Lynda F. Delph

Gender and sexual dimorphism in flowering plants

1 Gender and Sexual Dimorphism in Flowering Plants: A Review of Terminology, Biogeographic patterns, Ecological Correlates, and Phylogenetic Approaches.- 1.1 Introduction.- 1.2 Terminology.- 1.3 Incidence of Dioecy.- 1.3.1 Overview.- 1.3.2 Ecological Associations.- 1.3.3 Geographic Patterns.- 1.4 Importance of Phylogenetic Approaches.- 1.5 Using Phylogenies to Understand Process and Pattern.- 1.5.1 Phylogenetic Distributions.- 1.5.2 Self-Incompatibility and Dioecy.- 1.5.3 Dioecy and Fleshy Fruits.- 1.5.4 Habitat Shifts, Pollination Biology, and Changes in Outcrossing Rates.- 1.6 Conclusions.- References.- 2 Theories of the Evolution of Dioecy.- 2.1 Introduction.- 2.2 Importance of Theoretical Models.- 2.3 Pathways to Dioecy.- 2.4 Theoretical Relationships Between Allocation of Reproductive Resources and Invasion of Populations by New Sex Morphs.- 2.4.1 Fitness in Outcrossing and Partially Selfing Cosexes and Allocation in Cosexes.- 2.4.2 Invasion of Populations by Females and Males.- 2.4.3 Effect of Cosex Allocations on Invasion by Unisexuals or Partially Sterile Types.- 2.4.4 Effects of Unisexuals on Cosex Allocations.- 2.4.5 Other Possible Routes to Dioecy.- 2.5 Testing the Theory.- 2.5.1 Comparative Tests.- 2.5.2 Gain Curves.- 2.5.3 Intraspecific Data.- 2.5.4 Genetic Data.- 2.6 Conclusions.- 2.7 References.- 3 Empirical Studies: Evolution and Maintenance of Dimorphic Breeding Systems.- 3.1 Introduction.- 3.2 Evolutionary Pathways to Gender Dimorphism.- 3.2.1 Approaches to the Study of Gender.- 3.2.1.1 Quantitative Description of Plant Gender.- 3.2.1.2 Theoretical Modelling.- 3.2.1.3 Phylogenetic Analysis.- 3.2.2 Overview of Pathways.- 3.2.3 From Cosexuality Via Gynodioecy to Dioecy.- 3.2.4 From Monoecy Via Paradioecy to Dioecy.- 3.2.5 From Cosexuality Via Androdioecy to Dioecy.- 3.2.6 From Heterostyly to Dioecy.- 3.2.7 From Duodichogamy or Heterodichogamy to Dioecy.- 3.2.8 The Evolution of Trioecy.- 3.3 Maintenance of Gender Dimorphism in Natural Populations.- 3.3.1 Sex Ratios.- 3.3.2 Evidence for an Outcrossing Advantage: Rates of Selfing and Levels of Inbreeding Depression.- 3.3.3 Relative Seed Fecundity of the Two Sexes.- 3.3.4 Relative Pollen Fecundity of the Two Sexes.- 3.3.5 Case Studies: Tests of Theoretical Models.- 3.3.5.1 Female Frequency and Habitat in Plantago lanceolata.- 3.3.5.2 Plant Vigour, Fruit Production and the Sex Ratio in Hebe strictissima.- 3.3.5.3 Rates of Selfing, Inbreeding Depression and the Sex Ratio.- 3.3.5.4 The Breakdown of Outcrossing Mechanism in Aralia.- 3.4 Directions for Future Research.- 3.4.1 Testable Predictions from Ecological Correlations.- 3.4.2 Other Research Gaps.- 3.5 Conclusions.- References.- 4 Theories of the Evolution of Sexual Dimorphism.- 4.1 Introduction.- 4.2 Models of Sexual Dimorphism.- 4.2.1 Types of Models.- 4.2.2 General Features.- 4.2.3 Sexual Dimorphism in a Dioecious Organism.- 4.2.3.1 Genetic Models.- 4.2.3.2 ESS Models.- 4.2.4 The Evolution of Gender and Sexual Dimorphism.- 4.2.4.1 ESS Models.- 4.2.4.2 Genetic Models.- 4.3 The Biology of Sexual Dimorphism.- 4.3.1 Disruptive Selection on Homologous Characters.- 4.3.1.1 Biological Circumstances.- 4.3.1.2 Theory on Disruptive Selection in Dioecious Organisms.- 4.3.1.3 Theory on the Evolution of Gender and Sexual Dimorphism.- 4.3.1.4 Disruptive Selection and Sexual Dimorphism in Plants.- 4.3.2 Ecological Competition.- 4.3.2.1 Biological Circumstances.- 4.3.2.2 Theory on Character Displacement Due to Intraspecific Competition.- 4.3.2.3 Competitive Character Displacement and SSS in Dioecious Plants.- 4.3.3 Intersexual Selection.- 4.3.3.1 Biological Circumstances.- 4.3.3.2 Theory on Intersexual Selection.- 4.3.3.3 Mate Choice and Sexual Dimorphism in Plants.- 4.4 Conclusions.- References.- 5 Sexual Dimorphism in Flowers and Inflorescences.- 5.1 Introduction.- 5.2 Patterns.- 5.2.1 Perianth Size.- 5.2.2 Perianth Shape.- 5.2.3 Nectar.- 5.2.4 Vestigial Characters.- 5.2.5 Other Flower Characters.- 5.2.6 Multi-Flower Characters.- 5.2.7 Questions.- 5.3 Evolutionary Hypotheses.- 5.3.1 Sexual Selection and Character Exaggeration.- 5.3.2 Specific Tests, Hypotheses, and Uncertainties.- 5.3.2.1 Perianth Size.- 5.3.2.2 Perianth Shape.- 5.3.2.3 Nectar.- 5.3.2.4 Vestigial Character.- 5.3.2.5 Other Flower Characters: Longevity.