Cruickshank Tami, Wade Michael J
Biology Department, Indiana University, Bloomington, IN, USA.
Evol Dev. 2008 Sep-Oct;10(5):583-90. doi: 10.1111/j.1525-142X.2008.00273.x.
A central goal of evolutionary developmental biology (Evo-Devo) is to synthesize comparative molecular developmental genetics and its description of the dynamic relationship between genotype and phenotype with the microevolutionary processes (mutation, random drift, and selection) of population genetics. To this end, we analyzed sequence variation of five gene classes that act sequentially to shape early embryo development in Drosophila: maternal, gap, pair-rule, segment polarity, and segment identity genes. We found two related patterns: (1) a microevolutionary pattern, wherein relative sequence variation within species is 2- to 3-fold higher for maternal-effect genes than for any other gene class; and, (2) a macroevolutionary pattern, wherein the relative sequence divergence among species for maternal-effect genes is 2- to 4-fold greater than for any other gene class. Both patterns are qualitatively and quantitatively consistent with the predictions of microevolutionary theory. Our findings connect within-species genetic variation to between-species divergence and shed light on the controversy over the existence of a "developmental hourglass," where mid-embryonic stages are more evolutionarily constrained than either earlier or later stages. Because maternal-effect genes experience relaxed selective constraint relative to zygotic-effect genes, they explore a wider mutational and phenotypic space. As a result, early acting maternal-effect genes diverge more widely across taxa and thereby broaden the base of the developmental hourglass. In contrast, later acting zygotic genes are relatively more constrained and limited in their diversification across taxa, narrowing the waist of the developmental hourglass. This pattern is obscured by genes with both maternal and zygotic expression, which experience the strongest evolutionary constraint.
进化发育生物学（Evo-Devo）的一个核心目标是将比较分子发育遗传学及其对基因型与表型之间动态关系的描述，与群体遗传学的微观进化过程（突变、随机漂变和选择）进行整合。为此，我们分析了果蝇中依次作用于塑造早期胚胎发育的五类基因的序列变异：母体效应基因、缺口基因、成对规则基因、体节极性基因和体节身份基因。我们发现了两种相关模式：（1）微观进化模式，即物种内母体效应基因的相对序列变异比其他任何基因类别高2至3倍；（2）宏观进化模式，即物种间母体效应基因的相对序列分歧比其他任何基因类别大2至4倍。这两种模式在定性和定量上都与微观进化理论的预测一致。我们的研究结果将物种内的遗传变异与物种间的分歧联系起来，并揭示了关于“发育沙漏”存在的争议，即在胚胎发育中期比早期或晚期阶段受到更强的进化限制。由于母体效应基因相对于合子效应基因受到的选择限制较宽松，它们探索了更广泛的突变和表型空间。因此，早期起作用的母体效应基因在不同分类群中分歧更大，从而拓宽了发育沙漏的底部。相比之下，后期起作用的合子基因在不同分类群中的多样化相对更受限制，从而使发育沙漏的腰部变窄。这种模式被同时具有母体和合子表达的基因所掩盖，这些基因受到最强的进化限制。
