Abstract
In contrast to the important role of hormones in the development of sexual traits in vertebrates (Cox RM, Stenquist DS, Calsbeek R. 2009. Testosterone, growth and the evolution of sexual size dimorphism. J Evol Biol. 22(8):1586–1598.), the differentiation of these traits in insects is attributed almost exclusively to cell-autonomous mechanisms controlled by members of the sex determination pathway (Verhulst EC, van de Zande L. 2015. Double nexus – doublesex is the connecting element in sex determination. Brief Funct Genomics 14(6):396–406.), such as doublesex. Although hormones can shape the development of sexual traits in insects, variation in hormone levels are not conclusively known to cause dimorphism in these traits (Prakash A, Monteiro A. 2016. Molecular mechanisms of secondary sexual trait development in insects. Curr Opin Insect Sci. 17:40–48.). Here, we show that butterflies use sex-specific differences in 20-hydroxyecdysone hormone titers to create sexually dimorphic wing ornaments. 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 form both sexes have similarly sized ornaments (Prudic KL, Jeon C, Cao H, Monteiro A. 2011. Developmental plasticity in sexual roles of butterfly species drives mutual sexual ornamentation. Science 331(6013):73–75.). High levels of circulating 20-hydroxyecdysone during larval development in DS females and wet season 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 hormones can play a role in the development of secondary sexual traits in insects, just like they do in vertebrates.
Highlights
Recent studies have shown that sexual traits are neither under constant, or even similar direction of selection over time and space (Cornwallis and Uller 2010; Stillwell et al 2010; Miller and Svensson 2014)
This study advances our understanding of how the environment regulates sex-specific patterns of plasticity of sexual ornaments and conclusively shows that hormones can play a role in the development of secondary sexual traits in insects, just like they do in vertebrates
Because ornament size in males is controlled by rearing temperature (Prudic et al 2011), we began by identifying the developmental window that is critical for eyespot size regulation using temperature shift experiments
Summary
Recent studies have shown that sexual traits are neither under constant, or even similar direction of selection over time and space (Cornwallis and Uller 2010; Stillwell et al 2010; Miller and Svensson 2014). This is because organisms do not live in stable biotic and abiotic environments. In the WS, both sexes develop large eyespots characteristic of the season and males avidly court choosy females This leads to a pattern of sexual dimorphism in the DS and plasticity in the sexual ornament that is male-limited This leads to a pattern of sexual dimorphism in the DS and plasticity in the sexual ornament that is male-limited (fig. 1) (Prudic et al 2011)
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