Tia-Lynn Ashman

POLLEN LIMITATION OF PLANT REPRODUCTION: ECOLOGICAL AND EVOLUTIONARY CAUSES AND CONSEQUENCES

Determining whether seed production is pollen limited has been an area of intensive empirical study over the last two decades. Yet current evidence does not allow satisfactory assessment of the causes or consequences of pollen limitation. Here, we critically evaluate existing theory and issues concerning pollen limitation. Our main conclusion is that a change in approach is needed to determine whether pollen limitation reflects random fluctuations around a pollen–resource equilibrium, an adaptation to stochastic pollination environments, or a chronic syndrome caused by an environmental perturbation. We formalize and extend D. Haig and M. Westobys conceptual model, and illustrate its use in guiding research on the evolutionary consequences of pollen limitation, i.e., whether plants evolve or have evolved to ameliorate pollen limitation. This synthesis also reveals that we are only beginning to understand when and how pollen limitation at the plant level translates into effects on plant population dynamics...

The genome of woodland strawberry ( Fragaria vesca )

The woodland strawberry, Fragaria vesca (2n = 2x = 14), is a versatile experimental plant system. This diminutive herbaceous perennial has a small genome (240 Mb), is amenable to genetic transformation and shares substantial sequence identity with the cultivated strawberry (Fragaria × ananassa) and other economically important rosaceous plants. Here we report the draft F. vesca genome, which was sequenced to ×39 coverage using second-generation technology, assembled de novo and then anchored to the genetic linkage map into seven pseudochromosomes. This diploid strawberry sequence lacks the large genome duplications seen in other rosids. Gene prediction modeling identified 34,809 genes, with most being supported by transcriptome mapping. Genes critical to valuable horticultural traits including flavor, nutritional value and flowering time were identified. Macrosyntenic relationships between Fragaria and Prunus predict a hypothetical ancestral Rosaceae genome that had nine chromosomes. New phylogenetic analysis of 154 protein-coding genes suggests that assignment of Populus to Malvidae, rather than Fabidae, is warranted.

Sex Determination: Why So Many Ways of Doing It?

Sex is universal amongst most eukaryotes, yet a remarkable diversity of sex determining mechanisms exists. We review our current understanding of how and why sex determination evolves in animals and plants.

A quantitative synthesis of pollen supplementation experiments highlights the contribution of resource reallocation to estimates of pollen limitation

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 plants 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.

Genetic constraints on floral evolution: a review and evaluation of patterns

The characteristics of flowers influence most aspects of angiosperm reproduction, including the agents of pollination and patterns of mating. Thus, a clear view of the forces that mediate floral phenotypic evolution is central to understanding angiosperm diversity. Here, we inform on the capacity for floral phenotype to respond to selection by reviewing published data on heritabilities and genetic correlations for several classes of floral traits (primary sexual, attraction, mating system) in hermaphroditic plants. We find significant heritability for all floral traits but also variation among them, as well as a tendency for heritability to vary with mating system, but not life history. We additionally test predictions stemming from life history theory (eg, negative covariation between male–female traits and flower size-flower number), and ideas concerning the extent and pattern of genetic integration between flowers and leaves, and between the sexes of dioecious and gynodioecious species. We find mixed evidence for life history tradeoffs. We find strong support for floral integration and its relation with floral morphology (actinomorphy vs zygomorphy) and for a decoupling of floral and vegetative traits, but no evidence that modular integration varies with floral morphology. Lastly, we find mixed evidence for a relationship between the level of sexual dimorphism in attraction traits and the between-sex correlation in gender dimorphic plants.

Explaining phenotypic selection on plant attractive characters: male function, gender balance or ecological context?

It is widely agreed that the flowers of hermaphrodite plants evolve in response to selection acting simultaneously through male and female sexual functions, but we know very little about the pattern of gender–specific selection. We review three current hypotheses for gender–specific selection by viewing them within a single phenotypic selection framework. We compile data from phenotypic selection and manipulative studies and evaluate the fit between empirical data and the hypotheses. In this preliminary analysis, we find that neither the male–function hypothesis nor the gender–balance hypothesis is well supported. However, the context–dependence hypothesis is supported by the documented diversity of gender–specific selection and by evidence that selection through female fertility is significantly correlated with pollen limitation of seed production. Future studies contributing to our understanding of selection through male and female function in plants need to quantify and manipulate the ecological context for reproduction, as well as describe male and female fitness responses to fine–scale trait manipulation.

