Department of Evolutionary Genetics, Göttingen Center of Molecular Biology, Georg-August-Universität Göttingen, Justus-von-Liebig-Weg 11, 37077 Göttingen, Germany.
Dev Biol. 2013 Feb 1;374(1):174-84. doi: 10.1016/j.ydbio.2012.11.026. Epub 2012 Nov 29.
Early embryonic stages differ significantly among related animal taxa while subsequent development converges at the conserved phylotypic stage before again diverging. Although this phenomenon has long been observed, its underlying genetic mechanisms remain enigmatic. The dipteran Drosophila melanogaster develops as a long germ embryo where the head anlagen form a cap at the anterior pole of the blastoderm. Consequently, the anterior and terminal maternal systems give crucial input for head patterning. However, in the short germ beetle Tribolium castaneum, as in most insects, the head anlagen is located at a ventral position distant from the anterior pole of the blastoderm. In line with these divergent embryonic anlagen, several differences in the axis formation between the insects have been discovered. We now ask to what extent patterning and morphogenesis of the anterior median region (AMR) of the head, including clypeolabral and stomodeal anlagen, differ among these insects. Unexpectedly, we find that Tc-huckebein is not a terminal gap gene and, unlike its Drosophila ortholog, is not involved in Tribolium head development. Instead, Tc-six3 acts upstream of Tc-crocodile and Tc-cap'n'collar to pattern posterior and anterior parts of the AMR, respectively. We further find that instead of huckebein, Tc-crocodile is required for stomodeum development by activating Tc-forkhead. Finally, a morphogenetic movement not found in Drosophila shapes the embryonic head of Tribolium. Apparently, with anterior displacement of the head anlagen during long germ evolution of Drosophila, the ancestral regulation by the bilaterian anterior control gene six3 was replaced by the anterior and terminal maternal systems, which were further elaborated by adding bicoid, tailless and huckebein as anterior regionalization genes.
早期胚胎阶段在相关动物类群中存在显著差异，而随后的发育在保守的表型阶段汇聚，然后再次分化。尽管这种现象早已被观察到，但它的潜在遗传机制仍然是个谜。双翅目果蝇作为长胚胚胎发育，头部原基在胚盘的前极形成一个帽状结构。因此，前极和末端母体系统为头部模式形成提供了关键输入。然而，在短胚甲虫拟步甲中，与大多数昆虫一样，头部原基位于远离胚盘前极的腹侧位置。与这些不同的胚胎原基一致，昆虫的轴形成之间已经发现了几个差异。我们现在想知道，头部前正中区域（AMR）的模式形成和形态发生，包括唇瓣和口原基，在这些昆虫中差异有多大。出乎意料的是，我们发现 Tc-huckebein 不是一个末端间隙基因，与它的果蝇同源物不同，它不参与拟步甲头部发育。相反，Tc-six3 在前部 crocodile 和 cap'n'collar 的上游作用，分别对 AMR 的后部和前部进行模式化。我们进一步发现，不是 huckebein，而是 Tc-crocodile 通过激活 Tc-forkhead 来激活 Tc-forkhead 来激活 stomodeum 的发育。最后，在果蝇中没有发现的形态发生运动塑造了拟步甲的胚胎头部。显然，在果蝇长胚进化过程中，头部原基的前位移导致了六基因的祖先调控被前极和末端母体系统所取代，然后通过添加 bicoid、tailless 和 huckebein 作为前区域化基因进一步细化。
