Evolvability and trait function predict phenotypic divergence of plant populations.

Understanding the causes and limits of population divergence in phenotypic traits is a fundamental aim of evolutionary biology, with the potential to yield predictions of adaptation to environmental change. Reciprocal transplant experiments and the evaluation of optimality models suggest that local adaptation is common but not universal, and some studies suggest that trait divergence is highly constrained by genetic variances and covariances of complex phenotypes. We analyze a large database of population divergence in plants and evaluate whether evolutionary divergence scales positively with standing genetic variation within populations (evolvability), as expected if genetic constraints are evolutionarily important. We further evaluate differences in divergence and evolvability-divergence relationships between reproductive and vegetative traits and between selfing, mixed-mating, and outcrossing species, as these factors are expected to influence both patterns of selection and evolutionary potentials. Evolutionary divergence scaled positively with evolvability. Furthermore, trait divergence was greater for vegetative traits than for floral (reproductive) traits, but largely independent of the mating system. Jointly, these factors explained ~40% of the variance in evolutionary divergence. The consistency of the evolvability-divergence relationships across diverse species suggests substantial predictability of trait divergence. The results are also consistent with genetic constraints playing a role in evolutionary divergence.

Parasitic manipulation or by-product of infection: an experimental approach using trematode-infected snails.

Natural selection should favour parasite genotypes that manipulate hosts in ways that enhance parasite fitness. However, it is also possible that the effects of infection are not adaptive. Here we experimentally examined the phenotypic effects of infection in a snail-trematode system. These trematodes (Atriophallophorus winterbourni) produce larval cysts within the snail's shell (Potamopyrgus antipodarum); hence the internal shell volume determines the total number of parasite cysts produced. Infected snails in the field tend to be larger than uninfected snails, suggesting the hypothesis that parasites manipulate host growth so as to increase the space available for trematode reproduction. To test the hypothesis, we exposed juvenile snails to trematode eggs. Snails were then left to grow for about one year in 800-l outdoor mesocosms. We found that uninfected males were smaller than uninfected females (sexual dimorphism). We also found that infection did not affect the shell dimensions of males. However, infected females were smaller than uninfected females. Hence, infection stunts the growth of females, and (contrary to the hypothesis) it results in a smaller internal volume for larval cysts. Finally, infected females resembled males in size and shape, suggesting the possibility that parasitic castration prevents the normal development of females. These results thus indicate that the parasite is not manipulating the growth of infected hosts so as to increase the number of larval cysts, although alternative adaptive explanations are possible.

Pollen competition is the mechanism underlying a variety of evolutionary phenomena in dioecious plants.

It has long been known that more pollen grains often arrive on stigmas than there are ovules to fertilize, resulting in pollen competition. Moreover, this competition among pollen grains (gametophytes) depends, in part, on their extensive haploid gene expression. Here I review how this leads to a variety of phenomena in dioecious plants of interest to evolutionary biologists. For example, pollen competition can lead to extreme female-biased sex ratios. In addition, gene expression by individual pollen grains can slow mutation accumulation and degeneration of the Y chromosome. Lastly, I review work on how the haploid selection resulting from pollen competition has been proposed to influence which alleles are linked to the Y chromosome, and some recent empirical evidence in support of this theory.

Water availability drives population divergence and sex-specific responses in a dioecious plant.

PREMISE: Water availability is an important abiotic factor, resulting in differences between plant species growing in xeric and mesic habitats. Species with populations occurring in both habitat types allow examination of whether water availability has acted as a selective force at the intraspecific level. Investigating responses to water availability with a dioecious species allows determination of whether males and females, which often have different physiologies and life histories, respond differently. METHODS: An experiment varying water availability was performed under an outdoor rain-out shelter using plants from two mesic and two xeric populations of the dioecious plant Silene latifolia. Early growth rate, flowering propensity, flower size, and specific leaf area were measured. At the end of the season, the plants were harvested, aboveground and root biomass were measured, and the total number of flowers and fruit produced were counted. RESULTS: Compared to the two mesic populations, plants from the two xeric populations grew more slowly, were less likely to flower, took longer to flower, had thicker leaves, invested less in aboveground biomass and more in root biomass, produced fewer flowers and fruit, but were more likely to live. Many traits exhibited significant habitat type × treatment interactions. Compared to the xeric populations, males-but not females-from mesic populations had less root biomass and greatly reduced their flower production in response to low water availability. CONCLUSIONS: Mesic and xeric populations responded in ways congruent with water availability being a selective force for among-population divergence, especially for males.

