STOCHASTIC LOSS OF STYLE MORPHS FROM POPULATIONS OF TRISTYLOUSLYTHRUM SALICARIAANDDECODON VERTICILLATUS(LYTHRACEAE)

Patterns of variation in distylous traits and reproductive consequences in Erythroxylum species and populations

PREMISE

Distylous species possess two floral morphs with reciprocal positioning of stigmas and anthers that is hypothesized to promote disassortative pollination. Theoretical models predict equal morph frequencies, but many populations depart from the expected 1:1 ratio, a pattern that often correlates with asymmetric mating between morphs and/or presence of a weak incompatibility system. Variation in reciprocity can also affect the likelihood of disassortative pollination and, hence, reproductive fitness.

METHODS

We described variation in incompatibility systems and morph ratio in four Erythroxylum species to test if greater deviations from 1:1 ratios occur in populations of self-compatible species. Using adaptive inaccuracy, we described upper and lower organ reciprocity in species and populations and assessed the relationship of reciprocity to population means and coefficients of variation for fruit set to test if reciprocity could predict female reproductive success.

RESULTS

Morphs occurred in 1:1 ratios in most populations of three Erythroxylum species with distylous self-incompatibility. In self-compatible E. campestre populations showed an excess of the long-styled morph, the short-styled morph, or were monomorphic for the short-styled morph. We detected deviations from reciprocity, with total inaccuracy ranging between 9.39% and 42.94%, and inaccuracy values were lowest in low organs. Across populations, we found a positive relationship between inaccuracy and the coefficient of variation of fruit set.

CONCLUSIONS

Erythroxylum species showed variation in the distylous syndrome, with changes in the incompatibility system that corresponded with deviations from 1:1 morph ratio, and variation in reciprocity that correlated with variation in female reproductive fitness.

The opposing effects of genetic drift and Haldane's sieve on floral-morph frequencies in tristylous metapopulations

Tristyly is a genetic floral polymorphism in which three floral morphs are maintained at equal frequencies by negative frequency-dependent selection on alleles at two interacting loci. Because dominant alleles at these loci are maintained at a lower frequency than their recessive counterparts, they are more likely to be lost by founder events and genetic drift. Here we examine the hypothesis that dominant alleles under negative frequency-dependent selection should also be more likely to re-invade populations than recessive alleles, due to Haldane's Sieve, because recessive alleles not expressed in a heterozygote state cannot benefit from positive selection when rare. We used computer simulations of tristylous metapopulations to verify that Haldane's Sieve acting on migrants into occupied demes can indeed reverse the bias in allele frequencies expected for small single tristylous populations, particularly in situations of rapid population growth following colonisation. This effect is manifest both locally and at the metapopulation level. Our study illustrates the potential effect of Haldane's Sieve in the novel context of an iconic plant sexual-system polymorphism under the influence of metapopulation dynamics.

'A most complex marriage arrangement': recent advances on heterostyly and unresolved questions

Heterostylous genetic polymorphisms provide paradigmatic systems for investigating adaptation and natural selection. Populations are usually comprised of two (distyly) or three (tristyly) mating types, maintained by negative frequency-dependent selection resulting from disassortative mating. Theory predicts this mating system should result in equal style-morph ratios (isoplethy) at equilibrium. Here, I review recent advances on heterostyly, focusing on examples challenging stereotypical depictions of the polymorphism and unresolved questions. Comparative analyses indicate multiple origins of heterostyly, often within lineages. Ecological studies demonstrate that structural components of heterostyly are adaptations improving the proficiency of animal-mediated cross-pollination and reducing pollen wastage. Both neutral and selective processes cause deviations from isoplethy in heterostylous populations, and, under some ecological and demographic conditions, cause breakdown of the polymorphism, resulting in either the evolution of autogamy and mixed mating, or transitions to alternative outcrossing systems, including dioecy. Earlier ideas on the genetic architecture of the S-locus supergene governing distyly have recently been overturned by discovery that the dominant S-haplotype is a hemizygous region absent from the s-haplotype. Ecological, phylogenetic and molecular genetic data have validated some features of theoretical models on the selection of the polymorphism. Although heterostyly is the best-understood floral polymorphism in angiosperms, many unanswered questions remain.

