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The metazoan super clade Lophotrochozoa includes mollusks, annelids, and several other animal phyla. It is reasonable to assume that this organismal diversity may be traced, in part, to changes in developmentally important genes, such as the homeobox genes. Although most comparative studies have focussed on ancient homeobox gene families conserved across bilaterians, there are also “novel” home obox genes that have a risen more recently in evolution, presumably by duplication followed by radical divergence and functional change. We classify 136homeobox genes in thegenome sequence of the Pacific oyster, Crassostrea gigas. The genome shows an unusually low degree of home obox gene clustering, with disruption of the NK, Hox, and Para Hoxgene clusters. Among the oyster genes, 31 do not fall into ancient metazoan or bilaterian homeobox gene families; we deduce that they originated in the lophotrochozoanclade. We compare deight lophotrochozoangenomes to trace the pattern of home obox gene evolution across this clade, allowing us to define 19 new lophotrochozoan-specific clades within the ANTP, PRD, TALE, ZF, SIX, and CUT classes. Using transcriptome data, we compared temporal expression of each homeobox gene in oyster development, and discovered that the lophotrochozoan-specific home obox genes have peak expression either in early development (eggtogastrula) or in late development (after the trochophorelarval stage), but rarely in between. This finding is consistent with the egg-timer, hour glass or phylotypic stage model of developmental evolution, in which there is a conserved central phase of development, but more evolutionarily labile early and late phases
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Kembali