Grenier J K, Garber T L, Warren R, Whitington P M, Carroll S
Howard Hughes Medical Institute, University of Wisconsin, Madison 53708, USA.
Curr Biol. 1997 Aug 1;7(8):547-53. doi: 10.1016/s0960-9822(06)00253-3.
Dramatic changes in body size and pattern occurred during the radiation of many taxa in the Cambrian, and these changes are best documented for the arthropods. The sudden appearance of such diverse body plans raises the fundamental question of when the genes and the developmental control systems that regulate these designs evolved. As Hox genes regulate arthropod body patterns, the evolution of these genes may have played a role in the origin and diversification of the arthropod body plan from a homonomous ancestor. To trace the origin of arthropod Hox genes, we examined their distribution in a myriapod and in the Onychophora, a sister group to the arthropods.
Despite the limited segmental diversity within myriapods and Onychophora, all insect Hox genes are present in both taxa, including the trunk Hox genes Ultrabithorax and abdominal-A as well as an ortholog of the fushi tarazu gene. Comparative analysis of Hox gene deployment revealed that the anterior boundary of expression of trunk Hox genes has shifted dramatically along the anteroposterior axis between Onychophora and different arthropod classes. Furthermore, we found that repression of expression of the Hox target gene Distal-less is unique to the insect lineage.
A complete arthropod Hox gene family existed in the ancestor of the onychophoran/arthropod clade. No new Hox genes were therefore required to catalyze the arthropod radiation; instead, arthropod body-plan diversity arose through changes in the regulation of Hox genes and their downstream targets.
在寒武纪许多生物类群的辐射演化过程中，身体大小和形态发生了显著变化，其中节肢动物的这些变化记录最为详尽。如此多样的身体结构突然出现，引发了一个基本问题：调控这些结构的基因和发育控制系统是何时演化出来的。由于Hox基因调控节肢动物的身体形态，这些基因的演化可能在节肢动物身体结构从同源祖先起源及多样化过程中发挥了作用。为了追溯节肢动物Hox基因的起源，我们研究了它们在多足动物以及与节肢动物亲缘关系较近的有爪动物中的分布情况。
尽管多足动物和有爪动物的体节多样性有限，但所有昆虫Hox基因在这两个类群中均有存在，包括躯干Hox基因超双胸和腹部A基因，以及ftz基因的一个直系同源基因。对Hox基因表达模式的比较分析表明，躯干Hox基因表达的前端边界在有爪动物和不同节肢动物类群之间沿前后轴发生了显著变化。此外，我们发现Hox靶基因Distal-less表达的抑制是昆虫谱系所特有的。
有爪动物/节肢动物分支的祖先中存在完整的节肢动物Hox基因家族。因此，节肢动物的辐射演化并不需要新的Hox基因；相反，节肢动物身体结构的多样性是通过Hox基因及其下游靶标的调控变化产生的。
