Early Hox Gene Expression in Echinoderms

Abstract

Breakages of sequential Hox expression in Echinodermata may reflect evolutionary changes in the body plan. We quantified Hox gene expression at early larval stages, i.e., blastula at 13 hpf, gastrula at 35 hpf, prism stage at 46 hpf, and echinoplutei at 4 and 9 dpf, in the sea urchin with indirect development, Strongylocentrotus intermedius. The design of specific primers for real-time PCR (q-RT-PCR) for S. intermedius was performed. At blastula stage, only SiHox7 (medial Hox group), SiHox11/13b and SiHox11/13c (posterior Hox group) were expressed. At gastrula and prism stages, the expression of SiHox genes was lower than that in unfertilized eggs. Significant increase in the expression of all SiHox genes was observed in 4-day pluteus with further increase of SiHox1, SiHox8, SiHox9/10, and SiHox11/13b expression in 9-day pluteus. In other echinoderms, the genes of anterior Hox group are nearly not expressed in the early stages of development. At blastula, gastrula and prism stages, the genes of medial Hox group (mainly Hox7 and Hox8) and posterior Hox group (mainly Hox11/13a, 11/13b, 11/13c) are significantly expressed. This breakage of sequential Hox expression in echinoderm ancestors may be caused by loss of metameric gill slits, which are the most important synapomorphy of deuterostomes.