Architectural constraints, male fertility variation and biased floral morph ratios in tristylous populations

Tristyly is a genetic polymorphism in which populations are comprised of three floral morphs (mating types) differing reciprocally in sex-organ height. Intermorph (disassortative) mating governed by a trimorphic incompatibility system should result in 1:1:1 morph ratios at equilibrium, but both deterministic and stochastic processes can cause skewed morph ratios in tristylous populations. Here, we investigate mechanisms causing morph-ratio bias in Pontederia parviflora, an emergent aquatic native to tropical America. We compared reproductive traits among morphs and surveyed 71 populations to determine patterns of morph-ratio bias. We then used simulation models of morph-frequency dynamics to test the hypothesis that morph-specific differences in pollen production and their influence on male fertility can explain patterns of morph-ratio bias. Ninety-seven percent of populations that we sampled were tristylous, but with a significant excess of the short-styled morph and a deficiency of the long-styled morph. Atypically for a tristylous species, mid-level anthers of the short-styled morph produced over twice as much pollen compared with the corresponding anthers of the long-styled morph. Our computer models incorporating this difference in male fertility resulted in morph ratios not significantly different from the average frequencies from our survey suggesting that the short-styled morph is more successful than the long-styled morph in siring ovules of the mid-styled morph. We propose that the difference in male fertility between morphs may be a non-adaptive consequence of a developmental constraint caused by the architecture of tristyly in Pontederiaceae.

INBREEDING DEPRESSION IN PARTIALLY SELF-FERTILIZING DECODON VERTICILLATUS (LYTHRACEAE): POPULATION-GENETIC AND EXPERIMENTAL ANALYSES

Inbreeding depression is a major selective force favoring outcrossing in flowering plants. Some self-fertilization, however, should weaken the harmful effects of inbreeding by exposing genetic load to selection. This study examines the maintenance of inbreeding depression in partially self-fertilizing populations of the long-lived, herbaceous wetland plant, Decodon verticillatus (L.) Ell. (Lythraceae). Estimates from ten populations indicate that 30% of offspring are produced through self-fertilization. Population-genetic estimates of inbreeding depression (δ = 1 - relative mean fitness of selfed progeny) based on changes in the inbreeding coefficient for the same ten populations were uniformly high, ranging from 0.49 to 1.79 and averaging 1.11 ± 0.29 SE. Although confidence intervals of individual population estimates were large, estimates were significantly greater than 0 in six populations and greater than 0.5 in four. Inbreeding depression was also estimated by comparing growth, survival, and flowering of experimentally selfed and outcrossed offspring from two of these populations in a 1-yr glasshouse experiment involving three density regimes; after which offspring were transplanted into garden arrays and two field sites and monitored for two consecutive growing seasons. Overall δ^ for survival averaged 0.27 ± 0.01 in the glasshouse, 0.33 ± 0.04 in the garden, and 0.46 ± 0.04 in the field. The glasshouse experiment also revealed strong inbreeding depression for growth variables, especially above-soil dry weight (δ^ = 0.42 ± 0.03). The fitness consequences of inbreeding depression for these growth variables approximately doubles if survival to maturity is determined by severe truncation selection. Despite substantial selfing, inbreeding depression appears to be a major selective force favoring the maintenance of outcrossing in D. verticillatus.

