Nuclear receptors in nematodes: themes and variations

Large-scale sequencing efforts are providing new perspectives on similarities and differences among species. Sequences encoding nuclear receptor (NR) transcription factors furnish one striking example of this. The three complete or nearly complete metazoan genome sequences - those of the nematode Caenorhabditis elegans, the fruit fly (Drosophila melanogaster) and the human - reveal dramatically different numbers of predicted NR genes: 270 for the nematode, 21 for the fruit fly and similar to 50 for the human. Although some classes of NRs present in insects and mammals are also represented among the nematode genes, most of the C. elegans NR sequences are distinct from those known in other phyla. Questions regarding the evolution and function of NR genes in nematodes, framed by the abundance and diversity of these genes in the C. elegans genome, are the focus of this article.