Plasticity of gene-regulatory networks controlling sex determination: of masters, slaves, usual suspects, newcomers, and usurpators

Abstract

Sexual dimorphism is one of the most pervasive and diverse features of animal morphology, physiology, and behavior. Despite the generality of the phenomenon itself, the mechanisms controlling how sex is determined differ considerably among various organismic groups, have evolved repeatedly and independently, and the underlying molecular pathways can change quickly during evolution. Even within closely related groups of organisms for which the development of gonads on the morphological, histological, and cell biological level is undistinguishable, the molecular control and the regulation of the factors involved in sex determination and gonad differentiation can be substantially different. The biological meaning of the high molecular plasticity of an otherwise common developmental program is unknown. While comparative studies suggest that the downstream effectors of sex-determining pathways tend to be more stable than the triggering mechanisms at the top, it is still unclear how conserved the downstream networks are and how all components work together. After many years of stasis, when the molecular basis of sex determination was amenable only in the few classical model organisms (fly, worm, mouse), recently, sex-determining genes from several animal species have been identified and new studies have elucidated some novel regulatory interactions and biological functions of the downstream network, particularly in vertebrates. These data have considerably changed our classical perception of a simple linear developmental cascade that makes the decision for the embryo to develop as male or female, and how it evolves.

Keywords: Dmrt1; SRY; ovary; testis; transcription factor.

Figure 1. Independent evolution of SOX3 genes toward a master sex‐determining function in mice and Indian rice fish

While SRY appears to be restricted to the therian mammals, evidence accumulates that SOX3 has independently been recruited as a “precursor” of master sex‐determining genes also outside mammals. Hence, although not a priori destined to have a direct function during sex determination, common mechanisms of evolution seem to be repeatedly employed. Given that SOX3 is not generally expressed during gonadal induction or during gonadal development, the first step toward a sex‐determining function is a transcriptional rewiring in order to acquire a timed pattern of expression compatible with sex determination. Such transcriptional rewiring, although not unique to SOX3 (see Dmrt1bY in medaka fish for example 56), generally involves either fusion of the gene to new promoters or insertions of transposable elements into their pre‐existing promoter, bringing in cis‐regulatory elements compatible with the timing of gonadal induction. Interestingly and surprisingly, it seems that at least in mice and rice fish, this step alone was sufficient to endow SOX3 with a sex‐determining function. Usually, the transcriptional rewiring steps seem to be accompanied by neo‐functionalization or functional specialization processes. These include specialization of the protein activity itself in therian mammals (adapted from reference 20) or more surprisingly adaptation of the downstream gene‐regulatory network (target genes) in the Indian rice fish.

Figure 2. Gene‐regulatory network of gonadal sex induction and maintenance in vertebrates

Schematic representation of main interactions within the regulatory network. In gonadal fate determination of mammals, Sry initiates activation of the male pathway (blue) through up‐regulation of Sox9. Dmrt1 is not only important for keeping the male pathway on but also in suppressing the two female networks (red). These two female networks involve Foxl2 as well as the Wnt/β‐catenin signaling pathways. Maintenance of gonadal identity in the differentiated gonads is a result of the cross‐inhibition activities of Dmrt1 and Foxl2. A critical equilibrium between these conflicting pathways underlies the bipotentiality of the gonadal somatic cells. Tipping the balance into one direction or the other will regulate the gonadal fate as a consequence of the activation of the male or female pathways. Solid lines define negative regulations. Dashed lines designate positive regulations. Beside the Sry ancestor Sox3 and Dmrt1, other genes (pink) can become the master sex‐determining genes by similarly impacting on the seesaw between the male and female programme.

Figure 3. Sex‐determining cascades in C. elegans and some insects

Molecular and genetic pathways leading to the formation of the gonad in the worm C. elegans, the mosquito Aedes aegypti, the fly D. melanogaster, the honey bee A. mellifera, and the silkworm B. mori. Conservation of the Dsx, Mab‐3 and Dmrt1, Tra‐like, (Tra‐2), or Fru homologs is designated with either pale brown, pale blue, pale green, or pale orange boxes, respectively. Tra‐(1, 2 or 3) of C. elegans are not phylogenetically related to Tra of Drosophila. Fem‐(1, 2 or 3) of C. elegans are not phylogenetically related to fem of Bombyx mori. In C. elegans and D. melanogaster, a ratio between X chromosomes and autosomes determines the sex. This leads to the on/off state of Xol or Sxl, respectively. Heterozygosity turns on Csd in the honeybee Apis melifera, leading to female development, and hemizygosity or homozygosity leaves Csd unexpressed and produces a drone. In the mosquito (Aedes aegypti), sex determination is triggered by a dominant male determiner (Nix). Nix is a distant homolog of the splicing factor Tra‐2 of Drosophila and likely regulates the sex‐specific splicing of Fru and Dsx. Sex in the silkworm Bombyx mori is controlled via a ZW sex chromosome system. Produced only from the sex‐determining locus on the W, the piRNAs suppress the male sex‐determining factor MASC.