|Abstract:||Sexually reproducing organisms must ensure correct specification and maintenance of germ cells for species survival. Germ cells must be carefully protected from inappropriate differentiation while simultaneously maintaining their identity as highly specialized totipotent cells. Several germ cell-intrinsic mechanisms, including transcriptional and post-transcriptional regulation, are key to executing germ cell-specific processes. Study of molecules necessary for the proper specification, maintenance, and differentiation of germ cells can lead to a deeper understanding of mechanisms underlying totipotency, infertility, tumorigenesis, evolutionary development, and even pathogenesis. Here we study germ cells in the context of both free-living and parasitic flatworms. We use the freshwater planarian Schmidtea mediterranea to understand the functions of germ cell-specific genes crucial for the development and proliferation of early male germ cells. Additionally, we also characterize genes essential for the tremendous reproductive output of the parasite Schisosoma mansoni, a causative agent of the a major neglected tropical disease, schistosomiasis.
It was recently shown that in S. mediterranea, a male germ cell-specific component of the ubiquitous Nuclear Factor-Y family of transcription factors, NF-YB, is necessary for testis maintenance. In Chapter 2, we characterize the cellular mechanism behind NF-YB-mediated maintenance of the male germline stem cells, known as spermatogonial stem cells, or SSCs. We demonstrate that NF-YB is not necessary for the specification of germ cells, but plays a role in the self-renewal and proliferation of SSCs. Furthermore, we show that other components of the NF-Y complex in S. mansoni are similarly necessary for the proliferation of male germ cells, indicating molecular and functional conservation of this family of transcription factors.
Next, we characterize the planarian homologs of boule, which belongs to a family of RNA-binding proteins known to play important roles in human infertility. In Chapter 3, we adopt an evolutionary-developmental approach and show that unlike other invertebrate boule genes, one of the planarian boule paralogs is required for SSC maintenance. Intriguingly, this early germ cell function, thought to be specific to vertebrate representatives of this gene family, appears to have evolved independently and earlier than previously thought. Our results demonstrate that the planarian can be a tractable invertebrate model system for understanding the germ cell functions of genes that are conserved between planarians and vertebrates.
Finally, in Chapter 4, we perform an unbiased transcriptomic screen to identify genes necessary for germ cell development in the parasite S. mansoni. The prodigious egg laying capacity of these parasites is the cause behindunderlies the morbidity of the disease schistosomiasis. Using a large-scale RNA interference-based functional approach, we identify genes functioning in different stages of schistosome male gametogenesis, as well as genes necessary for egg production in the mammalian host.
Together, our studies uncover novel roles for numerous genes critical for germ cell development in free-living and parasitic flatworms.