相关物种中胚胎的出现在发育过程中途发生融合，之后又分开，这个现象被称为“发育沙漏”。沙漏模型由Karl Ernst von Baer建立在经典观察结果之上，后来由Ernst Haeckel（达尔文在德国的支持者）广泛普及。但这种所谓的系统发生阶段是否真的存在？两个研究小组利用对比方法对这个模型进行了验证，并为其提供了支持。在该系统发生阶段所表达的基因在演化上比在其他阶段所表达的基因更古老、更保守。本期封面为Haeckel在1879年根据果蝇胚胎形成过程中的表达模式的数千幅图像画出的一幅图。（生物谷Bioon.com）
生物谷推荐原文出处：
Nature doi:10.1038/nature09634
Gene expression divergence recapitulates the developmental hourglass model
Alex T. Kalinka,Karolina M. Varga,Dave T. Gerrard,Stephan Preibisch,David L. Corcoran,Julia Jarrells,Uwe Ohler,Casey M. Bergman& Pavel Tomancak
The observation that animal morphology tends to be conserved during the embryonic phylotypic period (a period of maximal similarity between the species within each animal phylum) led to the proposition that embryogenesis diverges more extensively early and late than in the middle, known as the hourglass model1, 2. This pattern of conservation is thought to reflect a major constraint on the evolution of animal body plans3. Despite a wealth of morphological data confirming that there is often remarkable divergence in the early and late embryos of species from the same phylum4, 5, 6, 7, it is not yet known to what extent gene expression evolution, which has a central role in the elaboration of different animal forms8, 9, underpins the morphological hourglass pattern. Here we address this question using species-specific microarrays designed from six sequenced Drosophila species separated by up to 40 million years. We quantify divergence at different times during embryogenesis, and show that expression is maximally conserved during the arthropod phylotypic period. By fitting different evolutionary models to each gene, we show that at each time point more than 80% of genes fit best to models incorporating stabilizing selection, and that for genes whose evolutionarily optimal expression level is the same across all species, selective constraint is maximized during the phylotypic period. The genes that conform most to the hourglass pattern are involved in key developmental processes. These results indicate that natural selection acts to conserve patterns of gene expression during mid-embryogenesis, and provide a genome-wide insight into the molecular basis of the hourglass pattern of developmental evolution.
