Expression of Hox Genes in Early Development of Irregular Sea Urchin Scaphechinus mirabilis (Echinodermata, Echinoidea)

Current data indicate that the Hox gene expression pattern differs between regular and irregular sea urchins. Expression of most genes of the Hox cluster is not activated until the pluteus forms, and only Hox7, 11/13b, and 11/13c are expressed at the blastula stage in two Regularia species examined previously, Strongylocentrotus purpuratus and St. intermedius. In contrast, in Peronella japonica, the only species studied in Irregularia, Hox7, 9/10, 11/13b, and 11/13c are expressed at the blastula and gastrula stages; Hox1, 7, 8, 9/10, 11/13a, and 11/13b are expressed at the prism stage; and most genes of the Hox cluster are activated at the pluteus stage. The irregular sea urchin Scaphechinus mirabilis was examined. Almost all of its Hox genes were silent at early stages. SmHox11/13b was an exception and was expressed at the gastrula stage. Most Hox genes were activated only at the pluteus stage. Thus, the expression pattern of the Hox cluster in Sc. mirabilis is similar to that of the regular sea urchins St. purpuratus and St. intermedius and differs from that of the other irregular sea urchin P. japonica.