Mineta Katsuhiko, Nakazawa Masumi, Cebria Francesc, Ikeo Kazuho, Agata Kiyokazu, Gojobori Takashi
Center for Information Biology and DNA Data Bank of Japan, National Institute of Genetics, Yata 1111, Mishima, Shizuoka 411-8540, Japan.
Proc Natl Acad Sci U S A. 2003 Jun 24;100(13):7666-71. doi: 10.1073/pnas.1332513100. Epub 2003 Jun 11.
Among the bilateral animals, a centralized nervous system is found in both the deuterostome and protostome. To address the question of whether the CNS was derived from a common ancestor of deuterostomes and protostomes, it is essential to know kinds of genes existed in the CNS of the putative common ancestor and to trace the evolutionary divergence of genes expressed in the CNS. To answer these questions, we took a comparative approach using different species, particularly focusing on one of the lower bilateral animals, the planarian (Platyhelminthes, Tricladida), which is known to possess a CNS. We determined the nucleotide sequence of ESTs from the head portion of planarians, obtaining 3,101 nonredundant EST clones. As a result of homology searches, we found that 116 clones had significant similarity to known genes related to the nervous system. Here, we compared these 116 planarian EST clones with all ORFs of the complete genome sequences of the human, fruit fly, and nematode, and showed that >95% of these 116 nervous system-related genes, including genes involved in brain or neural morphogenesis, were commonly shared among these organisms, thus providing evidence at the molecular level for the existence of a common ancestral CNS. Interestingly, we found that approximately 30% of planarian nervous system-related genes had homologous sequences in Arabidopsis and yeast, which do not possess a nervous system. This implies that the origin of nervous system-related genes greatly predated the emergence of the nervous system, and that these genes might have been recruited toward the nervous system.
在两侧对称动物中，后口动物和原口动物都有一个集中的神经系统。为了解决中枢神经系统是否起源于后口动物和原口动物的共同祖先这一问题，有必要了解假定共同祖先的中枢神经系统中存在的基因种类，并追踪中枢神经系统中表达的基因的进化分歧。为了回答这些问题，我们采用了比较不同物种的方法，特别关注一种较低等的两侧对称动物——涡虫（扁形动物门，三肠目），已知它拥有中枢神经系统。我们测定了涡虫头部的EST核苷酸序列，获得了3101个非冗余EST克隆。同源性搜索的结果表明，有116个克隆与已知的神经系统相关基因有显著相似性。在这里，我们将这116个涡虫EST克隆与人类、果蝇和线虫完整基因组序列的所有开放阅读框进行了比较，结果表明，这116个与神经系统相关的基因中，超过95%，包括参与脑或神经形态发生的基因，在这些生物中是共同存在的，从而在分子水平上为共同祖先中枢神经系统的存在提供了证据。有趣的是，我们发现大约30%的涡虫神经系统相关基因在拟南芥和酵母中具有同源序列，而拟南芥和酵母并不拥有神经系统。这意味着神经系统相关基因的起源远远早于神经系统的出现，并且这些基因可能是被招募到神经系统中的。