Seasonal Variation in Pollination Dynamics of Sexually Dimorphic Sidalcea Oregana SSP. Spicata (Malvaceae)

We explored the combined effects of seasonal variation in both pollinator assemblage and availability of pollen donors on pollination in a gynodioecious species, Sidalcea oregana ssp. spicata (Malvaceae). Hermaphrodites produced flowers with signif- icantly larger petals but maintained fewer open flowers per inflorescence than females. Flowers of hermaphrodites produced 50% more nectar sugar in the 24 h after anthesis than the flowers of females. Nectar sugar production was also significantly and positively cor- related with petal length. Pollinator visitation rates were influenced more by differences in petal length than by differences in flower number per inflorescence. Consequently, her- maphrodites experienced higher visitation rates on a per-flower basis. Female flowers tended to receive pollen at a lower rate than hermaphrodites, but remained in the receptive female-phase longer than hermaphrodites. On average, the length of the period of flower receptivity declined as pollen deposition rate increased. These opposing processes resulted in the sex morphs receiving equivalent levels of pollination. Seasonal variation in the rate of pollen reception was more strongly influenced by the efficiency of the available pollinator pool than by rates of visitation. Dramatic seasonal shifts in the composition of the pollinator assemblage and pollen availability were correlated with increased pollination intensity as the season progressed. Not only were more pollen grains received, but they arrived in a shorter period of time and the number of potential pollen donors (hermaphrodites) in- creased. These findings suggest that pollen competition in both sex morphs may be more intense late in the season.

THE ROLE OF HERBIVORES IN THE EVOLUTION OF SEPARATE SEXES FROM HERMAPHRODITISM

Evolutionary biologists have repeatedly argued that environmental factors have influenced the evolution of dioecy (males and females) from hermaphroditism in plants. While many researchers have focused on the role of pollinators and abiotic factors, far fewer have considered the enemy component of the environment. In this paper, I explore how herbivory may impact the evolution of dioecy from hermaphroditism through gynodioecy. This synthesis shows that the widespread occurrence of male-biased enemy attack in sexually dimorphic species is likely to have significant consequences for both the first and second steps in the evolution of dioecy through gynodioecy. I identify several ways that herbivory can influence sexual-system evolution, review the handful of studies that have explored them, and highlight exciting new avenues for research. From this review and synthesis, I conclude that consideration of enemies will provide a fresh dimension to our understanding of plant sexual systems.

POLLINATOR SELECTIVITY AND ITS IMPLICATIONS FOR THE EVOLUTION OF DIOECY AND SEXUAL DIMORPHISM

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.

Genetic mapping of sex determination in a wild strawberry, Fragaria virginiana, reveals earliest form of sex chromosome

The evolution of separate sexes (dioecy) from hermaphroditism is one of the major evolutionary transitions in plants, and this transition can be accompanied by the development of sex chromosomes. Studies in species with intermediate sexual systems are providing unprecedented insight into the initial stages of sex chromosome evolution. Here, we describe the genetic mechanism of sex determination in the octoploid, subdioecious wild strawberry, Fragaria virginiana Mill., based on a whole-genome simple sequence repeat (SSR)-based genetic map and on mapping sex determination as two qualitative traits, male and female function. The resultant total map length is 2373 cM and includes 212 markers on 42 linkage groups (mean marker spacing: 14 cM). We estimated that approximately 70 and 90% of the total F. virginiana genetic map resides within 10 and 20 cM of a marker on this map, respectively. Both sex expression traits mapped to the same linkage group, separated by approximately 6 cM, along with two SSR markers. Together, our phenotypic and genetic mapping results support a model of gender determination in subdioecious F. virginiana with at least two linked loci (or gene regions) with major effects. Reconstruction of parental genotypes at these loci reveals that both female and hermaphrodite heterogamety exist in this species. Evidence of recombination between the sex-determining loci, an important hallmark of incipient sex chromosomes, suggest that F. virginiana is an example of the youngest sex chromosome in plants and thus a novel model system for the study of sex chromosome evolution.