Title from title page of PDF (University of Missouri--St. Louis, viewed February 10, 2010). Includes bibliographical references.

Unequal sex ratios are widespread in dioecious plants and understanding their cause is important to understanding fundamental aspects of their population dynamics, and yet what causes biased sex ratios in plants is still poorly understood. Competition experiments have been used in plants to predict the outcome of species interactions, but they have rarely been used to help explain sex ratio bias. This study used a response surface competition design to measure the relative competitive abilities of the sexes of the bryophyte Marchantia inflexa (a thallus liverwort of Marchantiaceae) to predict the outcome of competition before the onset of sexual reproductive structures. In bryophytes, dioecy and sex ratio bias is especially common, making them effective organisms for studying sex ratio bias. Given the frequency of female bias in bryophytes, the hypothesis was that females will show a higher competitive ability relative to males. The experiment was conducted in greenhouse conditions at several densities and proportions over the course of seven months. As individuals grew and formed clumps, identities were tracked, and growth measurements were made using photographs and computer imaging software. Both sexes grew on average 41% more with the opposite sex relative to their single-sex cultures. A model predicting future sex ratios showed coexistence between the sexes and predicted a male biased sex ratio of 3.2 males to 1 female. A trade-off was observed for males where single-sex cultures contained more asexual structures than mixed-sex cultures and the reverse for growth rate. Higher levels of asexual reproduction in males in single-sex cultures might be selected for to increase male dispersal for a higher probability of encountering females. This pattern was not found for females. The overyielding results suggest an interaction effect may exist due to niche differentiation between the sexes. In addition, the results suggest that in some dioecious plants a change in sex ratio can occur before differences in their allocation to sexual reproduction.

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.

Plants are threatened by global change, increasing variability in weather patterns, and associated abiotic stress. Consequently, there is an urgent need to enhance our ability to predict plant community dynamics, shifts in species distributions, and physiological responses to environmental challenges. By building a fundamental understanding of plant stress tolerance, it may be possibly to protect the ecological services, economic industries, and communities that depend on plants. Dehydration tolerance (DhT) is an important mechanism of water stress tolerance with promising translational applications. Here, I take advantage natural variation in DhT to gain a deeper insight into this complex trait. In addition, I address questions related to the causes and consequences of sexual dimorphisms in DhT. Understanding sexual dimorphisms in stress tolerance is critical because these dimorphisms can drive spatial segregation of the sexes, biased sex ratios, and may ultimately reduce sexual reproduction and population persistence. This work takes an integrated approach, addressing DhT on multiple scales from ecology, to physiology, to genomics in the tropical liverwort Marchantia inflexa. Initially, I tested for correlations between DhT and environmental dryness, sex differences in DhT, and genetic vs. plastic contributions to DhT variability. I found that patterns of variation in DhT are associated with environmental variability, including complex sexual dimorphisms, and derive from a combination of plasticity and genetic differences in DhT. Subsequently, I leveraged the variability in DhT to identify candidate DhT enhancing genes. In M. inflexa intraspecific differences in DhT are impacted by baseline variability among plants, as well as unique gene expression responses initiated during drying. In parallel, I assembled a draft genome assembly for M. inflexa, which was employed to investigate questions of sex chromosome evolution and sexual dimorphism in DhT. Finally, the bacteriome of M. inflexa was characterized and found to be extremely diverse and variable. Collectively, this work adds to a growing understanding of DhT and highlights the importance of sampling approaches that seek to comprehensively describe variability in DhT. I detected complex patterns of variability in DhT among populations and the sexes of M. inflexa, which were used to gain insight into the genetic intricacies of DhT.