- 5.3.2.6 Multi-Flower Characters.- 5.4 Conclusions.- 5.4.1 Towards Quantitative Understanding.- 5.4.2 Size-Number Trade-Offs.- 5.4.3 Costs of Exaggeration.- 5.4.4 Variation in Costs and Benefits.- 5.4.5 Macro evolution.- References.- 6 Sexual Dimorphism in Live History.- 6.1 Introduction.- 6.2 Predictions Based on Sex-Differential Reproductive Investment.- 6.3 Patterns of Sexual Dimorphism in Life-History Traits.- 6.3.1 Response to Stress.- 6.3.2 Case Studies of Two Species in which the Cost of Reproduction Is Higher for Females.- 6.4 Factors Offsetting Between-Sex Differences in the Cost of Reproduction.- 6.4.1 Sexual Dimorphism in the Timing of Investment in Reproduction Versus Growth Within a Season.- 6.4.2 Sexual Dimorphism in the Timing of Flowering Within a Season.- 6.4.3 Sexual Dimorphism in the Frequency of Flowering.- 6.4.4 Sexual Dimorphism in Age of Maturation.- 6.4.5 Sexual Dimorphism in Physiological Traits.- 6.4.6 Sex-Differential Herbivory.- 6.5 The Contrary Case of Silene latifolia.- 6.6 Conclusions.- References.- 7 Dimorphism in Physiology and Morphology.- 7.1 Introduction.- 7.1.1 Causes of Sexual Dimorphism in Physiology and Vegetative Morphology.- 7.1.2 Physiological and Morphological Responses to Natural Selection.- 7.1.3 Physiological and Morphological Responses to Sexual Selection.- 7.1.4 Functional Significance of Dimorphism in Physiology and Morphology.- 7.2 History of Studies on Sexual Dimorphism in Plants.- 7.3 Sexual Dimorphism in Plant Form and Function in Species with SSS.- 7.3.1 Salix (Willow Salicaceae).- 7.3.2 Acer negundo (Boxelder Aceraceae).- 7.3.3 Simmondsia chinensis (Jojoba/Goat Nut Buxaceae).- 7.3.4 Phoradendron juniperinum (Mistletoe Viscaceae).- 7.3.5 Other Species.- 7.4 Sexual Dimorphism in Plant Form and Function in Species Without SSS.- 7.4.1 Silene latifolia (White Campion Caryophyllaceae).- 7.4.2 Leucadendron (Proteaceae).- 7.4.3 Other Species.- 7.4.3.1 Agricultural and Weedy Species.- 7.4.3.2 Populus (Aspen Salicaceae).- 7.5 Conclusions and Future Directions.- References.- 8 Sexual Dimorphism and Biotic Interactions.- 8.1 Introduction.- 8.1.1 Reproductive Allocation and Biotic Interactions.- 8.2 Sexual Differences in Competitive Ability.- 8.3 Sexual Differences in Herbivory.- 8.3.1 Herbivore Preference.- 8.3.2 Correlates of Sexual Differences in Herbivore Damage.- 8.3.3 Herbivore Performance on Male and Female Hosts.- 8.3.4 Sexual Differences in Response to Herbivory.- 8.4 Sexual Differences in Parasitism.- 8.4.1 Foliar Pathogens.- 8.4.2 Flower-Infecting Pathogens.- 8.4.3 Nonfungal Parasites.- 8.5 General Discussion.- 8.5.1 Biotic Interactions and Biased Sex Ratios.- 8.5.2 Evolution of Sexual Differences in Herbivory.- 8.5.3 Future Studies.- References.- 9 Genetics of Gender Dimorphism in Higher Plants.- 9.1 Introduction.- 9.2 Monoecious Plants.- 9.2.1 Gender Dimorphism in Cucumber.- 9.2.2 Molecular Biology of Gender Dimorphism in Maize.- 9.2.2.1 Tasselseed2.- 9.2.2.2 Gibberellin and gender dimorphism in maize.- 9.2.2.3 The Anther ear1 gene.- 9.3 Multigenic gender determination systems in dioecious plants.- 9.3.1 Mercurialis annua.- 9.3.2 A single gender determination locus.- 9.3.3 Sex chromosomes.- 9.3.3.1 Morphologically distinct sex chromosomes.- 9.3.3.2 Structure of sex chromosomes in plants.- 9.3.3.3 X/autosome balance can regulate gender dimorphism.- 9.3.3.4 X/autosome balance in Drosophila melanogaster.- 9.3.4 Comparison of Active Y Sex Chromosomes in Plants and Animals.- 9.3.4.1 The active-Y gender determination of white campion.- 9.3.4.2 The mammalian active-Y gender determination mechanism.- 9.3.4.3 Does dosage compensation occur in white campion?.- 9.3.5 Evolution of the active-Y chromosome: Male sterility.- 9.3.5.1 Cytoplasmic male sterility.- 9.3.5.2 Suppression of carpel or pistil development.- 9.4 Expression of MADS-box genes in unisexual flowers.- 9.5 Conclusions.- References.- 10 Quantitative Genetics of Sexual Dimorphism.- 10.1 Introduction.- 10.2 Quantitative Genetic Models of Sexual Dimorphism.- 10.3 Integration of Quantitative Genetics with Sexual Selection.- 10.4 Correlated Evolution and Divergence of Male and Female Traits in Dioecious Plants.- 10.5 Correlated Evolution and Divergence of Male and Female Function in Hermaphroditic Plants.- 10.6 Conclusions.- 10.7 References.- Taxonomic Index.

