UC Davis

The precise regulation of gene expression is especially important during the earliest stages of development, when critical processes lay the foundation for the rest of development. At this time, embryogenesis is dependent on the transcriptional products of two genomes, from the mother and the zygote. Maternally deposited transcripts are supplied to the oocyte during oogenesis and carry out all initial developmental processes from the beginning of embryogenesis until the zygotic genome is activated. Over time, new transcripts are produced from the zygote as maternally deposited transcripts are degraded. The coordinated handoff of developmental control from the maternal to the zygotic genome is tightly regulated and highly conserved. In my dissertation, I investigate two aspects of gene regulation during early development and ask how they compare and evolve across species of Drosophila. In Chapter 1, I looked at how gene regulation evolves for differentially maternally deposited and zygotically transcribed genes by using hybrid crosses between three closely related species, D. simulans, D. sechellia, and D. mauritania. Surprisingly, the mechanisms of gene regulatory change differed substantially between the maternal and zygotic genome. There were more differences in maternal deposition resulting from changes in trans regulation while differences in zygotic transcription resulted from a combination of cis, trans, and the joint action of cis and trans changes. This is indicative that the maternal and zygotic genomes are under different sets of regulatory constraints and likely evolve via different mechanisms. Another critical aspect of gene regulation that I address in my dissertation is the trajectory of maternal transcript degradation throughout development, until all maternal transcripts are degraded, and how it compares across species of Drosophila. We chose four species of Drosophila, D. melanogaster, D. persimilis, D. virilis, and D. yakuba, which vary in developmental time and geographic origin and represent a range of divergence times. Looking at transcripts that are maternally transcribed and not later transcribed by the zygote (maternal-only), we found that a similar proportion degrades by the end of stage 5 in each of the species examined. This suggests that maternal transcripts are stable for a longer absolute amount of time in species that develop more slowly. We also find that relatively few maternal-only transcripts are common across all species examined while a larger proportion are unique to a specific species, indicating a lack of conservation of maternal-only genes, which is especially surprising given the high degree of conservation of maternal genes overall. Future work will investigate whether these transcripts, particularly those that degrade later in development than have previously been examined, contain motifs that may act as signatures for degradation and look at how those compare across species. Overall, the work in this dissertation gives a better understanding of transcriptional and post-transcriptional gene regulation during early embryogenesis and how it evolves across species of Drosophila.