Periodic, Parasite-Mediated Selection For and Against Sex.

Asexual lineages should rapidly replace sexual populations. Why sex then? The Red Queen hypothesis proposes that parasite-mediated selection against common host genotypes could counteract the per capita birth rate advantage of asexuals. Under the Red Queen hypothesis, fluctuations in parasite-mediated selection can drive fluctuations in the asexual population, leading to the coexistence of sexual and asexual reproduction. Does shifting selection by parasites drive fluctuations in the fitness and frequency of asexuals in nature? Combining long-term field data with mesocosm experiments, we detected a shift in the direction of parasite selection in the snail Potamopyrgus antipodarum and its coevolving parasite, Microphallus sp. In the early 2000s, asexuals were more infected than sexuals. A decade later, the asexuals had declined in frequency and were less infected than sexuals. Over time, the mean infection prevalence of asexuals equaled that of sexuals but varied far more. This variation in asexual infection prevalence suggests the potential for parasite-mediated fluctuations in asexual fitness. Accordingly, we detected fitness consequences of the shift in parasite selection: when they were less infected than sexuals, asexuals increased in frequency in the field and in paired mesocosms that isolated the effect of parasites. The match between field and experiment argues that coevolving parasites drive temporal change in the relative fitness and frequency of asexuals, potentially promoting the coexistence of reproductive modes in P. antipodarum.

Haldane's Rule: Genetic Bases and Their Empirical Support.

There are few patterns in evolution that are as rigidly held as Haldane's rule (HR), which states, "When in the first generation between hybrids between 2 species, 1 sex is absent, rare, or sterile, that sex is always the heterogametic sex." Yet despite considerable attention for almost a century, questions persist as to how many independent examples exist and what is (are) the underlying genetic cause(s). Here, we review recent evidence extending HR to plants, where previously it has only been documented in animals. We also discuss recent comparative analyses that show much more variation in sex-chromosome composition than previously recognized, thus increasing the number of potential independent origins of HR dramatically. Finally, we review the standing of genetic theories proposed to explain HR in light of the new examples and new molecular understanding.

About PAR: the distinct evolutionary dynamics of the pseudoautosomal region.

Sex chromosomes differ from other chromosomes in the striking divergence they often show in size, structure, and gene content. Not only do they possess genes controlling sex determination that are restricted to either the X or Y (or Z or W) chromosomes, but in many taxa they also include recombining regions. In these 'pseudoautosomal regions' (PARs), sequence homology is maintained by meiotic pairing and exchange in the heterogametic sex. PARs are unique genomic regions, exhibiting some features of autosomes, but they are also influenced by their partial sex linkage. Here we review the distribution and structure of PARs among animals and plants, the theoretical predictions concerning their evolutionary dynamics, the reasons for their persistence, and the diversity and content of genes that reside within them. It is now clear that the evolution of the PAR differs in important ways from that of genes in either the non-recombining regions of sex chromosomes or the autosomes.

On the importance of balancing selection in plants.

Balancing selection refers to a variety of selective regimes that maintain advantageous genetic diversity within populations. We review the history of the ideas regarding the types of selection that maintain such polymorphism in flowering plants, notably heterozygote advantage, negative frequency-dependent selection, and spatial heterogeneity. One shared feature of these mechanisms is that whether an allele is beneficial or detrimental is conditional on its frequency in the population. We highlight examples of balancing selection on a variety of discrete traits. These include the well-referenced case of self-incompatibility and recent evidence from species with nuclear-cytoplasmic gynodioecy, both of which exhibit trans-specific polymorphism, a hallmark of balancing selection. We also discuss and give examples of how spatial heterogeneity in particular, which is often thought unlikely to allow protected polymorphism, can maintain genetic variation in plants (which are rooted in place) as a result of microhabitat selection. Lastly, we discuss limitations of the protected polymorphism concept for quantitative traits, where selection can inflate the genetic variance without maintaining specific alleles indefinitely. We conclude that while discrete-morph variation provides the most unambiguous cases of protected polymorphism, they represent only a fraction of the balancing selection at work in plants.

