The genetic components of sex expression and their fitness effects in a gynodioecious species

Gynodioecy is a breeding system common in plants in which populations consist of female and hermaphrodite individuals. In most gynodioecious species, it appears that both cytoplasmic male-sterility (CMS) and nuclear restorers-of-fertility genes interact to determine the sex of an individual. Theoretical research on gynodioecy has hypothesized that the nuclear restorers must have deleterious effects on fitness (costs of restoration) that prevent them from spreading to fixation within populations. Models developed to show the plausibility of this prediction have focused on a single mode of expression for this deleterious effect of nuclear restorer genes. By extending these models to simulate other modes of deleterious effects, my coauthors and I have shown that many such modes are also plausible. In particular, overdominance for pollen or seed production caused by nuclear restorer alleles can maintain females in gynodioecious populations under strictly nuclear or nuclear-cytoplasmic gynodioecy. I also conducted a field test to find a cost of restoration using a native Indiana gynodioecious species, Lobelia siphilitica. I estimated the number of nuclear restorers carried by a set of individuals and measured the fitness of the same individuals in a natural setting. I have found a relationship between a certain class of nuclear restorers and pollen viability. This is the first empirical evidence that a cost of restoration occurs in nature. The final portion of my dissertation is a molecular study to identify restriction fragment length polymorphism (RFLP) markers for the cytoplasmic genes in L. siphilitica. Using these markers, we have demonstrated the high diversity of mitochondrial genotypes present in four populations. These findings are consistent with the view that negative frequency-dependent selection at nuclear and cytoplasmic sex-determining loci can explain the frequency of females in L. siphilitica populations. Recent work on gynodioecy has focused on stochastic forces such as founder effect, inter-population selection, and ecological factors to explain the existence of gynodioecious species. My research shows that there are two other plausible ways to maintain gynodioecy, overdominance for fitness and negative frequency-dependent selection at sex-determining loci, and that the assumptions and predictions of these theories match the patterns found in natural populations.