Embryonic expression of juvenile hormone binding protein and its relationship to the toxic effects of juvenile hormone in Manduca sexta

The juvenile hormones (JHs) regulate a diverse array of insect developmental and reproductive processes. One molecular target of JH action is its transporter, hemolymph JH binding protein (hJHBP); in the larva of the tobacco hornworm, Manduca sexta, low doses of JH can immediately increase hJHBP gene expression. Less explored are the effects of JH on embryological development, where early hormonal treatment has been shown to affect embryonic development and pupation. This study examines the egg form of JHBP and its gene expression during embryogenesis of M. sexta, as well as the phenotypic effect JH treatment has on embryos and on JHBP gene expression. We here demonstrate that the preponderance of JHBP found in the egg is maternally derived and that the embryonic gene and protein appear identical to those found in the larva. Expression of the JHBP gene begins in both the embryo itself and extra-embryonic tissues 15 h after fertilization, long before emergence of a functional fat body and circulatory system. Topical application of low JH doses to early embryos resulted in larval abnormalities while high doses of the hormone induced embryonic mortality. These effects are not mediated through regulation of the JHBP gene, since embryonic expression appears invariant in response to JH challenge. The toxicity of JH is tightly correlated with the concentration of unbound hormone.