The relative importance of reproductive assurance and automatic selection as hypotheses for the evolution of self-fertilization.

BACKGROUND: The field of plant mating-system evolution has long been interested in understanding why selfing evolves from outcrossing. Many possible mechanisms drive this evolutionary trend, but most research has focused upon the transmission advantage of selfing and its ability to provide reproductive assurance when cross-pollination is uncertain. We discuss the shared conceptual framework of these ideas and their empirical support that is emerging from tests of their predictions over the last 25 years. SCOPE: These two hypotheses are derived from the same strategic framework. The transmission advantage hypothesis involves purely gene-level selection, with reproductive assurance involving an added component of individual-level selection. Support for both of these ideas has been garnered from population-genetic tests of their predictions. Studies in natural populations often show that selfing increases seed production, but it is not clear if this benefit is sufficient to favour the evolution of selfing, and the ecological agents limiting outcross pollen are often not identified. Pollen discounting appears to be highly variable and important in systems where selfing involves multiple floral adaptations, yet seed discounting has rarely been investigated. Although reproductive assurance appears likely as a leading factor facilitating the evolution of selfing, studies must account for both seed and pollen discounting to adequately test this hypothesis. CONCLUSIONS: The transmission advantage and reproductive assurance ideas describe components of gene transmission that favour selfing. Future work should move beyond their dichotomous presentation and focus upon understanding whether selection through pollen, seed or both explains the spread of selfing-rate modifiers in plant populations.

Sex-specific natural selection on SNPs in Silene latifolia.

Selection that acts in a sex-specific manner causes the evolution of sexual dimorphism. Sex-specific phenotypic selection has been demonstrated in many taxa and can be in the same direction in the two sexes (differing only in magnitude), limited to one sex, or in opposing directions (antagonistic). Attempts to detect the signal of sex-specific selection from genomic data have confronted numerous difficulties. These challenges highlight the utility of "direct approaches," in which fitness is predicted from individual genotype within each sex. Here, we directly measured selection on Single Nucleotide Polymorphisms (SNPs) in a natural population of the sexually dimorphic, dioecious plant, Silene latifolia. We measured flowering phenotypes, estimated fitness over one reproductive season, as well as survival to the next year, and genotyped all adults and a subset of their offspring for SNPs across the genome. We found that while phenotypic selection was congruent (fitness covaried similarly with flowering traits in both sexes), SNPs showed clear evidence for sex-specific selection. SNP-level selection was particularly strong in males and may involve an important gametic component (e.g., pollen competition). While the most significant SNPs under selection in males differed from those under selection in females, paternity selection showed a highly polygenic tradeoff with female survival. Alleles that increased male mating success tended to reduce female survival, indicating sexual antagonism at the genomic level. Perhaps most importantly, this experiment demonstrates that selection within natural populations can be strong enough to measure sex-specific fitness effects of individual loci. Males and females typically differ phenotypically, a phenomenon known as sexual dimorphism. These differences arise when selection on males differs from selection on females, either in magnitude or direction. Estimated relationships between traits and fitness indicate that sex-specific selection is widespread, occurring in both plants and animals, and explains why so many species exhibit sexual dimorphism. Finding the specific loci experiencing sex-specific selection is a challenging prospect but one worth undertaking given the extensive evolutionary consequences. Flowering plants with separate sexes are ideal organisms for such studies, given that the fitness of females can be estimated by counting the number of seeds they produce. Determination of fitness for males has been made easier as thousands of genetic markers can now be used to assign paternity to seeds. We undertook just such a study in S. latifolia, a short-lived, herbaceous plant. We identified loci under sex-specific selection in this species and found more loci affecting fitness in males than females. Importantly, loci with major effects on male fitness were distinct from the loci with major effects on females. We detected sexual antagonism only when considering the aggregate effect of many loci. Hence, even though males and females share the same genome, this does not necessarily impose a constraint on their independent evolution.

