Factors affecting the magnitude of inbreeding depression in male and female function in Cucurbita pepo ssp. texana (Cucurbitaceae) /

by Hayes, C. Nelson.

I conducted a series of studies to investigate the nature of inbreeding depression in Cucurbita pepo ssp. texana L. (Cucurbitaceae) and identify factors that exacerbate or ameliorate the severity of inbreeding depression. Overall, I found that the magnitude of inbreeding depression varies with sexual function, with the coefficient of inbreeding, with the coefficient of inbreeding of the maternal parent, and with the level of environmental stress in which individuals are grown. In the first study (Chapter 2), I compared the magnitude of inbreeding depression within male and female functions at an intermediate level of inbreeding. I found that male and female functions respond independently to inbreeding, and that there was a slight tendency for inbreeding depression to be more severe for female function than for male function, implying that estimates of inbreeding depression based solely on female fitness traits might be misleading. In the second study (Chapter 3), I examined the relationship between coefficient of inbreeding (f) and fitness across a range of f for both male and female fitness. Fitness tended to decline linearly with f for most traits, suggesting a simple additive among-locus genetic model for inbreeding depression in C. pepo ssp. texana and a gradual decline in fitness as homozygosity increases with the level of inbreeding, suggesting the action of many alleles of small effect and modest purging of deleterious recessives. Inbreeding depression in maternal parents was found to affect fitness for vegetative and reproductive traits in progeny, suggesting that maternal effects may compound the effects of inbreeding depression in multigenerational studies. iv In the third study (Chapter 4), I grew inbred and outcrossed plants under high and low nitrogen conditions to examine the effect of environmental stress on the magnitude of inbreeding depression. I found that differences among inbred and outcrossed individuals were more pronounced under the more stressful low nitrogen conditions. I also found evidence of fluctuating asymmetry for leaf shape among inbred plants, suggesting that inbreeding reduces developmental stability in C. pepo ssp. texana. In the final study (Chapter 5), I examined the relationship between inbreeding and herbivory by diabroticite beetles. Beetles preferentially attacked inbred plants and damaged a larger proportion of the leaves on inbred plants, reducing the resources available for growth and reproduction. When inbred plants were sprayed with pesticides, they had higher fitness and less severe inbreeding depression than untreated control plants, although the increase in female fitness was more pronounced than in male fitness, mainly as a result of increased fruit production. I conclude that the magnitude of inbreeding depression is not a fixed property of an individual having a particular coefficient of inbreeding but rather a function of the severity of the environment in which the individual grows and matures. The coefficient of inbreeding is in some sense a better predictor of environmental sensitivity than a predictor of fitness, and this sensitivity may vary depending on whether male or female function is being examined. As a consequence, models of mating system evolution and conservation management programs involving hermaphrodites should consider inbreeding depression as a dynamic property of the interaction between an individual and its maternal and external environments and pay separate attention to male and female sexual roles. v