On the Importance of Male Fitness in Plants: Patterns of Fruit‐Set

A hypothesis is presented which states that flower production in hermaphroditic flow- ering plants is primarily controlled by male function. The male function hypothesis predicts a lower fruit-to-flower ratio for hermaphrodites as compared to monoecious or dioecious plants. The hy- pothesis also predicts that self-compatible hermaphrodites should exhibit a higher percent fruit-set than self-incompatible hermaphrodites. These predictions are supported by fruit-set data compiled from the literature. An alternative hypothesis relating fruit-set to the probability of self-fertilization also predicts low fruit-set for hermaphrodites as compared to monoecious or dioecious plants. The self-incompatibility hypothesis is tested and rejected on the basis of fruit-set patterns in self-incom- patible andromonoecious, self-incompatible monoecious, and self-compatible monoecious species. The effect of the male function hypothesis on current ideas concerning low fruit-set in hermaphrodites is then examined.

HOW ENVIRONMENTAL FACTORS AFFECT POLLEN PERFORMANCE: ECOLOGICAL AND EVOLUTIONARY PERSPECTIVES

We review the effects of herbivory and other environmental factors on pollen performance in plants. We conclude that natural levels of variation in herbivory and other environmental factors during pollen development are often sufficient to cause significant differences in pollen performance, and that the differences in pollen performance are likely to be caused by differences in the provisioning of pollen grains. From an evolutionary perspective, we discuss how pollen and ovule provisioning may be negatively genetically correlated and how this would maintain genetic variation for pollen performance within populations. Furthermore, the highly plastic nature of pollen performance provides the potential for genotypes to respond differently to environmental variation (genotype–environment interactions), which would also promote the maintenance of genetic variation in pollen performance.

