are historically non‐native is nowadays common. From an evolutionary standpoint, invasions are paradoxical: not only non‐native environments could be different from native ones for which introduced individuals would be ill‐adapted, but also small founding population size should be associated with reduced adaptive potential. As such, biological invasions are considered valuable real‐time evolutionary experiments. Here, we investigated the population structure and adaptive potential of the highly invasive topmouth gudgeon (Pseudorasbora parva) across Europe and East Asia. We RAD‐sequenced 301 specimens from sixteen populations and three distinct within‐catchment invaded regions as well as two locations in the native range. With 13,785 single nucleotide polymorphisms, we provide conclusive evidence for a genome‐wide signature of two distinct invasion events, in Slovakia and Turkey, each originating from a specific area in the native range. A third invaded area, in France, appears to be the result of dispersal within the invasive range. Few loci showed signs of selection, the vast majority of which being identified in the Slovakian region. Functional annotation suggests that faster early stage development, resistance to pollution and immunocompetence contribute to the invasion success of the local habitats. By showing that populations in the invasive range have different evolutionary histories, our study reinforces the idea that populations, rather than species, are the units to consider in invasion biology.     

The joint evolution of mating system and pollen performance: Predictions regarding male gametophytic evolution in selfers vs. outcrossers

Studies of sexual selection in plants historically have focused on pollinator attraction, pollen transfer, gametophytic competition, and post-fertilization discrimination by maternal plants. Pollen performance (the speeds of germination and pollen tube growth) in particular is thought to be strongly subject to intrasexual selection, but the effect of mating system on this process has not been rigorously evaluated. Here we propose four predictions derived from the logic that pollen performance should evolve with mating system as an adaptive response to: (1) the competitive environment among pollen genotypes and (2) variation among female genotypes regularly encountered by a given pollen genotype. First, as previously predicted, due to the higher potential for intense selection among diverse pollen genotypes in outcrossing relative to selfing taxa, pollen should evolve to germinate and/or to grow more rapidly in outcrossers than in selfers. Second, due to stronger selection on pollen performance in outcrossing than in selfing taxa, heritable variation in pollen tube growth rate is more likely to be purged in outcrossers. In selfers, by contrast, genetic variation in pollen tube growth rates may readily accumulate because selfing reduces the number of genetically distinct male gametophytes likely to be deposited on any given stigma, thereby relaxing selection on male gametophytic traits. A summary of published studies presented here provides preliminary support for this prediction. Third, due to the high probability that the pollen of outcrossing individuals will be exposed to multiple pistil genotypes, we predict that the pollen of habitually outcrossing taxa will evolve to perform more consistently across female genotypes than the pollen of selfing taxa. Fourth, we predict that epistatic interactions between pollen and pistil genotypes are more likely to evolve in selfers than in outcrossers. We suggest several empirical approaches that may be used to test these predictions.     

Correlation between molecular markers and adaptively significant genetic variation in Bromus tectorum (Poaceae), an inbreedingannual grass

Single sequence repeat (SSR) and amplified fragment length polymorphic (AFLP) molecular marker genotypes in cheatgrass (Bromus tectorum) were compared to published data on phenotypic variation in seed dormancy, vernalization requirement, and resistance to the pathogen Ustilago bullata. Several features of cheatgrass facilitated this study: it is a recent invader in the western United States, has considerable phenotypic polymorphism, and is an obligate self-pollinator. Forty self-pollinating lines from four populations common to the three phenotypic data sets were analyzed for molecular genetic variation using seven SSR loci and 31 AFLP loci. We examined correlations between distance matrices using the Mantel test for each pair of studies. The two molecular data sets were significantly correlated (r = 0.636). The AFLP markers often distinguished among several lines with identical SSR genotypes. The AFLP data were also significantly correlated with the phenotypic data (r values from 0.4640 to 0.5658), but the SSR data were much more highly correlated (r values from 0.677 to 0.844). The difference between molecular marker systems was especially notable when an outlier population from Potosi Pass, Nevada, was excluded from the analysis. These results suggest that SSR markers may be good surrogates for phenotypic traits in population genetic studies of strongly inbreeding species such as cheatgrass.     

Landscape genetics of an early successional specialist in a disturbance‐prone environment

Species that specialize in disturbed habitats may have considerably different dispersal strategies than those adapted to more stable environments. However, little is known of the dispersal patterns and population structure of such species. This information is important for conservation because many postfire specialists are at risk from anthropogenic changes to natural disturbance regimes. We used microsatellite markers to assess the effect of landscape variation and recent disturbance history on dispersal by a small mammal species that occupies the early seral stage of vegetation regeneration in burnt environments. We predicted that a postfire specialist would be able to disperse over multiple habitat types (generalist) and not exhibit sex‐biased dispersal, as such strategies should enable effective colonization of spatially and temporally variable habitat. We found significant differentiation between sites that fitted an isolation‐by‐distance pattern and spatial autocorrelation of multilocus genotypes to a distance of 2–3 km. There was no consistent genetic evidence for sex‐biased dispersal. We tested the influence of different habitat‐ and fire‐specific landscape resistance scenarios on genetic distance between individuals and found a significant effect of fire. Our genetic data supported recently burned vegetation having greater conductance for gene flow than unburnt habitat, but variation in habitat quality between vegetation types and occupied patches had no effect on gene flow. Postfire specialists must evolve an effective dispersal ability to move over distances that would ensure access to early successional stage vegetation. Natural disturbance and natural heterogeneity may therefore not influence population genetic structure as negatively as expected.     

