Hox expression in the direct‐type developing sand dollar Peronella japonica

Background: Echinoderms are a curious group of deuterostomes that forms a clade with hemichordates but has a pentameral body plan. Hox complex plays a pivotal role in axial patterning in bilaterians and often occurs in a cluster on the chromosome. In contrast to hemichordates with an organized Hox cluster, the sea urchin Strongylocentrotus purpuratus has a Hox cluster with an atypical organization. However, the current data on hox expression in sea urchin rudiments are fragmentary. Results: We report a comprehensive examination of hox expression in a sand dollar echinoid. Nine hox genes are expressed in the adult rudiment, which are classified into two groups, but hox11/13b belongs to both: one with linear expression in the coelomic mesoderm and another with radial expression around the adult mouth. The linear genes may endow the coelom/mesentery with axial information to direct postmetamorphic transformation of the digestive tract, whereas the radial genes developmentally correlate with the morphological novelties of echinoderms and/or sea urchins. Recruitment of the radial genes except hox11/13b appears to be accompanied by the loss of ancestral/axial roles. Conclusions: This in toto co‐option of the hox genes provides insight into the molecular mechanisms underlying the evolution of echinoderms from a bilateral ancestor. Developmental Dynamics 243:1020–1029, 2014. © 2014 Wiley Periodicals, Inc. Key Findings Nine hox genes are dynamically expressed in the adult rudiment of the sand dollar Peronella japonica. The hox genes are classified into two groups in expression, but hox11/13b belongs to both: one with radial expression around the adult mouth and another with linear expression in the somatocoel. The genes with radial expression developmentally correlate with the morphological novelties of echinoderms and/or sea urchins.