Influences of clonality on plant sexual reproduction

Flowering plants possess an unrivaled diversity of mechanisms for achieving sexual and asexual reproduction, often simultaneously. The commonest type of asexual reproduction is clonal growth (vegetative propagation) in which parental genotypes (genets) produce vegetative modules (ramets) that are capable of independent growth, reproduction, and often dispersal. Clonal growth leads to an expansion in the size of genets and increased fitness because large floral displays increase fertility and opportunities for outcrossing. Moreover, the clonal dispersal of vegetative propagules can assist "mate finding," particularly in aquatic plants. However, there are ecological circumstances in which functional antagonism between sexual and asexual reproductive modes can negatively affect the fitness of clonal plants. Populations of heterostylous and dioecious species have a small number of mating groups (two or three), which should occur at equal frequency in equilibrium populations. Extensive clonal growth and vegetative dispersal can disrupt the functioning of these sexual polymorphisms, resulting in biased morph ratios and populations with a single mating group, with consequences for fertility and mating. In populations in which clonal propagation predominates, mutations reducing fertility may lead to sexual dysfunction and even the loss of sex. Recent evidence suggests that somatic mutations can play a significant role in influencing fitness in clonal plants and may also help explain the occurrence of genetic diversity in sterile clonal populations. Highly polymorphic genetic markers offer outstanding opportunities for gaining novel insights into functional interactions between sexual and clonal reproduction in flowering plants.

Indirect effects of herbivory on plant-pollinator interactions in invasive Lythrum salicaria

PREMISE OF THE STUDY

Herbivory can affect a plant's fitness in a variety of ways, including modifying the biotic interactions of the plant. In particular, when herbivory influences floral display, we hypothesize that pollinator visitation will be altered accordingly. Here we studied the indirect effects of feeding by two beetles, Neogalerucella calmariensis and N. pusilla, released as a biological control, on plant-pollinator interactions and fitness in the invasive plant, purple loosestrife (Lythrum salicaria).

METHODS

Two herbivory treatments (ambient and simulated) were applied to plants in a naturally occurring population of purple loosestrife. During flowering, traits of plants in the treatment and control groups were recorded. Data on pollinator visitation behavior was then collected after intense larval herbivory had ended.

KEY RESULTS

Plants exposed to herbivory treatments produced more flowers and inflorescences but flowered significantly later than those in the control group. Moreover, we found a significant, positive association of herbivory with the number of flowers probed by bumblebees and with the number of times a foraging pollinator moved among inflorescences on a single plant. No differences in female fitness (fruit or seed production) were detected.

CONCLUSIONS

We conclude that herbivore-mediated differences in floral display traits impacted pollinator visitation behavior. However, as we discuss, differences in pollinator visitation did not translate into detectable differences in female reproductive success. We postulate that herbivory could influence other unmeasured aspects of fitness, such as seed quality or the number of seeds sired.

Range-wide population genetics and variation in morph ratio in style-dimorphic Narcissus papyraceus

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PREMISE OF THE STUDY

Theoretical models state that natural selection and mating patterns account for floral morph ratio in style-polymorphic plants. However, the demographic history of populations can also influence variation in morph ratios. If so, we hypothesize an association between the morph ratios and the genetic structure across populations.•

METHODS

We used nuclear microsatellites to assess genetic variation and structure in populations of Narcissus papyraceus, a style-dimorphic plant whose floral morph ratios (L-morph to S-morph) gradually vary throughout its distribution range in the southwestern Mediterranean Basin. We implemented analyses to relate the genetic features of populations with their morph ratios.•

KEY RESULTS

We found greater frequencies of the S-morph in central populations and declining frequencies toward the periphery. This geographic pattern was not associated with the genetic structure of populations. Instead, we found two distinct genetic groups, mainly separated by the Strait of Gibraltar, with a mixture of morph ratios within each one. Overall, there was a weak genetic structure. Genetic diversity was greater in central and southern dimorphic populations than in northern L-monomorphic populations.•

CONCLUSIONS

Altogether, our results do not support the hypothesis that the demographic history of populations can account for the observed geographical pattern of morph ratios in N. papyraceus. We suggest that adaptive processes shown in previous studies in the species are the main determinant of the existing variation in the morph composition of populations.

Strong differences in genetic structure across disjunct, edge, and core populations of the distylous forest herb Pulmonaria officinalis (Boraginaceae)

PREMISE OF THE STUDY

Populations at the edge of a species' distribution area are often small and have low levels of gene flow resulting in lower genetic variation and higher differentiation compared to core populations. This study examined genetic variation among populations of the distylous temperate forest herb Pulmonaria officinalis located in the core, the edge, and outside the species' main distribution range.