Environment-dependent intralocus sexual conflict in a dioecious plant.

Intralocus sexual conflict is a form of conflict that does not involve direct interactions between males and females. It arises when selection on a shared trait with a common genetic basis differs between the sexes. Environmental factors, such as resource availability, may influence the expression and evolutionary outcome of such conflict. We quantified the genetic variance-covariance matrix, G, for both sexes of Silene latifolia for floral and leaf traits, as well as the between-sex matrix, B. We also quantified selection on the sexes via survival for 2 yr in four natural populations that varied in water availability. Environment-dependent intralocus sexual conflict exists for specific leaf area, a trait that is genetically correlated between the sexes. Males experienced significant negative selection, but only in populations with relatively limited water availability. Females experienced weakly positive or significant stabilizing selection on the same trait. Specific leaf area is genetically correlated with flower size and number, which are sexually dimorphic in this species. The extent of intralocus sexual conflict varied with the environment. Resolution of such conflict is likely to be confounded, given that specific leaf area is highly genetically integrated with other traits that are also divergent between the sexes.

Observational evidence of herbivore-specific associational effects between neighboring conspecifics in natural, dimorphic populations of Datura wrightii.

Associational effects-in which the vulnerability of a plant to herbivores is influenced by its neighbors-have been widely implicated in mediating plant-herbivore interactions. Studies of associational effects typically focus on interspecific interactions or pest-crop dynamics. However, associational effects may also be important for species with intraspecific variation in defensive traits. In this study, we observed hundreds of Datura wrightii-which exhibits dimorphism in its trichome phenotype-from over 30 dimorphic populations across California. Our aim was to determine whether a relationship existed between the trichome phenotype of neighboring conspecifics and the likelihood of being damaged by four species of herbivorous insects. We visited plants at three timepoints to assess how these effects vary both within and between growing seasons. We hypothesized that the pattern of associational effects would provide rare morphs (i.e., focal plants that are a different morph than their neighbors) with an advantage in the form of reduced herbivory, thereby contributing to the negative frequency-dependent selection previously documented in this system. We found the best predictor of herbivory/herbivore presence on focal plants was the phenotype of the focal plant. However, we also found some important neighborhood effects. The total number of plants near a focal individual predicted the likelihood and/or magnitude of herbivory by Tupiochoris notatus, Lema daturaphila, and Manduca sexta. We also found that velvety focal plants with primarily sticky neighbors are more susceptible to infestation by Tupiochoris notatus and Lema daturaphila. This does not align with the hypothesis that associational effects at the near-neighbor scale contribute to a rare-morph advantage in this system. Overall, the results of our study show that the number and trichome-morph composition of neighboring conspecifics impact interactions between D. wrightii and insect herbivores.

The effect of breeding system on polymorphism in mitochondrial genes of Silene.

Gynodioecy is a breeding system characterized by the co-occurrence of hermaphrodite and female individuals, generally as the result of nuclear-cytoplasmic interactions. The question remains whether the genetic factors controlling gynodioecy are maintained in species over long evolutionary timescales by balancing selection or are continually arising and being replaced in epidemic sweeps. If balancing selection maintains these factors, then neutral cytoplasmic diversity should be greater in gynodioecious than hermaphroditic species. In contrast, epidemic sweeps of factors controlling gynodioecy should decrease cytoplasmic diversity in gynodioecious relative to hermaphroditic species. We took a comparative approach in which we sequenced two mitochondrial genes, cytochrome b (cob) and cytochrome oxidase (cox1), for multiple populations of several hermaphroditic, gynodioecious, and dioecious species in the genus Silene. Breeding system was predictive of polymorphism. Gynodioecious species harbor many old haplotypes while hermaphroditic and dioecious species have little to no nucleotide diversity. The genealogical structure of neither gene departed from neutral expectations. Taken together, our results suggest that balancing selection acts on cytoplasmic male-sterility factors in several gynodioecious species in the genus.

