Pollinator selectivity is thought to influence the evolution of separate sexes in plants because of its potential to limit plant reproductive success. Selective visitation could also constrain or ...promote the phenotypic divergence of the sexes. In this study, I explored the causes and consequences of selectivity by generalist pollinators of a gynodioecious wild strawberry (Fragaria virginiana) and thus provide insight into potential pollinator-mediated selection for dioecy and sexual dimorphism. I found that flowers of F. virginiana show pronounced sexual dimorphism in petal length, stamen length, nectar and pollen production, and that this results in dramatic and consistent levels of sex-differential visitation by ants, bees, and flies. I performed manipulations of hermaphrodite flowers to understand the basis of selectivity and found that much of bee and fly preference for hermaphrodite flowers derived from their strong preference for longer petals, but also from a more subtle preference for pollen-filled anthers. These studies also revealed that other traits contribute to the observed discrimination against females. A stronger relationship existed between bee visitation and pollen receipt in females than between bee visitation and pollen removal from hermaphrodites. An analysis of natural variation in petal and stamen length confirmed the central role of petal length and also showed a lack of an effect of vestigial stamen length in pollination success of females. It also revealed a significant effect of stamen length, but not of petal length, on pollen removal. The data suggest that pollinator selectivity may affect the evolution of floral sexual dimorphism, both by exerting selection that could lead to the maintenance of stamens in females and by exerting selection to increase petal length in females.
We studied the natural hybrid (Fragaria × ananassa subsp. cuneifolia) between two sexually dimorphic octoploid strawberry species (Fragaria virginiana and Fragaria chiloensis) to gain insight into ...the dynamics of sex chromosomes and the genesis of sexual dimorphism. Male sterility is dominant in both the parental species and thus will be inherited maternally, but the chromosome that houses the sex-determining region differs. Thus, we asked whether (1) the cytotypic composition of hybrid populations represents one or both maternal species, (2) the sex-determining chromosome of the hybrid reflects the location of male sterility within the maternal donor species and (3) crosses from the hybrid species show less sexual dimorphism than the parental species. We found that F. × ananassa subsp. cuneifolia populations consisted of both parental cytotypes but one predominated within each population. Genetic linkage mapping of two crosses showed dominance of male sterility similar to the parental species, however, the map location of male sterility reflected the maternal donor in one cross, but not the other. Moreover, female function mapped to a single region in the first cross, but to two regions in the second cross. Aside from components of female function (fruit set and seed set), other traits that have been found to be significantly sexually dimorphic in the pure species were either not dimorphic or were dimorphic in the opposite direction to the parental species. These results suggest that hybrids experience some disruption of dimorphism in secondary sexual traits, as well as novel location and number of quantitative trait locus (QTL) affecting sex function.
The objectives of the present study were to develop microsatellite markers for the wild strawberry, Fragaria virginiana, to evaluate segregation patterns of microsatellite alleles in this octoploid ...species, and assess genetic variability at microsatellite loci in a wild population. A genomic library was screened for microsatellite repeats and several PCR primers were designed and tested. We also tested the use of heterologous primers and found that F. virginiana primers amplified products in cultivated strawberry, Fragaria x ananassa Duch. and Fragaria chiloensis. Similarly, microsatellite loci developed from cultivated strawberry also successfully amplified F. virginiana loci. We investigated four microsatellite loci in detail, three developed from F. virginiana and one from cultivated strawberry. A survey of 100 individuals from a population of F. virginiana in Pennsylvania demonstrated high heterozygosities (H(e) or gene diversity ranged from 0.80 to 0.88 per locus) and allelic diversity (12-17 alleles per locus), but individual plants had no more than two alleles per locus. Segregation patterns in parents and progeny of two controlled crosses at these four loci were consistent with disomic Mendelian inheritance. Together these findings suggest that the genome of F. virginiana is "highly diploidized" and at least a subset of microsatellite loci can be treated as codominant, diploid markers. Significant heterozygote deficiencies were found at three of the four loci for hermaphroditic individuals but for only one locus among females in this gynodioecious species.
One evolutionary pathway from plants with combined male and female functions (hermaphroditism) to those with separate sexes (dioecy) involves females coexisting with hermaphrodites (gynodioecy). The ...research presented here explores sex allocation in Fragaria virginiana (a gynodioecious wild strawberry), within the context of theory on the gynodioecy–dioecy transition. By growing clonally replicated plants in the greenhouse and surveying six populations in situ, I evaluated the effects of plant size, genotype, sexual identity, population of origin and female frequency on sex allocation. I found significant positive effects of plant size on most sex allocation traits studied. In addition to strong sex‐specific allocation patterns, I found significant broad‐sense heritabilities for all traits, suggesting that plants could respond to selection. Moreover, there was a negative genetic correlation between pollen production and fruit set per flower within hermaphrodites, lending support to a basic assumption of sex allocation theory. On the other hand, several sex allocation traits, namely pollen and ovules per flower in hermaphrodites, were positively genetically correlated, suggesting that they may act to constrain the evolution of sexual dimorphism. Populations differed in the frequency of females, and females were more prevalent on sites with lower soil moisture and where hermaphrodites were least likely to produce fruit, suggesting that females’ seed fitness relative to that of hermaphrodites may be strongly environment‐dependent in this species.
