Investigating the role of transcription factor , Trl , during germline development in the Drosophila ovary

Oogenesis is the process by which an egg develops from undifferentiated cells in the ovary. The Drosophila melanogaster ovary is an assembly line for egg production , making it a great system to investigate initial oocyte specification and oocyte growth and maturity. Trl , GAGA factor , encodes a transcription factor that blocks repressive histones by modifying chromatin accessibility. Trl is expressed throughout the fly and during all stages of development. Trl is necessary for eye and wing development , embryogenesis , and male germ cell development in spermatogenesis. Other studies have begun to characterize Trl in oogenesis; however , the specific role Trl plays in the germline has yet to be determined. In global mutants , Trl functions in oogenesis for follicle survival , proper germ cell number , nurse cell size , oocyte fate , and oocyte localization. Since Trl is necessary for early events in fly development , homozygous null mutants are lethal , I utilized the Flippase/Flippase Recognition Target (Flp/FRT)-mediated clonal analysis and tissue-specific RNAi to investigate how Trl germline-specific mutants regulate oocyte fate in the adult fly ovary. In this study , I found that Trl is not necessary for germline stem cell maintenance. Loss of Trl in stages 2-4 and stages 7-9 results in condensed , pyknotic , fragmented nuclei in cysts , an indicator of premature cyst death. Trl13C(40) mutant cysts have a delay in Orb expression , whereas Trls2325 clones lack Orb altogether , indicating that Trl is necessary for Orb in the oocyte. However , Trl is not necessary for entry into meiosis or oocyte localization. Further , knock-down of Trl in stage 2 cysts using a germline-specific RNAi driven by OtuGal4 did not block Orb expression , suggesting that Trl is not required for oocyte maintenance. Taken together , I conclude that Trl is necessary for Orb accumulation in the oocyte before 16-cell cyst formation. Understanding the role Trl plays in the oocyte can help us understand how chromatin modification affects the critical set of conserved , developmental steps a mature oocyte must undergo. In turn , it could also help us understand these developmental steps in other species