The nearness of you: the effect of population structure on siring success in a gynodioecious species.

Theoretically, both balancing selection and genetic drift can contribute to the maintenance of gender polymorphism within and/or among populations. However, if strong differences exist among genotypes in the quantity of viable gametes they produce, then it is expected that these differences will play an important role in determining the relative frequency of the genotypes and contribute to whether or not such polymorphism is maintained. In this issue, De Cauwer et al. (2010) describe an investigation of gynodioecious wild sea beet, which in addition to containing females, contain two types of hermaphrodites: restored hermaphrodites carrying a cytoplasm that causes pollen sterility and a nuclear gene that restores pollen fertility, and hermaphrodites without the sterilizing cytoplasm. The results show that restored hermaphrodites, who have relatively low pollen viability, achieve disproportionately high siring success simply because of where they are located in a patchy population (Fig. 1). Notably, these individuals tend to be close to females because of the genetics of sex determination. These results indicate that population structure caused by drift processes can have an unexpectedly large effect on the fitness of these low quality hermaphrodites, thereby contributing in the short term to the maintenance of gynodioecy in this population. While these results indicate that population structure caused by drift processes can have a large effect on the relative fitness of genetic variants, whether these effects promote or discourage the maintenance of polymorphism in the long term is still up for debate.

Asymmetrical conspecific seed-siring advantage between Silene latifolia and S. dioica.

BACKGROUND AND AIMS: Silene dioica and S. latifolia experience only limited introgression despite overlapping flowering phenologies, geographical distributions, and some pollinator sharing. Conspecific pollen precedence and other reproductive barriers operating between pollination and seed germination may limit hybridization. This study investigates whether barriers at this stage contribute to reproductive isolation between these species and, if so, which mechanisms are responsible. METHODS: Pollen-tube lengths for pollen of both species in styles of both species were compared. Additionally, both species were pollinated with majority S. latifolia and majority S. dioica pollen mixes; then seed set, seed germination rates and hybridity of the resulting seedlings were determined using species-specific molecular markers. KEY RESULTS: The longest pollen tubes were significantly longer for conspecific than heterospecific pollen in both species, indicating conspecific pollen precedence. Seed set but not seed germination was lower for flowers pollinated with pure heterospecific versus pure conspecific pollen. Mixed-species pollinations resulted in disproportionately high representation of nonhybrid offspring for pollinations of S. latifolia but not S. dioica flowers. CONCLUSIONS: The finding of conspecific pollen precedence for pollen-tube growth but not seed siring in S. dioica flowers may be explained by variation in pollen-tube growth rates, either at different locations in the style or between leading and trailing pollen tubes. Additionally, this study finds a barrier to hybridization operating between pollination and seed germination against S. dioica but not S. latifolia pollen. The results are consistent with the underlying cause of this barrier being attrition of S. dioica pollen tubes or reduced success of heterospecifically fertilized ovules, rather than time-variant mechanisms. Post-pollination, pre-germination barriers to hybridization thus play a partial role in limiting introgression between these species.

Rapid reversal of a potentially constraining genetic covariance between leaf and flower traits in Silene latifolia.

Genetic covariance between two traits generates correlated responses to selection, and may either enhance or constrain adaptation. Silene latifolia exhibits potentially constraining genetic covariance between specific leaf area (SLA) and flower number in males. Flower number is likely to increase via fecundity selection but the correlated increase in SLA increases mortality, and SLA is under selection to decrease in dry habitats. We selected on trait combinations in two selection lines for four generations to test whether genetic covariance could be reduced without significantly altering trait means. In one selection line, the genetic covariance changed sign and eigenstructure changed significantly, while in the other selection line eigenstructure remained similar to the control line. Changes in genetic variance-covariance structure are therefore possible without the introduction of new alleles, and the responses we observed suggest that founder effects and changes in frequency of alleles of major effect may be acting to produce the changes.