Native perennial grasses show evolutionary response to Bromus tectorum (cheatgrass) invasion

Invasive species can change selective pressures on native plants by altering biotic and abiotic conditions in invaded habitats. Although invasions can lead to native species extirpation, they may also induce rapid evolutionary changes in remnant native plants. We investigated whether adult plants of five native perennial grasses exhibited trait shifts consistent with evolution in response to invasion by the introduced annual grass Bromus tectorum L. (cheatgrass), and asked how much variation there was among species and populations in the ability to grow successfully with the invader. Three hundred and twenty adult plants were collected from invaded and uninvaded communities from four locations near Reno, Nevada, USA. Each plant was divided in two and transplanted into the greenhouse. One clone was grown with B. tectorum while the other was grown alone, and we measured tolerance (ability to maintain size) and the ability to reduce size of B. tectorum for each plant. Plants from invaded populations consistently had earlier phenology than those from uninvaded populations, and in two out of four sites, invaded populations were more tolerant of B. tectorum competition than uninvaded populations. Poa secunda and one population of E. multisetus had the strongest suppressive effect on B. tectorum, and these two species were the only ones that flowered in competition with B. tectorum. Our study indicates that response to B. tectorum is a function of both location and species identity, with some, but not all, populations of native grasses showing trait shifts consistent with evolution in response to B. tectorum invasion within the Great Basin.     

Hybridization and the colonization of novel habitats by annual sunflowers

Although invasive plant species often have a hybrid ancestry, unambiguous evidence that hybridization has stimulated the evolution of invasive behaviors has been difficult to come by. Here, we briefly review how hybridization might contribute to the colonization of novel habitats, range expansions, and invasiveness and then describe work on hybrid sunflowers that forges a direct link between hybridization and ecological divergence. We first discuss the invasion of Texas by the common sunflower and show that the introgression of chromosomal segments from a locally adapted species may have facilitated range expansion. We then present evidence that the colonization of sand dune, desert floor, and salt marsh habitats by three hybrid sunflower species was made possible by selection on extreme or “transgressive” phenotypes generated by hybridization. This body of work corroborates earlier claims regarding the role of hybridization in adaptive evolution and provides an experimental and conceptual framework for ongoing studies in this area.     

Rapid evolution of Medicago polymorpha during invasion shifts interactions with the soybean looper

The Enemy Release Hypothesis posits that invasion of novel habitats can be facilitated by the absence of coevolved herbivores. However, a new environment and interactions with unfamiliar herbivores may impose selection on invading plants for traits that reduce their attractiveness to herbivores or for enhanced defenses compared to native host plants, leading to a pattern similar to enemy release but driven by evolutionary change rather than ecological differences. The Shifting Defense Hypothesis posits that plants in novel habitats will shift from specialized defense mechanisms to defense mechanisms effective against generalist herbivores in the new range. We tested these ideas by comparing herbivore preference and performance of native (Eurasia)‐ and invasive (New World)‐range Medicago polymorpha, using a generalist herbivore, the soybean looper, that co‐occurs with M. polymorpha in its New World invaded range. We found that soybean loopers varied in preference and performance depending on host genotype and that overall the herbivore preferred to consume plant genotypes from naïve populations from Eurasia. This potentially suggests that range expansion of M. polymorpha into the New World has led to rapid evolution of a variety of traits that have helped multiple populations become established, including those that may allow invasive populations to resist herbivory. Thus, enemy release in a novel range can occur through rapid evolution by the plant during invasion, as predicted by the Shifting Defense Hypothesis, rather than via historical divergence.     

The invasion history of the exotic freshwater zooplankter Daphnia lumholtzi (Cladocera, Crustacea) in North America: a genetic analysis

Daphnia lumholtzi is a planktonic crustacean native to subtropical regions in Africa, Asia and Australia. Since its invasion to the southern USA in ~1990 it has spread across North America as far north as the Laurentian Great Lakes. We assessed invasion history using microsatellite makers and to explore the influence of mean annual temperature on the genetic structure along a latitudinal gradient in North America. Genotypic data were obtained from 9 microsatellite markers for 178 individuals from 13 populations (eight populations introduced to North America and five populations in the native range). Pairwise F_st values as well as Bayesian clustering showed a strong subdivision between native and introduced populations. Bayesian clustering identified multiple genetic clusters in recently invaded locations, suggestive of multiple invasions from various sources, including Asia and Africa. Using variation partitioning, we determined the amount of variation for genetic clusters of populations in the invaded range due to mean annual air temperature and the year of first detection. The results point to a primary introduction into the southern range of North America, with a subsequent northward expansion, and multiple introductions possibly from both the native range and by secondary spread from previously-invaded locations. Separate analysis of genetic clusters within the invaded range suggests additional effects of temperature conditions on geographic genetic structure, possibly as a consequence of D. lumholtzi’s tropical origin.     