Extensive variation in synonymous substitution rates in mitochondrial genes of seed plants

BackgroundIt has long been known that rates of synonymous substitutions are unusually low in mitochondrial genes of flowering and other land plants. Although two dramatic exceptions to this pattern have recently been reported, it is unclear how often major increases in substitution rates occur during plant mitochondrial evolution and what the overall magnitude of substitution rate variation is across plants.ResultsA broad survey was undertaken to evaluate synonymous substitution rates in mitochondrial genes of angiosperms and gymnosperms. Although most taxa conform to the generality that plant mitochondrial sequences evolve slowly, additional cases of highly accelerated rates were found. We explore in detail one of these new cases, within the genus Silene. A roughly 100-fold increase in synonymous substitution rate is estimated to have taken place within the last 5 million years and involves only one of ten species of Silene sampled in this study. Examples of unusually slow sequence evolution were also identified. Comparison of the fastest and slowest lineages shows that synonymous substitution rates vary by four orders of magnitude across seed plants. In other words, some plant mitochondrial lineages accumulate more synonymous change in 10,000 years than do others in 100 million years. Several perplexing cases of gene-to-gene variation in sequence divergence within a plant were uncovered. Some of these probably reflect interesting biological phenomena, such as horizontal gene transfer, mitochondrial-to-nucleus transfer, and intragenomic variation in mitochondrial substitution rates, whereas others are likely the result of various kinds of errors.ConclusionThe extremes of synonymous substitution rates measured here constitute by far the largest known range of rate variation for any group of organisms. These results highlight the utility of examining absolute substitution rates in a phylogenetic context rather than by traditional pairwise methods. Why substitution rates are generally so low in plant mitochondrial genomes yet occasionally increase dramatically remains mysterious.

Pattern and process: evidence for the evolution of photosynthetic traits in natural populations

The patterns of interspecific variation identified by comparative studies provide valuable hypotheses about the role of physiological traits in evolutionary adaptation. This review covers tests of these hypotheses for photosynthetic traits that have used a microevolutionary perspective to characterize physiological variation among and within populations. Studies of physiological differentiation among populations show that evolutionary divergence in photosynthetic traits is common within species, and has a pattern that supports many adaptive hypotheses. These among-population studies imply that selection has influenced photosynthetic traits in some way, but they are not designed to identify the traits targeted by selection or the environmental agents that cause selection. Analyses of genetic and phenotypic variation within populations address these questions. Studies that have quantified genetic variation within populations show that levels of heritable variation can be adequate for evolutionary change in photosynthetic traits. Other studies have measured phenotypic selection for these traits by analyzing how the variation within populations is correlated with fitness. This work has shown that selection for photosynthetic traits may often operate indirectly via correlations with other traits, and emphasizes the importance of viewing the phenotype as an integrated function of growth, morphology, life-history and physiology. We also outline some methodological problems that may be encountered for ecophysiological traits by these types of studies, provide some potential solutions, and discuss future directions for the field of plant evolutionary ecophysiology.

Silene as a model system in ecology and evolution.

The genus Silene, studied by Darwin, Mendel and other early scientists, is re-emerging as a system for studying interrelated questions in ecology, evolution and developmental biology. These questions include sex chromosome evolution, epigenetic control of sex expression, genomic conflict and speciation. Its well-studied interactions with the pathogen Microbotryum has made Silene a model for the evolution and dynamics of disease in natural systems, and its interactions with herbivores have increased our understanding of multi-trophic ecological processes and the evolution of invasiveness. Molecular tools are now providing new approaches to many of these classical yet unresolved problems, and new progress is being made through combining phylogenetic, genomic and molecular evolutionary studies with ecological and phenotypic data.

Sexual dimorphism in flower size

Sexual dimorphism in the secondary characters of unisexual flowers is often observed. Most prior reports indicate that male flowers have larger petals than female flowers. In this article we examine hypotheses regarding patterns of perianth (sepals and petals) size dimorphism. (1) Developmental associations between the corolla and the stamens constrain the independent evolution of these characters. (2) The role of the perianth in enclosing the reproductive structures in the bud results in a correlation between the size of the enclosed structures and the perianth parts. (3) In animal-pollinated species, the perianth serves to attract pollinators; fitness gains achieved through allocation to attractive structures differ between the sexes. To test these hypotheses, we compared 919 species with unisexual flowers; quantitative measurements of floral parts were made for 84 of these species. Unlike most previous studies, this study found that the petals of male flowers were larger than those of females in less than one-half of the cases, demonstrating that developmental associations are not strictly responsible for patterns of sexual dimorphism. Relative perianth size appears to be evolutionarily labile, even within genera. Patterns of perianth-size dimorphism differed between temperate and tropical species and between animal- and wind-pollinated species. Perianth size was strongly associated with the size of the reproductive structures enclosed in each flower, suggesting that the function of petals and sepals to protect developing reproductive structures may determine their size. However, the size of the reproductive structures was a better predictor of perianth size in wind-than in animal-pollinated species, which supports the attractive function hypothesis of the perianth. We conclude that the two functional roles of the perianth, protection and attraction, are the primary determinants of perianth size in unisexual flowers.

