| I studied the causes of sex ratio variation in the gynodioecious plant Nemophila menziesii.; I first tested whether a region of high female frequencies may be due to hybridization between regionally-distributed populations with different corolla colors. N. menziesii has nuclear-cytoplasmic sex inheritance, thus if populations with different colors are fixed for different male-sterile cytoplasms and matching nuclear restorer alleles, hybridization between them should yield high female frequencies. Hybrid crosses yielded higher female frequencies than within-color crosses. In the field, populations with high female frequencies had intermediate means and higher variances in corolla color, suggesting hybridization. Two all-hermaphroditic field populations segregated females in hybrid crosses suggesting that field populations contain sex ratio distorters but appear undistorted, a prediction of genomic conflict theory.; Second, I investigated whether soil moisture affects the relative fitnesses of females and hermaphrodites, and whether fitness differences translate into sex ratio differences. I found a relatively greater increase in hermaphrodite versus female seed production in a manipulative watering experiment, suggesting a female fitness advantage in low soil moisture conditions. Over a natural soil moisture gradient, the ratio of female to hermaphrodite seeds was higher in dry than in wet sites. There was no significant effect of soil moisture on the sex ratio of 23 populations. These results demonstrate a sex-dependent effect of soil moisture on resource allocation to seeds which does not translate into a simple effect on the sex ratio.; Third, I tested whether hermaphrodite sex allocation is correlated with female frequency. In species with nuclear-cytoplasmic sex inheritance, female frequencies that cycle through time are expected, and increased male allocation in hermaphrodites may lag behind actual population female frequencies, with no observable correlation. In a multi-population study, I found no significant effect of female frequency on hermaphrodite sex allocation, confirming expectations. In a more in-depth two-population study, a frequency-dependent effect of females on hermaphrodite sex allocation was found but this appears to be due to anomalously high hermaphrodite seed production at one population.; In conclusion, N. menziesii sex ratio dynamics are affected by both genetic and environmental factors and are consistent with ideas from genomic conflict theory. |
