Author ORCID Identifier

Date Available


Year of Publication


Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation


Arts and Sciences



First Advisor

Dr. D. Nicholas McLetchie


In dioecious plants, selection due to sex function differences has produced sex-specific life histories, morphologies, and physiologies. In many dioecious seed plants, dimorphisms and population sex ratios have been plausibly linked, but similar links are not yet apparent in dioecious bryophytes. Population sex ratio bias is often expected to favor the sex with lower investment in sexual reproduction, especially in resource-poor environments. Unlike in seed plants, bryophyte males may have higher average reproductive investment than females, which typically have low offspring production rates due to sperm limitation. However, traits aside from reproductive investment such as shoot and leaf arrangement may be differentially selected and could influence life history and sex ratio, but these are rarely tested. My questions concentrated on the dimorphic traits responsible for sex ratio bias and their links to sex function. My studies, using the moss Bryum argenteum, included field and greenhouse experiments investigating sex ratio bias and morphological plasticity along a light/canopy openness (exposure) gradient, a greenhouse comparison of clump morphology and water-holding capacity, and a field and growth chamber study on sex-specific responses to stress (high temperature and desiccation). The sex ratio of urban Lexington, KY was highly female-biased, did not correlate with exposure, and was not linked with pre-zygotic reproductive investment. Leaf characteristics of B. argenteum plastically responded to exposure but were not sex-specific. However, juvenile females produced shoots at a faster rate and grew taller in high light. Juvenile male shoots held more external water than female shoots, but this did not predict mature clump water-holding capacity. Male clumps were shorter, denser, and held less water than females likely to shed sperm-laden water for sexual reproduction. Clump height did not trade off with reproductive investment, adding evidence that sex-specific size is linked with other aspects of sex function. Although chlorophyll fluorescence data (a measure of the status of photosystem II) from both field and growth chamber experiments indicated subtle sex-specific stress recovery responses among sexually immature and mature plants, differences were weaker than predicted and sexually mature shoots did not fare worse than vegetative shoots. The sex differences in size, clump morphology, and clump water-holding capacity very likely affect survival, growth, competitive ability, and ultimately adult sex ratio bias.

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