Johnson Andrew D, Crother Brian, White Mary E, Patient Roger, Bachvarova Rosemary F, Drum Matthew, Masi Thomas
Department of Genetics, University of Nottingham, Queen's Medical Centre, Nottingham NG7 2UH, UK.
Philos Trans R Soc Lond B Biol Sci. 2003 Aug 29;358(1436):1371-9. doi: 10.1098/rstb.2003.1331.
How germ cells are specified in the embryos of animals has been a mystery for decades. Unlike most developmental processes, which are highly conserved, embryos specify germ cells in very different ways. Curiously, in mouse embryos germ cells are specified by extracellular signals; they are not autonomously specified by maternal germ cell determinants (germ plasm), as are the germ cells in most animal model systems. We have developed the axolotl (Ambystoma mexicanum), a salamander, as an experimental system, because classic experiments have shown that the germ cells in this species are induced by extracellular signals in the absence of germ plasm. Here, we provide evidence that the germ cells in axolotls arise from naive mesoderm in response to simple inducing agents. In addition, by analysing the sequences of axolotl germ-cell-specific genes, we provide evidence that mice and urodele amphibians share a common mechanism of germ cell development that is ancestral to tetrapods. Our results imply that germ plasm, as found in species such as frogs and teleosts, is the result of convergent evolution. We discuss the evolutionary implications of our findings.
几十年来，动物胚胎中的生殖细胞是如何被指定的一直是个谜。与大多数高度保守的发育过程不同，胚胎以非常不同的方式指定生殖细胞。奇怪的是，在小鼠胚胎中，生殖细胞是由细胞外信号指定的；它们不像大多数动物模型系统中的生殖细胞那样由母体生殖细胞决定因素（生殖质）自主指定。我们已经开发了一种蝾螈（美西钝口螈）作为实验系统，因为经典实验表明，在没有生殖质的情况下，该物种的生殖细胞是由细胞外信号诱导产生的。在这里，我们提供证据表明，蝾螈的生殖细胞是由原始中胚层在简单诱导因子的作用下产生的。此外，通过分析蝾螈生殖细胞特异性基因的序列，我们提供证据表明，小鼠和有尾两栖动物共享一种生殖细胞发育的共同机制，这种机制是四足动物祖先所具有的。我们的结果表明，青蛙和硬骨鱼等物种中发现的生殖质是趋同进化的结果。我们讨论了我们研究结果的进化意义。