Herbivore-mediated negative frequency-dependent selection underlies a trichome dimorphism in nature.

Negative frequency-dependent selection (NFDS) has been shown to maintain polymorphism in a diverse array of traits. The action of NFDS has been confirmed through modeling, experimental approaches, and genetic analyses. In this study, we investigated NFDS in the wild using morph-frequency changes spanning a 20-year period from over 30 dimorphic populations of Datura wrightii. In these populations, plants either possess glandular (sticky) or non-glandular (velvety) trichomes, and the ratio of these morphs varies substantially among populations. Our method provided evidence that NFDS, rather than drift or migration, is the primary force maintaining this dimorphism. Most populations that were initially dimorphic remained dimorphic, and the overall mean and variance in morph frequency did not change over time. Furthermore, morph-frequency differences were not related to geographic distances. Together, these results indicate that neither directional selection, drift, or migration played a substantial role in determining morph frequencies. However, as predicted by negative frequency-dependent selection, we found that the rare morph tended to increase in frequency, leading to a negative relationship between the change in the frequency of the sticky morph and its initial frequency. In addition, we found that morph-frequency change over time was significantly correlated with the damage inflicted by two herbivores: Lema daturaphila and Tupiochoris notatus. The latter is a specialist on the sticky morph and damage by this herbivore was greatest when the sticky morph was common. The reverse was true for L. daturaphila, such that damage increased with the frequency of the velvety morph. These findings suggest that these herbivores contribute to balancing selection on the observed trichome dimorphism.

Parthenogenesis maintains male sterility in a gynodioecious orchid.

The invasion of male-sterile (female) individuals into hermaphroditic populations, leading to gynodioecy, is common in flowering plants. Both theoretical and empirical studies have shown that as the frequency of females increases in a population, pollen limitation reduces seed production more in females than in hermaphrodites, leading to higher fitness for hermaphrodites and a consequent decrease in female frequency. Here we show that contrary to this expectation, females of the gynodioecious orchid Satyrium ciliatum are maintained only in populations that experience high pollen limitation caused by low pollinator service and high pollen herbivory. This species avoids the typical problem of pollen limitation for seed production and can therefore maintain high frequencies of females in pollen-limited populations because females produce more seeds than hermaphrodites via facultative parthenogenesis in the absence of pollinia. Our results therefore demonstrate that parthenogenesis is a novel mechanism favoring the maintenance of gynodioecy.

Benefits and costs to pollinating, seed-eating insects: the effect of flower size and fruit abortion on larval performance.

Plant-pollinator interactions are well-known examples of mutualism, but are not free of antagonism. Antagonistic interactions and defenses or counter-defenses are expected particularly in nursery pollination. In these systems, adult insects, while pollinating, lay their eggs in flowers, and juveniles consume the seeds from one or several fruits, thereby substantially reducing plant fitness. The outcome of such interactions will depend, for the plant, on the balance between pollination versus seed predation and for the larvae on the balance between the food and shelter provided versus the costs imposed by plant defenses, e.g., through abortion of infested fruits. Here, we examine the costs and benefits to the larvae in the nursery-pollination system Silene latifolia/Hadena bicruris. Using selection lines that varied in flower size (large- vs. small-flowered plants), we investigated the effects of variation in flower and fruit size and of a potential defense, fruit abortion, on larval performance. In this system, infested fruits are significantly more likely to be aborted than non-infested fruits; however, it is unclear whether fruit abortion is effective as a defense. Larger flowers gave rise to larger fruits with more seeds, and larvae that were heavier at emergence. Fruit abortion was frequently observed (ca. 40% of the infested fruits). From aborted fruits, larvae emerged earlier and were substantially lighter than larvae emerging from non-aborted fruits. The lower mass at emergence of larvae from aborted fruits indicates that abortion is a resistance mechanism. Assuming that lower larval mass implies fewer resources invested in the frugivore, these results also suggest that abortion is likely to benefit the plant as a defense mechanism, by limiting both resources invested in attacked fruits, as well as the risk of secondary attack. This suggests that selective fruit abortion may contribute to the stability of mutualism also in this non-obligate system.