Abstract
Sex determination was a founding topic of C. elegans research. After three decades of research, a complex signal transduction pathway with multiple layers of regulation has been elucidated. This pathway links karyotype to phenotype by coordinating the development of sexually dimorphic tissues. In this article, this pathway is placed in two broader contexts. The first is that of nematodes and animals in general. The important role of C. elegans studies in revealing the first universally conserved component of metazoan sex determination is discussed, as is the role of cooption of genes into the sex determination and dosage compensation pathways. The second context is that of a subset of more closely related species, with emphasis on other members of the genus Caenorhabditis. Studies reviewed here have determined the gene-level conservation of the known pathway and the relative rates of molecular evolution in conserved components, and made substantial progress in the manipulation of gene activity in non-elegans species. Special attention is paid to the origins of hermaphroditism, which evolved from gonochorism through germline-specific changes in sex determination. Recent studies suggest that the most rapidly evolving aspects of sex determination are germline functions related to evolutionary shifts in mating systems, while somatic sex determination is relatively conservative. From all of these studies, a picture emerges in which C. elegans utilizes an intriguing mixture of general and species-specific genes and regulatory mechanisms.
Highlights
Sex determination was one of the first aspects of C. elegans biology to be systematically characterized with genetic analysis (Hodgkin and Brenner, 1977), and the depth of this analysis has made it a major topic in developmental biology
In 1949, Nigon demonstrated that C. elegans used an XX/XO sex chromosome system, and the importance of the X:autosome ratio was clarified by Madl & Herman (1979)
Among the small number of nematode fossil specimens that exist are mermithids emerging from their insect hosts, preserved in 40 million year old Baltic amber (Poinar, 2002). Their striking similarity to extant insect-parasitic species (Figure 1) suggests a minimum age for the origin of environmental sex determination (ESD) in this group, it is possible that ancient mermithids used genetic sex determination (GSD)
Summary
Sex determination was one of the first aspects of C. elegans biology to be systematically characterized with genetic analysis (Hodgkin and Brenner, 1977), and the depth of this analysis has made it a major topic in developmental biology. It is both fair and interesting to ask how general the C. elegans model is likely to be, both for other nematodes as well as for animals in general. Comparative studies of sex determination based on the C. elgans model have revealed both rapidly evolving and surprisingly well conserved features. This mixture of old and new (or slow and fast), along with its obvious relevance to evolution, ecology, and applied fields like agricultural and medical parasitology, has made the evolution of nematode sex determination an increasingly active research area
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