METHODS

We compared patterns of genetic variation for eight microsatellite loci between disjunct (Belgium), edge (western Germany), and core (eastern Germany) populations of P. officinalis.

KEY RESULTS

Disjunct populations contained only a subset of alleles found in edge and core populations and had significantly lower within-population genetic variation. No significant differences, however, in within-population genetic variation were found between edge and core populations, except for allelic and genotypic richness. Genetic differentiation was highest among disjunct (F(ST) = 0.11) and lowest among core populations (F(ST) = 0.03). Significant (P < 0.01) isolation by distance was found for disjunct and edge populations (r(M) = 0.29 and 0.50, respectively), but not for core populations (r(M) = 0.18).

CONCLUSIONS

The results are best interpreted from a "dynamic range" point of view in which the observed low levels of genetic diversity and high genetic differentiation in disjunct populations are best explained through historical processes, most likely the introduction of the species in medieval times. Lower levels of gene flow caused by the pronounced fragmentation of forests in Belgium may further have contributed to the genetic structure of P. officinalis in these disjunct populations.

Selective loss of polymorphic mating types is associated with rapid phenotypic evolution during morphic speciation

Polymorphism may play an important role in speciation because new species could originate from the distinctive morphs observed in polymorphic populations. However, much remains to be understood about the process by which morphs found new species. To detail the steps of this mode of speciation, we studied the geographic variation and evolutionary history of a throat color polymorphism that distinguishes the "rock-paper-scissors" mating strategies of the side-blotched lizard, Uta stansburiana. We found that the polymorphism is geographically widespread and has been maintained for millions of years. However, there are many populations with reduced numbers of throat color morphs. Phylogenetic reconstruction showed that the polymorphism is ancestral, but it has been independently lost eight times, often giving rise to morphologically distinct subspecies/species. Changes to the polymorphism likely involved selection because the allele for one particular male strategy, the "sneaker" morph, has been lost in all cases. Polymorphism loss was associated with accelerated evolution of male size, female size, and sexual dimorphism, which suggests that polymorphism loss can promote rapid divergence among populations and aid species formation.

Changes in sexual organ reciprocity and phenotypic floral integration during the tristyly-distyly transition in Oxalis alpina

Although the 6 magnitude and pattern of correlation among floral traits (phenotypic integration) is usually conceived as an adaptation for successful pollination and reproduction, studies on the evolution of plant reproductive systems have generally focused on one or a few characters. If evolutionary transitions between reproductive systems involve morphological floral adjustments, changes in the magnitude and pattern of phenotypic integration of floral traits may be expected. In this study, we focused on the evolutionary dynamics of a complex adaptive trait, the extent of reciprocity (reciprocal placement) among sexual organs in a heterostylous species, and explored the associated changes in phenotypic floral integration during the transition from tristyly to distyly. The extent of reciprocity and both the magnitude and pattern of floral integration were characterized in 12 populations of Oxalis alpina representing the tristyly-distyly gradient. Although the extent of reciprocity increased along the tristyly-distyly transition, the flower size diminished. These adjustments did not affect the magnitude, but did affect the pattern, of floral integration. *Changes in the pattern of floral integration suggested that allometric, functional and pleiotropic relationships among floral traits were affected during this evolutionary transition.

Geographic variation, frequency-dependent selection, and the maintenance of a female-limited polymorphism

A central problem in evolutionary biology is to understand how spatial and temporal variation in selection maintain genetic variation within and among populations. Brown anole lizards (Anolis sagrei) exhibit a dorsal pattern polymorphism that is expressed only in females, which occur in "diamond,""bar," and intermediate "diamond-bar" morphs. To understand the inheritance of this polymorphism, we conducted a captive breeding study that refuted several single-locus models and supported a two-locus mode of inheritance. To describe geographic variation in morph frequencies, we surveyed 13 populations from two major islands in The Bahamas. Morph frequencies differed substantially between major islands but were highly congruent within each island. Finally, we measured viability selection on each island to test two hypotheses regarding the maintenance of the polymorphism: (1) that spatial variation in selection maintains variation in morph frequencies between islands, and (2) that temporal variation in selection across years maintains variation within islands. Although bar females had relatively lower survival where they were rare, our data do not otherwise suggest that selection varies spatially between islands. However, diamond-bar females were subject to positive frequency-dependent selection across years, and the relative fitness of bar and diamond females alternated across years. We propose that this polymorphism is maintained by temporal variation in selection coupled with the sheltering of alleles via a two-locus inheritance pattern and sex-limited expression.

