Tsuchimoto Jun, Yamaguchi Masaaki
Division of Life Science, Graduate School of Natural Science and Technology, Kanazawa University, Kanazawa, Japan; Institute for Molecular Science of Medicine, Aichi Medical University, Nagakute, Japan.
Dev Dyn. 2014 Aug;243(8):1020-9. doi: 10.1002/dvdy.24135. Epub 2014 Apr 23.
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.
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.
This in toto co-option of the hox genes provides insight into the molecular mechanisms underlying the evolution of echinoderms from a bilateral ancestor.
棘皮动物是一类奇特的后口动物，与半索动物构成一个进化枝，但具有五辐射对称的身体结构。Hox基因复合体在两侧对称动物的轴向模式形成中起关键作用，且通常在染色体上成簇出现。与具有组织化Hox簇的半索动物不同，海胆紫球海胆的Hox簇具有非典型的组织形式。然而，目前关于海胆幼体中hox基因表达的数据并不完整。
我们报告了对一种海胆类棘皮动物hox基因表达的全面研究。九个hox基因在成体幼体中表达，可分为两组，但hox11/13b同时属于两组：一组在体腔中胚层呈线性表达，另一组在成体口周围呈放射状表达。线性基因可能赋予体腔/肠系膜轴向信息，以指导消化道的变态后转化，而放射状基因在发育上与棘皮动物和/或海胆的形态新奇性相关。除hox11/13b外，放射状基因的招募似乎伴随着祖先/轴向功能的丧失。
hox基因的这种整体共选现象为深入了解棘皮动物从两侧对称祖先进化而来的分子机制提供了线索。
