The molecular mechanism of the larval mesenchyme cell specification in echinoids has been well analyzed. However, most of the data have been provided by studies of a single group of echinoids, the order Camarodonta. Little is known about this mechanism in other echinoid orders. We examined the expression patterns of mesenchyme specification genes, micro1, hesC, alx1, tbr, ets1, cyp1, and gcm, in the two non-Camarodonta echinoids, Glyptocidaris crenularis and Echinocardium cordatum. We found that the expression patterns of some genes contained characteristics that were unique to one of the species; others were shared by the two species. Some of the shared characteristics of G.crenularis and E.cordatum are not found in the species belonging to Camarodonta, suggesting the derived status of this order. The expression of ets1 in E.cordatum aboral ectoderm is one of the molecular level modifications possibly related to an evolutionarily novel larval structure, the posterior process. Our results suggest that a considerable number of modifications in the mesenchyme specification mechanisms have been introduced during the echinoid evolution.
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