Richardson M K
Department of Anatomy, St. George's Hospital Medical School, London, England.
Dev Biol. 1995 Dec;172(2):412-21. doi: 10.1006/dbio.1995.8041.
There has been a resurgence of interest in comparative embryology. It is now important to be able to compare gene expression in different species at similar developmental stages. One phenomenon which may make it difficult to compare embryos in this way is heterochrony--a change in developmental timing during evolution. It is not clear whether heterochrony can affect the intermediate stages of embryonic development, when many important genes involved in pattern formation are expressed. A prevalent view is that these so-called phylotypic stages are resistant to evolutionary change because they are when the body plan is laid down. Haeckel's famous drawings, which show different vertebrates developing from virtually identical somite-stage embryos, are still used to support this idea. I have reexamined the morphological data relating to developmental timing in somite-stage embryos. The data reveal striking patterns of heterochrony during vertebrate evolution. These shifts in developmental timing have strongly affected the phylotypic stage, which is therefore poorly conserved and is more appropriately described as the phylotypic period. This is contrary to the impression created by Haeckel's drawings, which I show to be inaccurate and misleading. The study of gene expression in embryos which show heterochrony could give important insights into evolutionary and developmental mechanisms.
对比较胚胎学的兴趣再度兴起。如今，能够比较不同物种在相似发育阶段的基因表达变得很重要。异时性——进化过程中发育时间的变化——可能会使以这种方式比较胚胎变得困难。目前尚不清楚异时性是否会影响胚胎发育的中间阶段，而在这个阶段许多参与模式形成的重要基因会表达。一种普遍的观点是，这些所谓的系统发育型阶段对进化变化具有抗性，因为它们是身体蓝图确定的时期。海克尔著名的绘图展示了不同脊椎动物从几乎相同的体节期胚胎发育而来，这些绘图至今仍被用来支持这一观点。我重新审视了与体节期胚胎发育时间相关的形态学数据。这些数据揭示了脊椎动物进化过程中显著的异时性模式。发育时间的这些变化对系统发育型阶段产生了强烈影响，因此该阶段的保守性较差，更适合被描述为系统发育型时期。这与海克尔绘图所营造的印象相反，我发现这些绘图不准确且具有误导性。对表现出异时性的胚胎进行基因表达研究，可能会为进化和发育机制提供重要见解。
