Expression patterns of Hox genes in larvae of the sea lily Metacrinus rotundus

Abstract

We cloned eight Hox genes (MrHox1, MrHox2, MrHox4, MrHox5, MrHox7, MrHox8, MrHox9/10, and MrHox11/13c) from the sea lily Metacrinus rotundus, a member of the most basal group of the extant echinoderms. At the auricularia stage, before the formation of the pentaradial rudiment, four MrHox genes were expressed sequentially along the anteroposterior (AP) axis in the straightened mesodermal somatocoels in the order MrHox5, MrHox7, MrHox8, and MrHox9/10. The expression of MrHox7 and MrHox8 was detected as early as the hatching stage in the presumptive somatocoel region of the archenteral sac. MrHox5 was expressed in the anteriormost region of the somatocoels, where a stalk-related structure (the chambered organ) forms later. In addition to the mesodermal somatocoels, MrHox7 was expressed in the oral hood ectoderm, which gives rise to the adhesive pit. The expression of four other MrHox genes (MrHox1, MrHox2, MrHox4, and MrHox11/13c) was not detected in any of the larval stages we examined. In comparison with the mesodermal sea urchin Hox genes, the MrHox genes are expressed more posteriorly along the AP (oral-anal) axis than the sea urchin orthologs, implying that the evolution of the eleutherozoans was accompanied by a posteriorization of the larval body. Our study illuminates the possible body plan and Hox expression patterns of the ancestral echinoderm and sheds light on the larval body plan of the last common ancestor of the echinoderms and chordates.

Deciphering deuterostome phylogeny: molecular, morphological and palaeontological perspectives

“…Although both tunicates and ambulacrarians have larvae that are planktotrophic, they are very different in form and in the gene networks that are deployed during development (Swalla 2006). Echinoderm and hemichordate dipleurula-type larvae feed using ciliary bands and are undoubtedly homologous (Hara et al 2006). The nonfeeding tunicate larva is very different and clearly non-homologous, being tadpole-like and sharing derived body plan features with vertebrates, including a notochord centred in the tail flanked dorsally by the nerve cord, laterally by muscles and ventrally by endoderm (Swalla 2007).…”

PCR survey of Xenoturbella bocki Hox genes

“…The sources of used sequence data: Echinodermata: Sea cucumber, Holothuria glaberrima (Mendéz, 2000) (PCR); Sea stars, Asterina minor (Mito and Endo, '97) (PCR) and Patiriella exigua (Long et al, 2000) (PCR), Hox4 gene (Long et al, 2003); Crinoid, Oxycomanthus japonicus (Mito and Endo, 2000) (PCR), Metacrinus rotundus (Hara et al, 2006); Ophiurid, Stegophiura sladeni (Mito and Endo, 2000) (PCR), Amphiura filiformis (PCR, P. Martinez et al, GenBank submissions). For echinoids we used the completely sequenced Hox cluster (Cameron et al, 2006) as references.…”

The amphioxus Hox cluster: characterization, comparative genomics, and evolution

“…This general approach has been very successful for many protostome and deuterostome taxa; see e.g. Hara et al (2006) and Manuel et al (2006). However, because the genes are not isolated in their genomic context, relationships with regard to co-linearity of physical arrangement and expression patterns during embryonic and larval development are not known or can only be inferred.…”