Marcel E. Dorken

THE EVOLUTION AND MAINTENANCE OF MONOECY AND DIOECY IN SAGITTARIA LATIFOLIA (ALISMATACEAE)

Abstract Plant species rarely exhibit both monoecious and dioecious sexual systems. This limits opportunities to investigate the consequences of combined versus separate sex function on mating patterns and genetic variation and the analysis of factors responsible for the evolution and maintenance of the two sexual systems. Populations of the North American clonal aquatic Sagittaria latifolia are usually either monoecious or dioecious and often grow in close geographic proximity. We investigated mating patterns, genetic structure, and relationships between the two sexual systems using allozyme variation in populations from southern Ontario, Canada. As predicted, selfing rates in monoecious populations (n= 6, mean = 0.41) were significantly higher than in dioecious populations (n= 6, mean = 0.11). Moreover, marker‐based estimates of inbreeding depression (δ) indicated strong selection against inbred offspring in both monoecious (mean δ= 0.83) and dioecious (mean δ= 0.84) populations. However, the difference in selfing rate between the sexual systems was not reflected in contrasting levels of genetic variation. Our surveys of 12 loci in 15 monoecious and 11 dioecious populations revealed no significant differences in the proportion of polymorphic loci (P), number of alleles per locus (A), or observed and expected heterozygosity (Ho and He, respectively). Strong inbreeding depression favoring survival of outcrossed offspring may act to maintain similar levels of diversity between monoecious and dioecious populations. Despite geographical overlap between the two sexual systems in southern Ontario, a dendrogram of genetic relationships indicated two distinct clusters of populations largely corresponding to monoecious and dioecious populations. Reproductive isolation between monoecious and dioecious populations appears to be governed, in part, by observed differences in habitat and life history. We suggest that selfing and inbreeding depression in monoecious populations are important in the transition from monoecy to dioecy and that the maintenance of distinct sexual systems in S. latifolia is governed by interactions between ecology, life history, and mating.

Colonisation as a common denominator in plant metapopulations and range expansions: effects on genetic diversity and sexual systems

Colonisation plays a central role in both the initial occupancy of a region through range expansions as well as in metapopulations, where local extinctions are balanced by re-colonisations. In this paper, we review the effects that colonisation is expected to have on patterns of genetic variation within a species, and we draw attention to the possibility of interpreting these patterns as signatures of colonisation in the past. We briefly review theoretical predictions for the effect of colonisation on both neutral genetic diversity and on variation at genetic loci that regulate the sexual system of plant populations. The sexual system represents a particularly important trait in this context because it is affected by both selection during colonisation, and because it influences gene flow amongst populations. Finally, we introduce four case studies of plant species that show variation in their sexual systems that is consistent with theoretical predictions.

LOSS OF SEX IN CLONAL POPULATIONS OF A FLOWERING PLANT, DECODON VERTICILLATUS (LYTHRACEAE)

The loss of traits that no longer increase fitness is a pervasive feature of evolution, although detailed studies of the genetic, developmental, and evolutionary factors involved are few. Most perennial plants practice both sexual and clonal reproduction, and it has been hypothesized that populations with little sexual recruitment may lose the capacity for sexual reproduction by fixing mutations that disable one or more of the many processes involved in sex. The clonal, tristylous aquatic plant, Decodon verticillatus, exhibits marked geographical variation in sexual recruitment. Populations at the northern limit of the range are usually monomorphic for style length consist of single genotypes, and produce almost no seed, due, in part, to environmental conditions that inhibit pollination, fertilization, and seed maturation. Controlled crosses in a greenhouse provided evidence for greatly reduced sexual capacity in an exclusively clonal, monomorphic population. Plants from this infertile population produced only 3–18% as many seeds per pollination as fertile populations. Observations of pollen tube growth indicated that infertility is due to severe reductions in pollen tube numbers both early after pollination and later when pollen tubes were traversing the ovary, due primarily to the inability of pistils to support normal tube growth. A three‐year greenhouse experiment comparing fertility, survival, and growth of F1 progenies produced from reciprocal crosses between plants from the infertile population and those from nearby fertile populations suggested that the genetic basis for infertility is simple and may involve a single recessive mutation. In addition, the results did not reveal any association between infertility and other aspects of survival and vegetative vigor. The infertile genotype was likely fixed in the population through founder effect rather than indirect selection resulting from antagonistic pleiotropy or direct selection of advantages associated with reduced investment in sexual reproduction. A broader comparison of sexual fertility in 15 clonal, monomorphic populations and five genotypically diverse, trimorphic populations under greenhouse conditions revealed substantial infertility in all but one monomorphic population. Populations varied somewhat in the stage at which infertility was expressed, however, pollen tube growth was impaired in all populations. These results provide strong support for the hypothesis that complex traits like sex are degraded by mutation when they no longer increase fitness.