Morph-ratio variation, population size and female reproductive success in distylous Pulmonaria officinalis (Boraginaceae)

Theory predicts that morph ratios in heterostylous populations are governed by negative frequency-dependent selection typically resulting in equal morph ratios at equilibrium. Previous work on the distylous perennial herb Pulmonaria officinalis, however, showed asymmetric mating between floral morphs and a weak self-incompatibility system, with the long-styled morph (L-morph) producing significantly higher seed set following intramorph crosses and even selfing than the short-styled morph (S-morph), two aspects thought to affect female fecundity and morph-ratio variation. Here, we evaluated morph ratios and population size of all known P. officinalis populations in the northern part of Belgium. Morph ratios deviated significantly from 1:1 (range 0.09-1 L-morph frequency, mean = 0.58). Relative fecundity of the S-morph (i.e. mean seed set of the S-morph/mean seed set of the L-morph) was on average 0.73, was positively related to the frequency of the L-morph, and reached 1 (similar levels of female fecundity) at an average L-morph frequency of 0.66 in the population. As some small populations had the S-morph in majority, our results suggest that local morph ratios are influenced both by the relative fecundity of L- and S-morph individuals and by stochastic processes in small populations.

Impact of mate availability, population size, and spatial aggregation of morphs on sexual reproduction in a distylous, aquatic plant

In distylous, self-incompatible plants, clonal propagation, unbalanced floral morph frequencies, and reduced population size can interfere with the functioning of distyly by compromising legitimate intermorph pollinations, resulting in reduced reproductive output. Here, we examined the mating system and the impact of mate availability, population size, and spatial aggregation of morphs on reproductive output in the distylous, clonal, aquatic plant Hottonia palustris. Controlled pollinations under greenhouse conditions detected no spontaneous selfing without the action of a pollen vector (autonomous autogamy) and demonstrated very low fruit and seed development after self-pollination. Intermorph (legitimate) crossings resulted in high reproductive output in both floral morphs (long- and short-styled individuals), whereas intramorph (illegitimate) crossings decreased fruit and seed development by more than 50%, indicating that the species has partial intramorph-incompatibility. In natural populations, small population size and increasing deviation of floral morph frequencies negatively affected reproductive outcome. Individuals of the majority morph type developed significantly fewer fruit and seeds than individuals of the minority morph type. This rapid decline in fecundity was symmetrical, indicating that regardless of which morph was in the majority, the same patterns of negative frequency-dependent mating occurred. Increasing spatial isolation between compatible morphs significantly reduced fruit and seed set in both morphs similarly. This study provides clear indications of frequency- and context-dependent mating in natural populations of a distylous plant species.

Female reproductive success and the evolution of mating-type frequencies in tristylous populations

In tristylous populations, mating-type frequencies are governed by negative frequency-dependent selection typically resulting in equal morph ratios at equilibrium. However, Narcissus triandrus generally exhibits long-styled (L)-biased populations with a deficiency of the mid-styled (M)-morph. Here we used a pollen-transfer model and measurements of female fertility in natural populations to investigate whether these uneven morph ratios were associated with variation in female reproductive success. Our theoretical analysis demonstrated that morph ratio bias can result from maternal fitness differences among the morphs, and that these effects were magnified by asymmetrical mating. In nine out of 15 populations of N. triandrus, seed set differed significantly among the morphs, but pollen limitation occurred in only two of 11 populations investigated. Average seed set of the M-morph was positively associated with its frequency in populations. Flower size was negatively correlated with the seed set of the M-morph. Our results suggest that interactions between mating patterns and female fertility are responsible for variation in morph frequencies and loss of the M-morph from tristylous populations of N. triandrus.

