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INTRODUCTION: Pristionchus pacificus is a nematode that has been established as a model system for evolutionary developmental biology. Initially, P. pacificus was used as a convenient nematode with which to compare the processes of vulva and gonad development as well as sex determination to Caenorhabditis elegans, one of the best-studied animal models. P. pacificus shares many features with C. elegans, including a short generation time, its ability to be easily cultured in the laboratory, and self-fertilization as a mode of reproduction. These features allowed forward and reverse genetic tools to be developed for this species. The application of these tools for genetic and molecular analysis of vulva formation revealed substantial differences between P. pacificus and C. elegans. The genome of P. pacificus has recently been sequenced and showed an expansion of protein-coding genes compared with C. elegans. Interestingly, the P. pacificus genome encodes some genes, such as cellulases, that are known to be present only in plant-parasitic nematodes. Many of the putative functions of the predicted genes in the genome are related to the ecology of P. pacificus and other Pristionchus species. Pristionchus nematodes can be isolated from beetles and soil, indicating that the ecology of P. pacificus is strikingly different from that of C. elegans. Generally, Pristionchus species show an unexpected level of species specificity in their beetle associations, providing a unique opportunity to study the genetic and molecular mechanisms underlying the interactions of organisms in the environment. Thus, P. pacificus is not only an established model system for evolutionary developmental biology, but also an emerging model system for the evolution of complex life-history traits.
