Date of Award

Fall 10-1-2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Genetics

First Advisor

Lin, Haifan

Abstract

The long-term survival of a species depends on the integrity of its genetic material and the inheritance of that genetic material across generations. In sexually-reproducing organisms, these processes are carried out by the germline: the tissue that produces eggs in females or sperm in males. PIWI proteins and their associated small noncoding piRNAs have been well-characterized for their roles in protecting genome integrity in the germline and promoting the self-renewal of germline stem cells, which support the continual production of eggs or sperm. In Drosophila, maternally-expressed PIWI proteins and piRNAs are also enriched in the germ plasm, the posterior region of the oocyte which is densely packed with proteins and mRNAs that direct the initial steps of germline development during embryogenesis. However, the role of Piwi – the founding member of the PIWI protein family – in embryonic germline development is not well understood, in part because of the severe and varied defects of piwi-null mutants. In this dissertation, I explore the role of maternal Piwi and piRNAs in the germline development of progeny beyond early embryogenesis. Piwi localizes to the germ plasm and is inherited by the embryonic germline, and I show that the maternal Piwi protein persists in primordial germ cells through the end of embryogenesis and to some larval stages. This expression pattern opens the possibility that maternal Piwi can regulate germline development beyond its previously-described roles in early embryogenesis. Strikingly, when the levels of maternal Piwi deposited into the early embryo were reduced, the fertility and normal gonad morphology of female, but not male, progeny was impaired. This phenotype does not appear to be related to Piwi’s well-studied role in transposon suppression; a few transposons were mildly derepressed in the early embryo, but transposons were fully repressed in the ovaries of adult progeny following maternal Piwi depletion. Instead, the female-specific fertility defect appears to be caused by a loss of maternally-deposited piRNAs and subsequent masculinization of the female germline in the absence of maternal Piwi. I propose that maternal Piwi ensures the fertility of female progeny by repressing the expression of male germline genes in the female germline. Together, these results reveal a previously unexplored role for the maternal PIWI/piRNA pathway in the germline development of female progeny and suggests that the PIWI/piRNA pathway may be involved in germline sex determination. This long-ranging maternal effect is striking in comparison to the vast majority of maternal-effect genes whose functions have mainly been studied in the early embryo. The genetic approaches I leveraged to investigate the longevity of maternal Piwi and its roles in progeny development will not only further our understanding of the PIWI/piRNA pathway, but can also be extended to reveal new developmental roles for other maternal factors far beyond early embryogenesis.

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