Sex determination and the evolution of dioecy from monoecy in Sagittaria latifolia (Alismataceae)

The role of mutations of small versus large effect in adaptive evolution is of considerable interest to evolutionary biologists. The major evolutionary pathways for the origin of dioecy in plants (the gynodioecy and monoecy–paradioecy pathways) are often distinguished by the number of mutations involved and the magnitude of their effects. Here, we investigate the genetic and environmental determinants of sex in Sagittaria latifolia, a species with both monoecious and dioecious populations, and evaluate evidence for the evolution of dioecy via gynodioecy or monoecy–paradioecy. We crossed plants of the two sexual systems to generate F1, F2 and backcross progeny, and grew clones from dioecious populations in low– and high–fertilizer conditions to examine sex inconstancy in females and males. Several lines of evidence implicate two–locus control of the sex phenotypes. In dioecious populations sex is determined by Mendelian segregation of alleles, with males heterozygous at both the male– and female–sterility loci. In monoecious populations, plants are homozygous for alleles dominant to male sterility in females and recessive to female sterility in males. Experimental manipulation of resources revealed sex inconstancy in males but not females. These results are consistent with predictions for the evolution of dioecy via gynodioecy, rather than the expected monoecy–paradioecy pathway, given the ancestral monoecious condition.

GENDER VARIATION AND TRANSITIONS BETWEEN SEXUAL SYSTEMS IN MERCURIALIS ANNUA (EUPHORBIACEAE)

Evolutionary transitions between hermaphroditism and dioecy have occurred numerous times in the land plants. We briefly review the factors thought to be responsible for these transitions, and we provide a synthesis of what has been learned from recent studies of the annual herb Mercurialis annua, in which dioecy (males and females), monoecy (functional hermaphrodites), and androdioecy (males and hermaphrodites) occur in different parts of its geographic range. Previous research on M. annua has revealed the importance of genome duplication and hybridization in the origin of much of the observed variation. Here we show, however, that spatial transitions in the sexual system also occur within the same ploidy level. In particular, we present an analysis, using flow cytometry data, of ploidy variation across a previously unstudied transition between hermaphroditism and androdioecy, in which we find that the sexual‐system transition is uncoupled from the shift in ploidy levels. We review recent research that shows that such transitions between sexual systems in M. annua are consistent with differential selection at the regional level for reproductive assurance during colonization. We also present new experimental data that highlight both the importance of the resource status of plants and that of their local mating context in regulating gender strategies and sex ratios. The studies reviewed and the new results presented emphasize the role that shifts in the ecological and genetic context of plant populations may play in causing transitions between sexual systems.

Trade-offs between clonal and sexual reproduction in Sagittaria latifolia (Alismataceae) scale up to affect the fitness of entire clones

Many plants combine sexual reproduction with vegetative propagation, but how trade-offs between these reproductive modes affect fitness is poorly understood. Although such trade-offs have been demonstrated at the level of individual shoots (ramets), there is little evidence that they scale up to affect genet fitness. For hermaphrodites, reproductive investment is further divided between female and male sexual functions. Female function should generally incur greater carbon costs than male function, which might involve greater nitrogen (N) costs. Using a common garden experiment with diclinous, clonal Sagittaria latifolia we manipulated investment in reproduction through female and male sex functions of 412 plants from monoecious and dioecious populations. We detected a 1:1 trade-off between biomass investment in female function and clonal reproduction. For male function, there was no apparent trade-off between clonal and sexual reproduction in terms of biomass investment. Instead, male function incurred a substantially higher N cost. Our results indicate that: trade-offs between investment in clonal propagation and sexual reproduction occur at the genet level in S. latifolia; and sexual reproduction interferes with clonal expansion, with investment in female function limiting the quantity of clonal propagules produced, and investment in male function limiting the nutrient content of clonal propagules.

