The
steroid hormone ecdysone controls genetic regulatory hierarchies underlying insect molting, metamorphosis and, in some insects, reproduction. Cytogenetic and molecular analysis of
ecdysone response in Drosophila larval salivary glands has revealed regulatory hierarchies including early genes which encode
transcription factors controlling late
ecdysone response. In order to determine whether similar hierarchies control reproductive
ecdysone response, we have investigated
ecdysone-regulated gene expression in vitellogenic mosquito ovaries and fat bodies. Here, we identify the homologue of the Drosophila E75 early
ecdysone inducible gene in the
yellow fever mosquito Aedes aegypti, and show that, as in Drosophila, the mosquito homologue, AaE75, consists of three overlapping transcription units with three
mRNA isoforms, AaE75A, AaE75B, and AaE75C, originating as a result of alternative splicing. All three AaE75
isoforms are induced at the onset of vitellogenesis by a blood meal-activated hormonal cascade, and highly expressed in the mosquito ovary and fat body, suggesting their involvement in the regulation of oogenesis and vitellogenesis, respectively. Furthermore, in vitro fat body culture experiments demonstrate that AaE75
isoforms are induced by
20-hydroxyecdysone, an active
ecdysteroid in the mosquito. These findings suggest that related
ecdysone-triggered regulatory hierarchies may be used reiteratively during developmental and reproductive
ecdysone responses.