The evolution of self-compatibility in geographically peripheral populations of Leavenworthia alabamica (Brassicaceae)

Self-compatibility and adaptations to self-fertilization are often found in plant populations at the periphery of species' ranges or on islands. Self-compatibility may predominate in these environments because it provides reproductive assurance when pollinators or availability of mates limits seed production. This possibility was studied in Leavenworthia alabamica, a flowering plant endemic to the southeastern United States. Populations at the center of the species' range retain sporophytic self-incompatibility, but peripheral populations are smaller, self-compatible, and have adaptations for self-fertilization. A reciprocal-transplant experiment was designed to test whether there is pollen limitation of seed set and to examine its strength in central and peripheral populations. Self-compatible genotypes produced more fruit and 17-22% more seed than self-incompatible genotypes in all environments, suggesting that the transition to self-compatibility may be favored by natural selection in all populations inhabited by L. alabamica. Sequence analyses demonstrated that two peripheral populations have 90-100% reductions in genetic variation, consistent with the effects of small population size or historical bottlenecks. Although pollen limitation of seed set occurs in all environments, self-compatibility may evolve at the periphery in L. alabamica because the benefits of reproductive assurance are influenced by population size or bottlenecks following extinction and colonization.

Evolutionary vestigialization of sex in a clonal plant: selection versus neutral mutation in geographically peripheral populations

The loss of traits that no longer contribute to fitness is widespread; however, the causative evolutionary mechanisms are poorly understood. Vestigialization could proceed through the fixation of selectively neutral degenerative mutations via genetic drift. Alternatively, selection may facilitate vestigialization if trait loss results in enhanced fitness. We tested these hypotheses using Decodon verticillatus, a clonal plant in which sexual sterility has arisen repeatedly in populations across the northern geographical range limit. We compared growth and survival of replicated genotypes from 7 sexually fertile and 18 sterile populations, over 3 years in a common environment. Survival of sterile genotypes was 53% greater than for fertile genotypes, but there was no difference in biomass accumulation. Almost all mortality, and hence increased performance of sterile genotypes, occurred during simulated overwinter dormancy. These observations suggest that selection has facilitated the vestigialization of sex, and thus do not support the neutral mutation hypothesis. The selective mechanism probably involves the relaxation of a genetic trade-off between sexual reproduction and survival: alleles that increase vegetative performance at the expense of sexual fertility are selected in geographically peripheral populations where sexual reproduction is suppressed by adverse environmental conditions.

Phylogenetic reconstruction of the evolution of stylar polymorphisms in Narcissus (Amaryllidaceae)

We investigated the origin of stylar polymorphisms in Narcissus, which possesses a remarkable range of stylar conditions and diverse types of floral morphology and pollination biology. Reconstruction of evolutionary change was complicated by incomplete resolution of trees inferred from two rapidly evolving chloroplast regions, but we bracketed reconstructions expected on the fully resolved plastid-based tree by considering all possible resolutions of polytomies on the shortest trees. Stigma-height dimorphism likely arose on several occasions in Narcissus and persisted across multiple speciation events. As proposed in published models, this rare type of stylar polymorphism is ancestral to distyly. While there is no evidence in Narcissus that dimorphism preceded tristyly, a rapid transition between them may explain the lack of a phylogenetic footprint for this evolutionary sequence. The single instances of distyly and tristyly in Narcissus albimarginatus and N. triandrus, respectively, are clearly not homologous, an evolutionary convergence unique to Amaryllidaceae. Floral morphology was likely an important trigger for the evolution of stylar polymorphisms: Concentrated-changes tests indicate that a long, narrow floral tube may have been associated with the emergence of stigma-height dimorphism and that this type of tube, in combination with a deep corona, likely promoted, or at least was associated with, the parallel origins of heterostyly.

