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要旨:
Pristionchus pacificus represents a nematode that shares with Caenorhabditis elegans many technical features and has been developed as a model system in evolutionary developmental biology. Many important differences on the genetic and molecular level of development, in particular vulva development, have been identified. Intriguingly, Pristionchus also occupies a distinct ecological niche. Pristionchus nematodes are associated with scarab beetles and have a necromenic life style: that is, worms invade the beetle as dauer larvae and wait for the insect's death to feed on the developing microbes on the carcass. It has been suggested that a close association with other organisms as seen in necromenic nematodes represent a pre-adaptation towards true parasitsm. With 169-Mb and 29,000 predicted protein-coding genes the P. pacificus genome is substantially larger than the genomes of C. elegans and the human parasite Brugia malayi. Comparative analysis with C. elegans revealed an elevated number of genes encoding cytochrome P450 enzymes, glucosyl-transferases and ABC transporters that were experimentally validated and confirmed. P. pacificus contains cellulase and diapausin genes and cellulase activity is found in P. pacificus secretions, the first time cellulases have been identified in nematodes beyond plant parasites. The increase in detoxification and degradation enzymes is consistent with the Pristionchus life-style and is a pre-requisite for parasitism. Thus, comparative genomics of three ecologically distinct nematodes offers a unique opportunity to investigate the correlation between genome structure and life- style. Current studies involve developmental and genetic analyses of various traits associated with the specific ecological setting of P. pacificus, such as dauer formation, insect recognition by olfaction, mouth form dimorphism and bacterial association. Specificity of the Pristionchus beetle association is achieved by nematode interception of the insect sex pheromone/sex attractant system. For example, P. maupasi recognizes its cockchafer host by olfaction towards phenol, one of the cockchafers´ sex attractants, whereas P. pacificus identifies the oriental beetle host by recognition of the sex pheromone (Z)-7-tetradecen-2-one. In contrast, analysis of the Pristionchus associated bacteria did not reveal any specificity so that the ultimate causation of the high specificity of the nematode beetle association remains open. Current analyses investigate a potential competetion between Pristionchus and entomopathogenic nematodes. Pristionchus, unlike many nematodes can feed on bacteria, fungi and other nematodes. This is because Pristionchus nematodes show a mouth dimorphism between a teethless bacterial feeding stenostomatous form and an eurystomatous form with teeth that can feed on fungi and nematodes as well. Genetic analysis suggests that the mouth dimorphism represents a case of phenotypic plasticity and that the ratio of the two forms is influenced by environmental factors, such as starvation. The genetic regulation of the mouth dimorphism is currently investigated and shares several genes with other plastic nematode traits.
