Sex Differences In 20-Hydroxyecdysone Hormone Levels Control Sexual Dimorphism In Bicyclus anynana Butterfly Wing Patterns

Abstract

In contrast to the important role of hormones in the development of sexual dimorphic traits in vertebrates [1], the differentiation of these traits in insects is attributed exclusively to variation in cell-autonomous mechanisms controlled by members of the sex determination pathway [2], such as doublesex (dsx). Although hormones can shape the development of sexual traits in insects, and interact with dsx to create dimorphisms, variation in hormone levels are not known to cause dimorphism in these traits [3]. Here we show that butterflies use sex-specific differences in 20-hydroxyecdysone (20E) hormone titers to create sexually dimorphic wing ornaments, without the local involvement of dsx. Females of the dry season (DS) form of Bicyclus anynana display a larger sexual ornament on their wings than males, whereas in the wet season (WS) form both sexes have similarly sized ornaments [4]. High levels of circulating 20E during larval development in DS females and WS forms cause proliferation of the cells fated to give rise to this wing ornament, and results in sexual dimorphism in the DS forms. This study advances our understanding of how the environment regulates sex-specific patterns of plasticity of sexual ornaments and conclusively shows that sex-specific variation in hormone titers can play a role in the development of secondary sexual traits in insects, just like they do in vertebrates.\n\nHighlights O_LISex-specific levels of 20E, an insect molting hormone, regulate secondary sexual trait dimorphism and plasticity in butterflies.\nC_LIO_LI20E levels above a threshold promote local patterns of cell division in one sex, but not in the other sex, to create sexually dimorphic eyespots.\nC_LI\n\neTOCSexual selection drives the evolution of ornaments for individuals to display to the opposite sex. Yet, the mechanisms by which sexual selection operates are still not well understood. Here Bhardwaj et al. provide conclusive evidence, for the first time, that male and female insects use variation in levels of hormones to create dimorphism in their sexual ornaments. Authors show that 20-hydroxyecdyone, the insect molting hormone, also functions as a sex hormone in a butterfly. They also show how the environment shapes the development of sexual ornaments at a proximate level.