LIFE-HISTORY DIFFERENTIATION AND THE MAINTENANCE OF MONOECY AND DIOECY IN SAGITTARIA LATIFOLIA (ALISMATACEAE)

Abstract The existence of monoecious and dioecious populations within plant species is rare. This limits opportunities to investigate the ecological mechanisms responsible for the evolution and maintenance of these contrasting sexual systems. In Sagittaria latifolia, an aquatic flowering plant, monoecious and dioecious populations exist in close geographic proximity but occupy distinct wetland habitats differing in the relative importance of disturbance and competition, respectively. Life‐history theory predicts contrasting evolutionary responses to these environmental conditions. We propose that the maintenance of monoecy and dioecy in S. latifolia is governed by ecological selection of divergent life‐history strategies in contrasting habitats. Here we evaluate this hypothesis by comparing components of growth and reproduction between monoecious and dioecious populations under four conditions: natural populations, a uniform glasshouse environment, a common garden in which monoecious and dioecious populations and their F1 progeny were compared, and a transplant experiment using shaded and unshaded plots in a freshwater marsh. Plants from dioecious populations were larger in size and produced heavier corms in comparison with monoecious populations. Monoecious populations flowered earlier and produced more flowers, clonal ramets, and corms than dioecious populations. The life‐history differences between the sexual systems were shown to have a quantitative genetic basis, with F1 progeny generally exhibiting intermediate trait values. Survival was highest for each sexual system in field plots that most closely resembled the habitats in which monoecious (unshaded) and dioecious (shaded) populations grow. These results demonstrate that monoecious and dioecious populations exhibit contrasting patterns of investment in traits involved with growth and reproduction. Selection for divergent life histories between monoecious and dioecious populations of S. latifolia appears to be the principal mechanism maintaining the integrity of the two sexual systems in areas of geographic overlap.

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.

Phenotypic Plasticity of Hermaphrodite Sex Allocation Promotes the Evolution of Separate Sexes: An Experimental Test of the Sex-Differential Plasticity Hypothesis Using Sagittaria latifolia (Alismataceae)

Abstract Separate sexes can evolve under nuclear inheritance when unisexuals have more than twice the reproductive fitness of hermaphrodites through one sex function (e.g., when females have more than twice the seed fertility of hermaphrodites). Because separate sexes are thought to evolve most commonly via a gynodioecious intermediate (i.e., populations in which females and hermaphrodites cooccur), the conditions under which females can become established in populations of hermaphrodites are of considerable interest. It has been proposed that resource-poor conditions could promote the establishment of females if hermaphrodites are plastic in their sex allocation and allocate fewer resources to seed production under these conditions. If this occurs, the seed fertility of females could exceed the doubling required for the evolution of unisexuality under low-, but not high-resource conditions (the sex-differential plasticity hypothesis). We tested this hypothesis using replicate experimental arrays of the aquatic herb Sagittaria latifolia grown under two fertilizer treatments. The results supported the sex-differential plasticity hypothesis, with females having more than twice the seed fertility of hermaphrodites under low-, but not high-fertilizer conditions. Our findings are consistent with the idea that separate sexes are more likely to evolve under unfavorable conditions.

Density-dependent regulation of the sex ratio in an annual plant.

Sex ratios are subject to strong frequency‐dependent selection regulated by the mating system and the relative male versus female investment. In androdioecious plant populations, where males co‐occur with hermaphrodites, the sex ratio depends on the rate of self‐fertilization by hermaphrodites and on the relative pollen production of males versus hermaphrodites. Here, we report evolutionary changes in the sex ratio from experimental mating arrays of the androdioecious plant Mercurialis annua. We found that the progeny sex ratio depended strongly on density, with fewer males in the progeny of plants grown under low density. This occurred in part because of a plastic adjustment in pollen production by hermaphrodites, which produced more pollen when grown at low density than at high density. Our results provide support for the prediction that environmental conditions govern sex ratios through their effects on the relative fertility of unisexual versus hermaphrodite individuals.