Department of Biological Sciences, Wayne State University, Detroit, MI 48202, USA.
Dev Biol. 2010 May 1;341(1):324-34. doi: 10.1016/j.ydbio.2010.02.018. Epub 2010 Feb 19.
The cockroach, Periplaneta americana represents a basal insect lineage that undergoes the ancestral hemimetabolous mode of development. Here, we examine the embryonic and post-embryonic functions of the hox gene Scr in Periplaneta as a way of better understanding the roles of this gene in the evolution of insect body plans. During embryogenesis, Scr function is strictly limited to the head with no role in the prothorax. This indicates that the ancestral embryonic function of Scr was likely restricted to the head, and that the posterior expansion of expression in the T1 legs may have preceded any apparent gain of function during evolution. In addition, Scr plays a pivotal role in the formation of the dorsal ridge, a structure that separates the head and thorax in all insects. This is evidenced by the presence of a supernumerary segment that occurs between the labial and T1 segments of RNAiScr first nymphs and is attributed to an alteration in engrailed (en) expression. The fact that similar Scr phenotypes are observed in Tribolium but not in Drosophila or Oncopeltus reveals the presence of lineage-specific variation in the genetic architecture that controls the formation of the dorsal ridge. In direct contrast to the embryonic roles, Scr has no function in the head region during post-embryogenesis in Periplaneta, and instead, strictly acts to provide identity to the T1 segment. Furthermore, the strongest Periplaneta RNAiScr phenotypes develop ectopic wing-like tissue that originates from the posterior region of the prothoracic segment. This finding provides a novel insight into the current debate on the morphological origin of insect wings.
美洲大蠊是一种原始的昆虫谱系，经历了祖先进化的半变态发育模式。在这里，我们研究了 hox 基因 Scr 在美洲大蠊中的胚胎和胚胎后功能，以更好地理解该基因在昆虫体节模式进化中的作用。在胚胎发生过程中，Scr 的功能严格局限于头部，而在前胸中没有作用。这表明 Scr 的祖先胚胎功能可能仅限于头部，并且 T1 腿中的表达后向扩张可能先于进化过程中的任何明显功能获得。此外，Scr 在背嵴的形成中起着关键作用，背嵴是所有昆虫中分隔头部和胸部的结构。这一点可以从 RNAiScr 第一代若虫的唇部和 T1 节之间存在多余的节段得到证明，这归因于 engrailed (en) 表达的改变。类似的 Scr 表型在 Tribolium 中观察到，但在 Drosophila 或 Oncopeltus 中没有观察到，这表明控制背嵴形成的遗传结构存在谱系特异性的变异。与胚胎作用形成鲜明对比的是，Scr 在美洲大蠊的胚胎后发育中在头部区域没有功能，而是严格作用于提供 T1 节的身份。此外，最强的美洲大蠊 RNAiScr 表型发育出起源于前胸节后部的异位翅膀状组织。这一发现为昆虫翅膀的形态学起源的当前争论提供了新的见解。
