The Hippo signaling pathway plays a role in homeostatic growth of soma and germ line in the D. melanogaster larval ovary. Didem P. Sarikaya, Cassandra G. Extavour. Organismic and Evolutionary Biology, Harvard University, Cambridge, MA.

   The Hippo (Hpo) pathway is conserved across animals, and regulates proliferation by altering the activity of the transcriptional coactivator Yorkie. The role of Hpo signaling during development of homogeneous cell types in various organs has been intensively investigated. However, it is not known whether Hippo may differently influence growth of distinct cell types within a single organ. The D. melanogaster larval ovary provides a useful model to study coordinated growth of different cell types during development of a single organ. The germ cells (GCs) are known to coordinate their proliferation homeostatically with the surrounding somatic intermingled cells (ICs). Some of the somatic cells anterior to the GCs differentiate into terminal filament cells (TFCs) and sort into stacks of cells (terminal filaments) that begin the process of dividing the ovary into functional units called ovarioles. The survival and proliferation of GCs, ICs, and TFCs are critical for ovariole morphogenesis and establishment of the functional adult ovary. Here we show that the Hpo pathway influences proliferation of both ICs and TFCs, and in contrast to previous reports, also plays a role in GC proliferation. Interestingly, the pathway appears to function differently in germ line and soma. While the Hpo pathway operates canonically in somatic ovarian cells, our data suggest that in the germ line yki may regulate proliferation of GCs in a hpo-independent manner. Previous studies had shown that reducing EGFR signaling from GCs to ICs reduced IC number and led to GC overproliferation, suggesting that ICs suppress GC. Surprisingly, we found that increasing IC number by abrogating Hpo pathway activity led to increased GC number. Conversely, reducing IC number via yki knockdown reduced GC number. This result contrasts with previous observations that IC reduction leads to germ cell overproliferation. Taken together, our results suggest that the Hippo pathway operates differently in distinct cell types of the ovary, and may play a role in regulating the homeostatic growth of germ line and soma.