E75 expression in mosquito ovary and fat body suggests reiterative use of ecdysone-regulated hierarchies in development and reproduction

The steroid hormone ecdysone controls genetic regulatory hierarchies underl ying insect molting, metamorphosis and, in some insects, reproduction. Cyto genetic and molecular analysis of ecdysone response in Drosophila larval sa livary glands has revealed regulatory hierarchies including early genes whi ch encode transcription factors controlling late ecdysone response. In orde r to determine whether similar hierarchies control reproductive ecdysone re sponse, we have investigated ecdysone-regulated gene expression in vitellog enic mosquito ovaries and fat bodies. Here, we identify the homologue of th e Drosophila E75 early ecdysone inducible gene in the yellow fever mosquito Aedes aegypti, and show that, as in Drosophila, the mosquito homologue, Aa E75, consists of three overlapping transcription units with three mRNA isof orms, AaE75A, AaE75B, and AaE75C, originating as a result of alternative sp licing. All three AaE75 isoforms are induced at the onset of vitellogenesis by a blood meal-activated hormonal cascade, and highly expressed in the mo squito ovary and fat body, suggesting their involvement in the regulation o f oogenesis and vitellogenesis, respectively. Furthermore, in vitro fat bod y culture experiments demonstrate that AaE75 isoforms are induced by 20-hyd roxyecdysone, an active ecdysteroid in the mosquito. These findings suggest that related ecdysone-triggered regulatory hierarchies may be used reitera tively during developmental and reproductive ecdysone responses. (C) 1999 P ublished by Elsevier Science Ireland Ltd. All rights reserved.