In this study we dissect the causes of variation in intra‐inflorescence allocation in a sexually polymorphic species, Fragaria virginiana. We separated out the effects of resource competition during ...flowering from those of inflorescence architecture, as well as identified the effects of sex morph and genotype. We found position‐based variation in petal length, ovule, pollen, and flower number to be influenced more by architecture than by our resource manipulations during flowering. We also found both genotype‐ and sex‐specific intra‐inflorescence patterns. Furthermore, our data indicate that the sex morph‐specific intra‐inflorescence patterns result from architectural modifications of the basic pattern. In fact, sex‐differential intra‐inflorescence patterns suggest that fitness through male and female function may be maximized by different resource distribution patterns within the inflorescence and may have been modified by past selection. Specifically, females invested heavily in ovules at positions where fruit set was most likely (primary and secondary), at the expense of flower number and allocation per flower at more distal positions. Whereas functional males invested minimally in ovules at all flower positions and produced the most abundantly flowered inflorescences, hermaphrodites, on the other hand, showed intermediate patterns, implying a compromise between sex functions. We suggest that consideration of intra‐inflorescence allocation and inflorescence architecture may reveal the mechanism underlying sexual dimorphism in flower allocation and number.
We explore the effects of gender dimorphism on resistance (damage avoidance) and tolerance (growth and fitness compensation) to a flower bud clipping weevil in a gynodioecious wild strawberry. Using ...both natural populations and a common garden, we document pervasive hermaphrodite-biased damage and identify several floral traits associated with resistance. Lower flower number, lower pollen production per flower, and earlier flowering date were associated with higher resistance. Because the sex morphs do not differ in flower number or flowering time, the presence of pollen is the main factor determining sex-differential resistance. We confirmed that simulated clipping was a good surrogate for weevil clipping, and we used simulated clipping to assess sex-morph- and sex-function-specific tolerance. We found that females were less tolerant than hermaphrodites in terms of seed and fruit production, but the sex morphs were similarly unable to compensate fully in flower production. Hermaphrodites showed some ability to compensate in pollen per flower, but this was not large enough to offset flower losses, leading to net undercompensation for pollen per plant. We evaluated potential mechanisms for tolerance and found that the number of reserve buds was the most consistent predictor of tolerance. Finally, we found that herbivory shifted sex expression of hermaphrodites toward greater femaleness via both plastic changes in allocation and direct effects of loss of male-functioning flowers. Our work indicates that the effects of herbivory on sexual system evolution can include sexual phenotype-biased herbivore damage and sex-dependent tolerance, as well as herbivory-induced changes in hermaphrodite sex allocation. In addition, this study suggests that pollen production is subject to selection via herbivores that opposes selection mediated by pollinators.
We explored the idea that sex ratio represents a unique context for selection on attractive traits by manipulating sex ratio and pollinator abundance in experimental populations of a gender‐dimorphic ...wild strawberryFragaria virginiana. We found that increasing the frequency of functional males (the pollen‐bearing morph) increased the frequency of pollen‐collecting syrphid flies in the pollinator assemblage, decreased pollinator visitation to less preferred morph (females), and decreased the degree of pollen limitation of females. Moreover, sex ratio influenced the strength of selection on petal size through female fitness but did not alter the strength of selection through male fitness components, suggesting that sex ratio can alter the gender bias of selection on an attractive trait. This study of context‐dependent selection has important implications for the evolution of sexual dimorphism in attractive traits. First, it suggests that only certain conditions generate male‐biased selection and, thus, could lead to selection‐driven male‐biased petal size dimorphism. Second, it suggests that flexible pollinator foraging may be an important mechanism by which sex ratio influences selection on attractive traits. Finally, it implies that variation in sex ratio could limit the evolution of sexual dimorphism and/or could maintain genetic variation in attractive traits.
J. H. Burns was omitted in error from the author list of the original version of this Data Descriptor. This omission has now been corrected in both the HTML and PDF versions.
Our understanding of pollen limitation depends on a realistic view of its magnitude. Previous reviews of pollen supplementation experiments concluded that a majority of plant species suffers from ...pollen limitation and that its magnitude is high. Here, we perform a meta-analysis and find evidence that publication bias, experimental design, and the response variable chosen all influence the magnitude of pollen limitation. Fail-safe numbers indicate that publication bias exists for some measures of pollen limitation; significant results are more likely to be published and therefore available for review. Moreover, experiments conducted on only a fraction of a plant's flowers and reproductive episodes report ~8-fold higher effect sizes than those on all flowers produced over the entire lifetime, likely because resource reallocation among flowers and across years contributes to estimates of pollen limitation. Studies measuring percentage fruit set report higher values of pollen limitation than those measuring other response variables, such as seeds per fruit, perhaps because many plant species will not produce fruits unless adequate pollen receipt occurs to fertilize most ovules. We offer suggestions for reducing the bias introduced by methodology in pollen supplementation experiments and discuss our results in the context of optimality theory.
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