Sex determination and gene expression in Daphnia magna exposed to juvenile hormone

Abstract

Daphnia spp. cycle between asexual and sexual reproduction depending on environmental conditions. Under favorable conditions, female offspring are produced, resulting in rapid population expansion; in less favorable conditions, males are produced, initiating the sexual portion of their lifestyle. Although mediated by environmental cues, sex determination is ultimately dependent on the expression of Doublesex genes, Dsx1 and Dsx2, in Daphnia. We exposed Daphnia magna to juvenile hormone III (JHIII), an insect hormone often mimicked by agricultural insecticides. Juvenile hormone (JH) and JH analogues disrupt the insect endocrine system, inhibiting reproduction; these chemicals pose an environmental threat to non-target organisms by leaching into surrounding waters. When Daphnia are exposed to JH or JH analogues, it causes a premature shift from asexual to sexual reproduction. We showed that Dsx1 is upregulated throughout vitellogenesis in female D. magna producing male offspring and Dsx2 is upregulated during late vitellogenesis, suggesting that both Dsx1 and Dsx2 contribute to male sex determination. In addition, Dsx2 expression increases during sexual maturation in male D. magna, suggesting that it also plays a role in spermatogenesis. In addition, exposure to JHIII decreased reproductive output, slowed growth and prematurely shifted D. magna reproductive strategy to the sexual portion of their lifecycle.