Electronic Thesis and Dissertation Repository


Doctor of Philosophy




Dr Amanda Moehring


Over time, genetic differences can accumulate between populations that are geographically separated. This genetic divergence can lead to the evolution of reproductive isolating mechanisms that reduce gene flow between the populations and, upon secondary contact, result in distinct species. The process of speciation is, thus, what accounts for the multitude of species that contribute to the rich biodiversity on Earth. Interspecies hybrid sterility is a postzygotic isolating mechanism that affects the development of hybrids, rendering them sterile. A notable trend, known as Haldane's Rule, describes that heterogametic individual (e.g. males in Drosophila) are more susceptible to sterility than homogametic hybrids. The genetic and cellular basis of this phenomenon is unknown. My objective was to describe the stage at which spermatogenesis fails in hybrids produced from three interspecies crosses in Drosophila. I found that chromosomes do not separate after meiosis I, leading to non-disjunction, and the formation of needle-eye sperm that are not dead. Secondly, sperm head length is aberrant in aged (6 days) sterile hybrid males, suggesting improper nuclear packaging, even with bi-allelic expression of sperm protamines. Third, individual sperm nuclei possess two sperm tails, with two undifferentiated, but active, mitochondria. Finally, I mapped for genetic factors that contribute to the formation of needle-eye sperm and identified possible genes. Together, these studies highlight that spermatogenesis fails at the same stage in sterile hybrid males in Drosophila, leading to the formation of paired sperm that are unable to fertilize. My findings suggest a potential universal genetic basis of hybrid sterility within the genus Drosophila.