Master of Science
Moehring, Amanda J.
Speciation is the underlying process that leads to formation of new species, and therefore is the basis of biodiversity. Genes involved in each stage of speciation, such as those involved in interspecies sterility, remain elusive. Male hybrid sterility and postzygotic isolation between Drosophila pseudoobscura and D. persimilis was examined in this study through backcrossing of female hybrids into each parental line (introgression), selecting for a sterile sperm phenotype, needle-eye sperm. Sperm phenotypes did not separate through backcrossing; instead, males presented with multiple sperm phenotypes. A relationship between the phenotypes observed and the potential genes involved was examined through whole genome sequencing and SNP analysis of the DNA of 20 introgressed male hybrid samples. One finding was SNPs for hybrid sperm sterility were species specific. Also, sperm sterility and heteromorphism appear to be controlled by many loci. Further analysis of SNPs isolated in this study has the strong potential to identify candidates for loci involved in formation of needle-eye sperm, and postzygotic male hybrid sterility in other species.
Summary for Lay Audience
Speciation is the process of two populations of organisms of the same species evolving over time until they are unable to reproduce with each other. Some species have not completely separated, and are still able to create viable, but oftentimes sterile, hybrid offspring. A common example of hybrid sterility comes from horses and donkeys, who separated approximately 7.7-15 million years ago (Huang et al. 2015). When a male donkey and a female horse reproduce, they sire a mule. All male mules are sterile and most female mules are sterile. In rare cases female mules are fertile when mated to a horse or donkey (Savory 1970).
Similar to horses and donkeys, the crossing of two species of fruit flies, Drosophila pseudoobscura and D. persimilis, produce all sterile male hybrids. However, in the case of these fruit flies, all female hybrids are fertile. These two species of fruit flies also diverged more recently, 0.55 million years ago. These sterile hybrid male fruit flies can still produce sperm, but these sperm are not able to fertilize female eggs to make more hybrids. Fruit flies are used because they are less expensive to maintain, have shorter life cycles, and can be in a tightly controlled environment. My research focused on genetic differences cause the male fruit flies to be sterile. Hybrids receive genetic material (DNA) from both parent species. The DNA of both fly species studied here is split into two pairs of five separate chromosomes, X/Y, 2, 3, 4, and dot. The pairs of each chromosome can interact with each other through proteins. Instead of ten separate assembly lines for proteins, pairs of chromosomes are connected to each other by networks integral to protein production and cell function. In hybrids, the chromosomes are unlikely to all function properly because each species has differentiated chromosomes that might not be able to form proper pairs. The failure of some of these networks could be the basis of sterility. My study supported the species-specific differences in the pieces of the network contributing to hybrid sterility. This work can be continued to identify specific points in the DNA that lead to hybrid sterility and applied to other species.
Mattice, Alannah J.P., "Genetic basis of hybrid sterility between Drosophila pseudoobscura and D. persimilis" (2020). Electronic Thesis and Dissertation Repository. 7565.