叶肢介颈器的超微形态特征及其演化意义

EVOLUTIONARY SIGNIFICANCE OF MORPHOLOGICAL DIVERSITY IN THE DORSAL ORGAN OF CLAM SHRIMPS

颈器的构造与功能在甲壳动物的演化过程中存在明显差异,其同源性受到质疑。本研究在叶肢介颈器的超微形态比较与分子系统学分析基础上,对该器官的同源性进行了讨论。颈器的超微形态分析表明,叶肢介的颈器形态多样,包括椭圆形、马蹄形、花边形、梨核形等,其中马蹄形、花边形、梨核形颈器为首次报道类型。颈器的形态在属与种分类级别上呈现一定的稳定性,如马蹄形颈器出现在2属4种个体上。基于分子数据的系统发生分析为评判颈器的同源性与演化意义提供了依据。系统分析表明,颈器并非鳃足纲与颚足类的共有衍征,而可能是泛甲壳动物的祖征;鳃足纲渔乡蚌虫科颈器存在3种状态特征:包括具颈器但不突出,颈器显著突出呈梨形,以及颈器不同程度的退化或丢失。颈器的形态特征可作为种或属级别上分类的辅征。

Comparative analyses of external morphology, internal structure, and functional interpretation suggest that dorsal organs of Crustaeea are probably not all homologous. In this study we have examined the morphology of the dorsal organ in eight species of Laevicaudata and Spinicaudata from China, and have analyzed our results together with previously documented occurrences of branchiopod dorsal organs within the phylogenetic framework established on the basis of the COI gene from 18 species of Spinicaudata. We illustrate five morphological forms of dorsal organ, oval, subrectangufar, horseshoe, petal and pear-core. These morphologies of dorsal organ are stable amongst individuals of species and genera; therefore we suggest that such features of dorsal organs may be used as auxiliary criteria in the classification of clamp shrimps. For example, the horseshoe type occurs in four species and two genera of Spinicaudata.Given the inter-generic relationships suggested by molecular phylogeny, the evolutionary history of the dorsal organ within the branchiopods is interpreted and illustrated. The most parsimonious reconstruction using MacClade suggests that an unelevated dorsal organ is the ancestral state for Limnadiidae and that the pyriform form is a synapomorphy for Metalimnadia ＋ Eulimnadia ＋ Limnadiopsis ＋ Limnadia AU. In contrast the shared presence at the next level higher in the hierarchy of a pyriform form of dorsal organ in Limnadia lenticularis and Metalimnadia ＋ Eulimnadia ＋ Limnadiopsis ＋ Limnadia AU is interpreted as a result of parallel evolution.