Evolution of sexual development and sexual dimorphism in insects

Abstract

Most animal species consist of two distinct sexes. At the morphological, physiological, and behavioral levels the differences between males and females are numerous and dramatic, yet at the genomic level they are often slight or absent. This disconnect is overcome because simple genetic differences or environmental signals are able to direct the sex-specific expression of a shared genome. A canonical picture of how this process works in insects emerged from decades of work on Drosophila. But recent years have seen an explosion of molecular-genetic and developmental work on a broad range of insects. Drawing these studies together, we describe the evolution of sexual dimorphism from a comparative perspective and argue that insect sex determination and differentiation systems are composites of rapidly evolving and highly conserved elements.

Figure 1.. Divergent primary sex determination signals in Diptera converge on sex-specific doublesex splicing.

In the 5 Dipterans shown, sex is specified at the level of the individual cell by factors associated with sex (or proto-sex) chromosomes. These male- and female-defining chromosomes vary between species from being highly similar to each other (homomorphic) to highly divergent (heteromorphic) in morphology and gene content. In D. melanogaster, the number of X chromosomes determines the dosage of a set of X-linked factors that regulate the expression state of Sex lethal (Sxl). High dosage (XX) activates Sxl expression, the protein product of which promotes female-specific splicing of transformer (tra). The resulting female-specific isoform of Transformer protein (Tra^F) is required for the female-specific splicing of the transcription factor doublesex (dsx). Maleness is defined by the lower dosage of X-linked factors, rather than the presence of a Y-chromosome (e.g., X0 individuals are males). Having a single X chromosome leaves Sxl inactive in males, and the male-specific isoform of Transformer is produced (Tra^M). The presence of a premature stop codon renders Tra^M non-functional, which in turn leads to the production of the male-specific isoform of dsx. Musca domestica, Ceratitis capitata, Aedes aegypti, and Anopheles gambiae each use independently evolved (non-homologous) dominant M-factors to determine maleness. These are encoded on the Y-chromosome in most cases, but translocations to autosomes (turning them into proto-sex chromosomes) have been detected in different M. domestica populations. Whether the M-factor found on chromosome 1 in one population of M. domestica (shown in white) is a derived Mdmd sequence or an independently evolved M-factor remains unclear. In M. domestica and C. capitata, the presence of M-factors leads to the production of non-functional Tra^M and therefore, as in D. melanogaster, the production of the male-specific isoform of Dsx. No tra homolog has been found in Ae. aegypti or An. gambiae. Their M-factors, Nix and Yob respectively, are therefore presumed to determine the male-specific splicing of dsx by an as of yet unknown, tra-independent mechanism. The male and female isoforms of Dsx share a DNA-binding N-terminus but bear different C-termini, allowing them to regulate downstream target genes in a sex-specific manner, leading to the development of sex-specific traits. Figure created using BioRender.

Figure 2.. The origin and diversification of a new sex-specific trait.

This schematic describes a four-part model for the origin and subsequent morphological diversification of a sex-specific structure, in this case a modified row of bristles (a ‘sex comb’) on the male Drosophila foreleg. Species 1 displays the ancestral state of monomorphism. Here, developing leg cells do not express the transcription factor doublesex (dsx) and therefore lack the capacity for sex-specific differentiation. In species 2, changes in the sequence of the regulatory region controlling dsx expression enable the binding of position- and stage-determining transcription factors (TF). These TFs activate dsx expression in a subset of leg cells during a particular developmental window. dsx is alternatively spliced to give rise to male- and female-specific isoforms (Dsx^M and Dsx^F), which bind to the regulatory regions of target genes via a shared DNA-binding domain and impart sex-specific effects on target gene expression through sex-specific C-termini. The localized, sex-specific regulation of gene expression that results enables the development of a novel structure only in males. In species 3, additional changes in the dsx enhancers generate changes in the binding of its upstream regulators. This leads to changes in the spatiotemporal pattern of dsx expression among developing leg cells, which in turn produces changes in the size and position of the male-specific structure. In species 4, Dsx has acquired new downstream target genes due to the gain of Dsx-binding sites in the regulatory regions of the new targets. Incorporation of the new targets into the gene regulatory network (‘GRN’) that controls the development of the male-specific structure leads to further morphological diversification. Figure created using BioRender.

Figure 3.. Hormonal inputs into insect sexual dimorphism.

Two principal mechanisms exist through which hormones can deliver sex-specific effects in insects. (A) Sex differences in hormone titer. Developing eyespot cells in the butterfly Bicyclus anynana express ecdysone receptor. The titer of circulating 20-hydroxyecdysone in females leads to a binding threshold being exceeded, which causes the cells to proliferate and the eyespot to grow. The lower titer in males fails to exceed the binding threshold and the cells fail to proliferate. What generates the divergence in hormone titer is unclear, but one potential mechanism is the direct or indirect regulation of enzymes in the ecdysone biosynthesis pathway by Dsx^M and/or Dsx^F. (B) Sex differences in sensitivity to hormones. Expression of dsx in the developing prepupal mandibles of the stag beetle Cyclommatus metallifer changes the sensitivity of mandibular cell proliferation to juvenile hormone. Dsx^M increases sensitivity, leading to enlarged mandibles in males. Dsx^F reduces sensitivity, leading to small mandibles in females. Figure created using BioRender.