Sex determination across evolution: connecting the dots

Abstract

Sexual differentiation appears to be an ancient, and potentially homologous, feature of animal biology, and yet the pathways that underlie the process exhibit bewildering variety

Figure 1. Diverse Genetic Factors Converge on a Conserved Regulator

The primary sex determination mechanisms are shown, from left to right, for Drosophila, Caenorhabditis, the box turtle Terrapene carolina, and humans. These proximate signals are then relayed by diverse signal transduction pathways that ultimately converge on a DM-family gene. The left image is from Muller [23]; the center-left image appears courtesy of Dr. Barbara Conradt, the center-right image appears courtesy of J.D. Willson, and the right image is from a plaque mounted on the NASA spacecraft Pioneer 11.

Figure 2. Evolutionary Dynamics of Sex-Determination Pathways

(A) Rapid sequence evolution. Shown are the genes in xol-1 region of C. elegans that have syntenic homologues in C. briggsae, with the amino-acid-level identity between them indicated below. (B) Pathway evolution and primary signal swapping (modified from Graham et al. [9]). In Drosophila (L), the X:A ratio indirectly regulates tra splicing through a requirement for Sxl. In the medfly Ceratitis (R), Sxl is not a sex determination gene, and the female-promoting positive regulation of tra is instead autonomous. Its inhibition by the dominant M gene allows an XX/XY system to replace one based on the X:A ratio. (C) Convergent evolution of nematode hermaphroditism in C. elegans and C. briggsae. fog-2 exists only in C. elegans, and although all species use the fem genes for male somatic development, only C. elegans requires them for hermaphrodite spermatogenesis.