Buzz-pollinated Dodecatheon originated from within the heterostylous Primula subgenus Auriculastrum (Primulaceae): a seven-region cpDNA phylogeny and its implications for floral evolution

We sequenced seven cpDNA regions from 70 spp. in Dodecatheon, Primula subgenus Auriculastrum, and outgroups, reconstructed their cpDNA phylogeny with maximum parsimony, and determined branch support with bootstrap frequencies and Bayesian posterior probabilities. Strongly supported conclusions include the (1) paraphyly of Primula subgenus Auriculastrum with respect to a monophyletic Dodecatheon, (2) sister relationship between the North American Dodecatheon and the Californian P. suffrutescens, (3) novel basal split in Dodecatheon to produce one clade with rugose and one clade with smooth anther connectives, (4) monophyly of all sections of Primula subgenus Auriculastrum, and (5) exclusion of the enigmatic Primula section Amethystina from the similar Primula subgenus Auriculastrum. These results support the origin of the monomorphic, buzz-pollinated flower of Dodecatheon from the heterostylous flower of Primula. We marshal evidence to support the novel hypothesis that the solanoid flower of Dodecatheon represents the fixation of recessive alleles at the heterostyly linkage group (pin phenotype). Of the remaining traits associated with their solanoid flowers, we recognize at least six likely to have arisen with the origin of Dodecatheon, one that preceded it (flower coloration, a transfer exaptation in Dodecatheon), and one that followed it (rugose anther connectives, an adaptation to buzz pollination).

Inbreeding depression in smooth cordgrass (Spartina alterniflora, Poaceae) invading San Francisco Bay

The magnitude of inbreeding depression in invading plant populations is often presumed to be small and of little consequence. The purpose of this study was to assess the magnitude of inbreeding depression in a pollen-limited, partially self-incompatible, invading plant population. The magnitude and timing of inbreeding depression were compared among ten maternal plants sampled from a population of smooth cordgrass (Spartina alterniflora) invading San Francisco Bay. Selfed and outcrossed progeny were compared for embryo abortion, survival of seedlings, and growth/survival at the end of the first growing season in three greenhouse environments. Estimates of inbreeding depression varied among environments, with competitive environment > high-nutrient environment > low-nutrient environment. Population-level estimates of inbreeding depression ranged from 0.61 to 0.81; however, maternal plants varied significantly in their magnitude of inbreeding depression, ranging from 0.1 to 0.97. The 95% confidence interval for inbreeding depression for some maternal plants included zero. There was a significant negative correlation between the overall magnitude of inbreeding depression and self-fertility rate among maternal plants. The few maternal plants with high self-fertility carried relatively little genetic load, and their selfed progeny are likely to survive on open mudflats. The noncompetitive, pollen-limited growing conditions associated with invasion may allow self-fertility to spread in this population.

Morph frequencies and floral variation in a heterostylous colonizing weed, Lythtum salicaria

Lythrum salicaria (purple loosestrife) is an exotic weed that arrived in North America from Europe during the early 1800s. It is a herbaceous perennial with a trimorphic breeding system. Seventy-four populations of L. salicaria were surveyed from Windsor, Ontario, to the Gaspé Peninsula in Quebec. Fifty of the populations were significantly anisoplethic (i.e., unequal frequencies of the three flower morphs), including 10 populations that were nontrimorphic. Populations with fewer than 100 plants tended to have one or even two morphs missing. Although larger populations rarely lacked a morph, they did show significantly skewed morph frequencies. Indices of clonal size, such as number of ramets per genet and genet diameter, differed significantly among sites, and clonal growth also showed significant interaction between morph and site. One-way analyses of variance indicated that morphs differed in terms of either number of ramets per genet or genet diameter in 16% of populations. Morphometric analyses of flowers from 49 populations showed significant variability in floral traits among genets, flower morphs, and sites. Results indicated frequent reduction in herkogamy (spatial separation between anther and stigma), with variant flowers having very little or no stigma–anther separation. Mean stigma–anther separation was lowest in the mid-morph individuals, followed by the short and long morphs. Correspondingly, the frequency of variant flowers was greatest in mid-morph individuals and least in long-morph individuals. Isoplethic and anisoplethic populations did not differ, for any morph, in the frequency of occurrence of these variant flowers. Key words: Lythrum salicaria, purple loosestrife, isoplethy, morph frequency, heterostyly, variant flowers, stigma–anther separation.