title: Reinforcing the egg-timer: recruitment of novel lophotrochozoa homeobox genes to early and late development in the pacific oyster.
abstract: The metazoan superclade 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" homeobox genes that have arisen more recently in evolution, presumably by duplication followed by radical divergence and functional change. We classify 136 homeobox genes in the genome sequence of the Pacific oyster, Crassostrea gigas. The genome shows an unusually low degree of homeobox gene clustering, with disruption of the NK, Hox, and ParaHox gene clusters. Among the oyster genes, 31 do not fall into ancient metazoan or bilaterian homeobox gene families; we deduce that they originated in the lophotrochozoan clade. We compared eight lophotrochozoan genomes to trace the pattern of homeobox 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 homeobox genes have peak expression either in early development (egg to gastrula) or in late development (after the trochophore larval stage), but rarely in between. This finding is consistent with the egg-timer, hourglass 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.
authors: Department of Zoology, University of Oxford, United Kingdom.
authors: Jordi
authors: J
authors: Paps
authors: National & Local Joint Engineering Laboratory of Ecological Mariculture, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China.
authors: Fei
authors: F
authors: Xu
authors: National & Local Joint Engineering Laboratory of Ecological Mariculture, Institute of Oceanology, Chinese Academy of Sciences, Qingdao, China.
authors: Guofan
authors: G
authors: Zhang
authors: Department of Zoology, University of Oxford, United Kingdom peter.holland@zoo.ox.ac.uk.
authors: Peter W H
authors: PW
authors: Holland
mesh_terms: D000818:Animals; D002875:Chromosomes; D019143:Evolution, Molecular; D005801:Genes, Homeobox; D023281:Genomics; D018398:Homeodomain Proteins / Q000378:metabolism; D005810:Multigene Family; D010125:Ostreidae / Q000196:embryology / Q000235:genetics* / Q000254:growth & development
chemical_list: D018398:Homeodomain Proteins
keywords: Annelida; Mollusca; Platyhelminthes; Rotifera; gene duplication; gene families; homeodomain
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