Olivia Pace

Many Lepidoptera (butterflies and moth) have a sperm dimorphism in which males produce two distinct types of sperm; nucleated (eupyrene) which can fertilize a female’s eggs and annucleated (apyrene) sperm which cannot fertilize a female’s eggs. Most of the sperm transferred to the female during copulation is annucleated, calling into question their significance. My objective was to better understand sperm dimorphism by testing the hypothesis that apyrene and eupyrene sperm function in competition between sperm of different male monarchs. Twenty-one monarchs were reared in late July/early August under varying conditions and treatments to induce migratory behavior and reproductive diapause. Sperm from each monarch was isolated for comparison of the densities of eupyrene and apyrene sperm across three treatment groups. I predicted that monarchs induced to reproduce after migration would have more eupyrene sperm, and therefore less apyrene sperm because of sperm competition, compared to the other experiment groups. Monarchs naturally experience reproductive diapause until they migrate north in February. By breaking reproductive diapause in the summer, one of the treatment groups compared to the other two was expected to invest more eupyrene and less apyrene sperm into their ejaculates in response to an increase in mating behavior. The results for the densities of eupyrene sperm did trend in the direction of my prediction, but not significantly. In addition, an increase in eupyrene sperm density did not cause a decrease in apyrene sperm, suggesting that there is no tradeoff between eupyrene and apyrene sperm. Results of this research contribute to the understanding of the function of apyrene sperm in sperm polymorphism and competition as well as the impact sperm competition has on diversification of males and consequently females of sexual species.