Sex-specific physiology and source-sink relations in the dioecious plant Silene latifolia

Differences in reproductive demands between the sexes of dioecious plants could cause divergence in physiology between the sexes. We found that the reproductive effort of female Silene latifolia plants increased to more than twice that of male plants or female plants that were prevented from setting fruit by lack of pollination after 4 weeks of flowering. Whole-plant source/sink ratios of pollinated females were significantly lower than those of males or unpollinated females because of investment in fruit. We hypothesized that these differences in source/sink ratio between the sexes and within females, depending on pollination, would lead to differences in leaf photosynthetic rates. Within females, we found that photosynthetic capacity was consistent with measurement of whole-plant source/sink ratio. Females that were setting fruit had 30% higher light-saturated photosynthetic rates by 28 days after flowering than females that were not setting fruit. Males, however, had consistently higher photosynthetic rates than females from 10 days after flowering onwards. Males also had approximately twice the dark respiration rates of fruiting females. We found that female reproductive structures are longer-lived and contribute more carbon to their own support than male reproductive structures. Despite the higher rates of leaf dark respiration and lower calyx photosynthetic rates, males fix more carbon than do females. We conclude that females have a sink-regulated mechanism of photosynthesis that allows them to respond to variations in fruit set. This mechanism is not, however, sufficient to explain why male S. latifolia plants have higher rates of photosynthesis, higher source/sink ratios, and lower reproductive allocation, but fail to grow larger than female plants.

Flower Size Dimorphism in Plants with Unisexual Flowers

Angiosperm species exhibit variation in every aspect of their flowers, from obvious differences in color, size, shape, and number to less conspicuous differences such as nectar production and the schedule of events that take place within each flower. Sprengel (1793) was perhaps the first to realize that floral characters are shaped by their function. Recent studies have shown that, for some species, characters that enhance male function may be different from those that enhance female function. For example, characters such as color (Stanton et al., 1986), color change (Cruzan et al., 1988), and shape (Campbell, 1989) have varying consequences for the success of the two gamete types. Campbell (1989) found that selection intensities through male function favored wide corollas, whereas narrow corollas were favored via female function. Studies such as this one indicate that selection through male function may actually be opposite in direction to selection through female function. Hence, investigations into how selection is operating on floral characters are made difficult in hermaphroditic plants because of the problems associated with quantifying male fitness (see Stanton et al., 1992). One approach to understanding how traits associated with male or female function are shaped by selection, therefore, is to investigate secondary floral structures (e.g., petals) in species with unisexual flowers (and see Chapter 11).

Host Sex and Local Adaptation by Parasites in a Snail-Trematode Interaction

One of the leading theories for the evolutionary stability of sex in eukaryotes relies on parasite‐mediated selection against locally common host genotypes (the Red Queen hypothesis). As such, parasites would be expected to be better at infecting sympatric host populations than allopatric host populations. Here we examined all published and unpublished infection experiments on a snail‐trematode system (Potamopyrgus antipodarum and Microphallus sp., respectively). A meta‐analysis demonstrated significant local adaptation by the parasite, and a variance components analysis showed that the variance due to the host‐parasite interaction far exceeded the variance due to the main effects of host source and parasite source. The meta‐analysis also indicated that asexual host populations were more resistant to allopatric sources of parasites than were (mostly) sexual host populations, but we found no significant differences among parasite populations in the strength of local adaptation. This result suggests that triploid asexual snails are more resistant to remote sources of parasites, but the parasite has, through coevolution, overcome the difference. Finally, we found that the degree of local adaptation did not depend on the genetic distance among host populations. Taken together, the results demonstrate that the parasites are adapted, on average, to infecting their local host populations and suggest that they may be a factor in selecting against common host genotypes in natural populations.