Juvenile hormone and insulin signaling pathways interact to mediate lipid metabolism during tsetse pregnancy

Tsetse flies, the sole vector of African trypanosomiasis, employ a unique, viviparous reproductive strategy wherein tsetse mothers nourish intrauterine larvae through a process analogous to mammalian lactation. The protein-and lipid-rich milk is synthesized by a modified accessory gland called the milk gland (MG). The hormonal control of tsetse pregnancy remains largely uncharacterized. Juvenile hormone (JH) signaling is critical for insect reproduction, but the mechanism of JH action remains unclear. Application of exogenous JH disrupts tsetse reproduction, possibly by perturbing MG physiology. Recent studies in a variety of insects indicate that JH interacts with Insulin/IGF signaling (IIS) to drive some aspects of insect reproduction. In this study, we examined JH/IIS interaction during tsetse pregnancy, specifically in coordinating lipid metabolism, which is a fundamental aspect of milk production. Genetic suppression of the JH receptor Met decreased mothers’ stored lipid content, lowered fecundity, and extended the length of the second gonotrophic cycle. Knockdown of Met, but not its paralog gce, modified expression of HDAC4, a modulator of FOXO activity, as well as Bmm, a gene whose activity was previously implicated in tsetse milk production. Insulin injection phenocopied JH application with respect to changes in lipid content, fecundity, an extension of the gonotrophic cycle, and the expression of several genes including HDAC4, Bmm, and the JH response gene Kr-h1. Furthermore, FOXO suppression resulted in concomitant increase of JHAMT, a key JH biosynthetic gene. From our results, we provide a preliminary model for JH/IIS interaction during tsetse pregnancy.