相關(guān)物種中胚胎的出現(xiàn)在發(fā)育過(guò)程中途發(fā)生融合,，之后又分開(kāi),，這個(gè)現(xiàn)象被稱(chēng)為“發(fā)育沙漏”。沙漏模型由Karl Ernst von Baer建立在經(jīng)典觀察結(jié)果之上,，后來(lái)由Ernst Haeckel（達(dá)爾文在德國(guó)的支持者）廣泛普及,。但這種所謂的系統(tǒng)發(fā)生階段是否真的存在？?jī)蓚€(gè)研究小組利用對(duì)比方法對(duì)這個(gè)模型進(jìn)行了驗(yàn)證,，并為其提供了支持,。在該系統(tǒng)發(fā)生階段所表達(dá)的基因在演化上比在其他階段所表達(dá)的基因更古老、更保守,。本期封面為Haeckel在1879年根據(jù)果蠅胚胎形成過(guò)程中的表達(dá)模式的數(shù)千幅圖像畫(huà)出的一幅圖,。（生物谷Bioon.com）
生物谷推薦原文出處：
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.
