Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire 03755, USA.
Nat Commun. 2010 Oct 19;1(7):99. doi: 10.1038/ncomms1102.
Concentration gradients of morphogenic proteins pattern the embryonic axes of Drosophila by activating different genes at different concentrations. The neurogenic ectoderm enhancers (NEEs) activate different genes at different threshold levels of the Dorsal (Dl) morphogen, which patterns the dorsal/ventral axis. NEEs share a unique arrangement of highly constrained DNA-binding sites for Dl, Twist (Twi), Snail (Sna) and Suppressor of Hairless (Su(H)), and encode the threshold variable in the precise length of DNA that separates one well-defined Dl element from a Twi element. However, NEEs also possess dense clusters of variant Dl sites. Here, we show that these increasingly variant sites are eclipsed relic elements, which were superseded by more recently evolved threshold encodings. Given the divergence in egg size during Drosophila lineage evolution, the observed characteristic clusters of divergent sites indicate a history of frequent selection for changes in threshold responses to the Dl morphogen gradient and confirm the NEE structure/function model.
形态发生蛋白的浓度梯度通过在不同浓度下激活不同的基因来塑造果蝇胚胎轴。神经胚外增强子（NEE）在 Dorsal（Dl）形态发生素的不同阈值水平下激活不同的基因，从而塑造背/腹轴。NEE 共享用于 Dl、Twist（Twi）、Snail（Sna）和 Hairless Suppressor（Su（H）的高度约束 DNA 结合位点的独特排列，并且在将一个定义明确的 Dl 元件与 Twi 元件分开的 DNA 精确长度上编码阈值变量。然而，NEE 也拥有密集的变体 Dl 位点簇。在这里，我们表明这些越来越多的变体位点是被取代的遗迹元素，它们被最近进化的阈值编码所取代。考虑到果蝇谱系进化过程中卵大小的差异，观察到的特征性发散位点簇表明，对 Dl 形态发生素梯度的阈值反应的频繁选择发生了变化，这证实了 NEE 结构/功能模型。
