Gene expression analysis of three homeobox genes throughout early and late development of a feather star Anneissia japonica.

Crinoids are considered as the most basal extant echinoderms. They retain aboral nervous system with a nerve center, which has been degraded in the eleutherozoan echinoderms. To investigate the evolution of patterning of the nervous systems in crinoids, we examined temporal and spatial expression patterns of three neural patterning-related homeobox genes, six3, pax6, and otx, throughout the development of a feather star Anneissia japonica. These genes were involved in the patterning of endomesodermal tissues instead of the ectodermal neural tissues in the early planktonic stages. In the stages after larval attachment, the expression of these genes was mainly observed in the podia and the oral nervous systems instead of the aboral nerve center. Our results indicate the involvement of these three genes in the formation of oral nervous system in the common ancestor of the echinoderms and suggest that the aboral nerve center is not evolutionally related to the brain of other bilaterians.