Specification of the embryonic precursors to the genital disc. A.E. Christiansen 1, E. Chen 1, 2, B.S. Baker 1. 1) Department of Biological Sciences, Stanford University, Stanford, CA; 2) current address: Department of Molecular Biology, Univeristy of Texas Southwestern Medical Center, Dallas, TX.
In adult Drosophila melanogaster the genitalia of either sex is derived from a single genital imaginal disc. The genital imaginal disc, unlike the leg and wing imaginal discs, is a compound disc founded by cells from three separate abdominal segments (A8, A9, and A10), which are the precursor cells for female genitalia, male genitalia, and analia, respectively. These cells are thought to be specified during blastoderm and subsequently aggregate to form a genital disc precursor cell (GDPC) cluster first detectible after germband retraction (GBR) using imaginal cell-specific markers such as escargot. We have examined the GDPCs in order to determine how many cells are present at the end of GBR and followed the development of these cells through the end of embryogenesis. Since imaginal precursors are thought to be quiescent during this time, we were surprised to see an increase in the number of GDPCs during the latter part of embryogenesis. During this period of development, we detect no cell proliferation in the GDPC cluster suggesting that cells are either recruited to the GDPC cluster, or that the apparent increase in cell number reflects late expression of escargot in some cells. Using both molecular and genetic tools, we have analyzed the roles of various patterning pathways in the specification of the GDPCs, paying particular attention to the contributions of the homeotic genes, AbdB-I, AbdB-II, and caudal, that determine the three abdominal segments, A8, A9, and A10, and their corresponding primoridia. Our results clarify earlier predictions of the number of cells that populate each primoridia. In addition, we elucidate the origin of the cells within each of these primordia in the GDPCs based on the expression and requirement of genes involved in the segment polarity and dorsal-ventral patterning pathways.