Molecular Evidence for an Extreme Genetic Bottleneck During Introduction of an Invading Grass to California

Although exotic species cause tremendous economic and ecological loss, we know relatively little about the post-introduction evolutionary dynamics of the invasive species themselves. Barbed goatgrass, Aegilops triuncialis L., is a cleistogamous annual grass with a native range throughout the Mediterranean Basin and Asia and introduced to California during the last century. It is considered a serious noxious range weed and is one of the few exotic plant species that is invading serpentine soil habitats. We examined whether patterns of molecular variation are consistent with a single or multiple introduction events into California and further, if individual populations show evidence for a genetic bottleneck during introduction. Fingerprinting patterns, using microsatellite loci derived from Triticum aestivum, were investigated for 57 Eurasian accessions, broadly spanning the native range and for 108 individuals from 11 localities in California. There is strong evidence for an extreme bottleneck in this species as it colonizes its new range because we detected only three multilocus genotypes occurring in California and 36 genotypes in Eurasia. In California one of the genotypes differs from one other by only one fragment and only occurs in one individual. This suggests two separate introductions. Each population is composed of highly uniform individuals and the two main genotypes are geographically separated. A. triuncialis is still expanding its range in California despite genomic uniformity after a strong bottleneck and its recently increased rate of spread is not correlated to a high within-population variability created by multiple introductions.     

Plasticity in salt tolerance traits allows for invasion of novel habitat by Japanese knotweed s. l. (Fallopia japonica and F.xbohemica, Polygonaceae)

Japanese knotweeds are among the most invasive organisms in the world. Their recent expansion into salt marsh habitat provides a unique opportunity to investigate how invasives establish in new environments. We used morphology, cytology, and AFLP genotyping to identify taxa and clonal diversity in roadside and salt marsh populations. We conducted a greenhouse study to determine the ability to tolerate salt and whether salt marsh populations are more salt tolerant than roadside populations as measured by the efficiency of PSII, leaf area, succulence, height, root-to-shoot ratio, and total biomass. Clonal diversity was extremely low with one F. japonica clone and five F. ×bohemica genotypes. The two taxa were significantly different in several traits, but did not vary in biomass or plasticity of any trait. All traits were highly plastic in response to salinity, but differed significantly among genets. Despite this variation, plants from the salt marsh habitats did not perform better in the salt treatment, suggesting that they are not better adapted to tolerate salt. Instead, our data support the hypothesis that plasticity in salt tolerance traits may allow these taxa to live in saline habitats without specific adaptation to tolerate salt.     

Contrasting variation within and covariation between gender-related traits in autogamous versus outcrossing species: Alternative evolutionary predictions

We present several predictions concerning the expression of genetic variation in, and covariation among, gender-related traits in perfect-flowered plant taxa with different breeding systems. We start with the inference that the pollen:ovule (P/O) ratio in obligately autogamous species (in which the ovules in a flower are fertilized only by the pollen it produces) should be under much stronger stabilizing selection than in outcrossing taxa. Consequently, we predict that obligately autogamous taxa should exhibit lower genetic coefficients of variation in the P/O ratio. Nevertheless, genetic variation in both pollen and ovule production per flower might persist within autogamous as well as outcrossing populations. In autogamous taxa, genotypes with relatively few pollen grains and ovules per flower (but producing relatively high numbers of flowers) and genotypes with comparatively high numbers of gametes per flower (but producing relatively few flowers) could co-exist if lifetime flower production is selectively neutral. In contrast, in outcrossers, the maintenance of genetic variation in ovule and pollen production per flower might result predominantly from their ability to maintain variation in phenotypic and functional gender. Given genetic variation in primary sexual traits, we predict that the genetic correlation between investment in male and female gametes per flower should qualitatively differ between selfers and outcrossers. We predict a positive genetic correlation between pollen and ovule production per flower in obligately autogamous taxa, primarily because strong stabilizing selection on the P/O ratio should select against the gender specialists that would be necessary to effect a negative genetic correlation between mean pollen and ovule production per flower. Moreover, the fact that autogamous individuals are 50% female and 50% male means that gender-biased phenotypes cannot be functionally gender-biased, preventing selection from favouring phenotypic extremes. In contrast, in outcrossing taxa, in which functionally male- and female-biased genotypes may co-exist, the maintenance of contrasting genders could contribute to the expression of negative genetic correlations between pollen and ovule production per flower. We discuss these and a number of corollary predictions, and we provide a preliminary empirical test of the first prediction. This revised version was published online in July 2006 with corrections to the Cover Date.