Year of Publication

2004

Document Type

Dissertation

College

Arts and Sciences

Department

Biology

First Advisor

Nicholas McLetchie

Second Advisor

R. Craig Sargent

Abstract

Sexual dimorphism in life history traits may influence the distribution of the sexes,population sex ratios, the maintenance of sex in populations, and the evolutionarypotential of a species. In bryophytes, sexual dimorphism in traits related to growth andreproduction may be responsible for female-biased population sex ratios and a lack ofsexual reproduction. I examined the roles of natural selection in maintaining sexualdimorphism in the context of impacts on bryophyte population sex ratios, usingMarchantia inflexa as a model system. My studies included an assessment of amongpopulationvariation in habitat use by the sexes, comparison of phenotypes betweensingle-sex and both-sex populations, a field study of natural selection, and a comparisonof the influence of selection on asexual and sexual fitness components.The sexes of M. inflexa were sexually dimorphic in investment in growth, asexualand sexual reproduction. The sexes were spatially separated in populations, but thesexes overlapped in habitat use. Populations differed in growth, asexual reproductionrates, degrees of sexual dimorphism, and strength of among-trait correlations. Plantsfrom single-sex and both-sex populations differed in investment in growth and asexualreproduction, but the two population types showed the same degree of sexualdimorphism. Thus, local environment may be more influential than the presence of theopposite sex in maintaining sexual dimorphism.Selection on sexually dimorphic traits was both sex-specific and environmentallydependent. Between-sex correlations were not significant in the greenhouse but weresignificant in the field thus, evolution and expression of sexual dimorphism in nature maybe constrained by among-trait and between-sex correlations. Additionally, femalesincurred a cost of plasticity that males did not. Because there was a negative trade-offbetween sexual and asexual fitness, overall lifetime selection may result in a differentpicture of how the sexes experience selection. The combination of sex-specific andenvironment-dependent selection, and sex-specific costs to plasticity may not onlymaintain sexually dimorphic traits but also ensure the persistence of both sexes in apopulation.

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