Genomic tests of body plan transitions from bilateral to pentameric symmetry in Echinoderms

Echinoderms are an exceptional group in bilaterians in that they develop pentameral adult symmetry from a bilaterally symmetric larva. However, the genetic basis in evolution and development of this unique transformation remains to be clarified. Here we report newly sequenced genomes, developmental transcriptomes, and proteomes of diverse echinoderms including the green sea urchin (L. variegatus), a sea cucumber (A. japonicus), and with particular emphasis on progeny of the earliest-diverged deuterostome, the feather star (A. japonica). We learned that the last common ancestor of echinoderms retained a well-organized Hox cluster genes reminiscent of the hemichordate, and had gene sets involved in endoskeleton development. Further, unlike in other animal groups, the most conserved developmental stages was not at the body plan establishing phase, and genes normally involved in bilaterality appear to function in pentameral axes development. These results enhance our understanding of the divergence of protostomes and deuterostomes almost 500 mya.

棘皮动物（Echinoderms）是两侧对称动物（bilaterians）中的特殊类群，它们可由两侧对称的幼虫发育出五辐射成体对称体制。然而，这一独特转变的演化与发育遗传基础仍有待阐明。本研究报道了多种棘皮动物的新测序基因组、发育转录组与蛋白质组，涵盖绿海胆（L. variegatus）、海参（A. japonicus），并特别聚焦于最早分化的后口动物（deuterostome）类群代表——海羽星（A. japonica）。研究发现，棘皮动物的最近共同祖先保留了结构规整、类似半索动物（hemichordate）的Hox基因簇（Hox cluster），且拥有参与内骨骼发育的基因集。此外，与其他动物类群不同，棘皮动物中最保守的发育阶段并非躯体构型建立阶段，而通常参与两侧对称发育的基因，似乎在五辐射轴的发育中发挥功能。这些结果加深了我们对近5亿年前原口动物（protostomes）与后口动物分化事件的认知。