Significance of heterochrony in ostracode higher taxonomy

这项研究的成果主要有以下几点：介形类系统发育关系的评估程序有了很大的改进。介形虫线粒体DNA分析的成功带来了第一个改进，并根据COI区DNA分析估计了3科6属11种之间的亲缘关系。然而，也证明了COI区对于近缘种的估计是足够好的，但由于碱基更替的速度太快，不适合于较高的类群。另一种新的评估方法，即通过个体发育对孔隙系统进行分化的过程，已被证明对较低和较高的分类群都非常有效。早期个体发育阶段和晚期个体发育阶段分别适用于较高和较低的系统发育关系。这两种方法首次为我们提供了定量和客观的关系。在重新排列了5个海鞘动物科的介形虫关系后，仔细分析了它们的形态。结果表明，铰链结构被认为是高等类群中稳定的特征之一，但异时性很容易改变。异时性在介形虫系统学中起着重要的作用，其转化本身具有独特的趋势。在晚中生代或早第三纪，各种具有特定铰链类型的类群爆炸性地出现。然后，发现其优势种具有足胚型铰链型的后代分类群在新近纪演化。这是一个罕见的成功描述了一种明显的进化趋势的案例研究。

Essence of the results of this project can be summarized as follows.The procedure to estimate the phylogenetic relationship of ostracodes has been drastically improved. The first improvement was brought by the success of the analysis of ostracode mtDNA,and the relatioship between 11 species belonging to 6 genera, 3 families was estimated based on the DNA analysis of COI region. It is, however, also proved that the COI region is good enough for the estimate of close species, but is not appropriate for higher taxa due to the rapid rate of the replacement of the bases. Another new procedure of the estimate, the process of differentiation of pore-systems through ontogeny, has been demonstrated to be very effective for both lower and higher taxa. Early and late ontogenetic stages are available for higher and lower phylogenetic relatioship, respectively. The two procedures give us a quantitative and objective relationship for the first time.After rearrangement of some ostracode relationship belonging to five cytheracean families, their morphology was carefully analyzed. Results showed that the structure of hingement, which was thought to be one of the stable characters in higher taxa, could be easily transformed by heterochrony. The heterochrony plays an important part in ostracode sytematics and the transformation itself has a unique tendency as follows. Various taxa with specific types of hingement explosively appeared during late Mesozoic or early Tertiary. Then, descendant taxa with paedomorphic type of hingement of their ascendants were found to have evolved during Neogene. This is a rare case study that succeeded to describe a clear evolutionary tendency.