Universität Rostock, Allgemeine & Spezielle Zoologie, Institut für Biowissenschaften, Universitaetsplatz 2, Rostock 18055, Germany.
Front Zool. 2013 Dec 11;10(1):76. doi: 10.1186/1742-9994-10-76.
Malacostracan evolutionary history has seen multiple transformations of ontogenetic mode. For example direct development in connection with extensive brood care and development involving planktotrophic nauplius larvae, as well as intermediate forms are found throughout this taxon. This makes the Malacostraca a promising group for study of evolutionary morphological diversification and the role of heterochrony therein. One candidate heterochronic phenomenon is represented by the concept of the 'egg-nauplius', in which the nauplius larva, considered plesiomorphic to all Crustacea, is recapitulated as an embryonic stage.
Here we present a comparative investigation of embryonic muscle differentiation in four representatives of Malacostraca: Gonodactylaceus falcatus (Stomatopoda), Neocaridina heteropoda (Decapoda), Neomysis integer (Mysida) and Parhyale hawaiensis (Amphipoda). We describe the patterns of muscle precursors in different embryonic stages to reconstruct the sequence of muscle development, until hatching of the larva or juvenile. Comparison of the developmental sequences between species reveals extensive heterochronic and heteromorphic variation. Clear anticipation of muscle differentiation in the nauplius segments, but also early formation of longitudinal trunk musculature independently of the teloblastic proliferation zone, are found to be characteristic to stomatopods and decapods, all of which share an egg-nauplius stage.
Our study provides a strong indication that the concept of nauplius recapitulation in Malacostraca is incomplete, because sequences of muscle tissue differentiation deviate from the chronological patterns observed in the ectoderm, on which the egg-nauplius is based. However, comparison of myogenic sequences between taxa supports the hypothesis of a zoea-like larva that was present in the last common ancestor of Eumalacostraca (Malacostraca without Leptostraca). We argue that much of the developmental sequences of larva muscle patterning were retained in the eumalacostracan lineage despite the reduction of free swimming nauplius larvae, but was severely reduced in the peracaridean clade.
软甲纲动物的进化历史经历了多种个体发生模式的转变。例如，与广泛的亲代抚育和涉及浮游幼体无节幼体幼虫的发育有关的直接发育，以及在整个这一类群中发现的中间形式。这使得软甲纲成为研究进化形态多样性和其中异时性作用的有前途的群体。一个候选的异时性现象是“卵-无节幼体”的概念，其中无节幼体幼虫被认为是所有甲壳动物的祖征，被重新概括为一个胚胎阶段。
在这里，我们对软甲纲的四个代表进行了胚胎肌肉分化的比较研究：Falcoctopus falcatus（十足目）、Neocaridina heteropoda（十足目）、Neomysis integer（十足目）和Parhyale hawaiensis（端足目）。我们描述了不同胚胎阶段的肌肉前体模式，以重建肌肉发育的顺序，直到幼虫或幼体孵化。对物种间发育序列的比较揭示了广泛的异时性和异形性变化。在十足目和十足目中，发现无节幼体节段的肌肉分化提前，并且独立于端细胞增殖区形成早期的纵向体壁肌肉，这是特征性的，所有这些都具有卵-无节幼体阶段。
我们的研究强烈表明，软甲纲中无节幼体的重演概念是不完整的，因为肌肉组织分化的序列偏离了基于无节幼体的外胚层的时间模式。然而，对分类群之间的肌原序列的比较支持了在真软甲类（没有软甲类的软甲类）的最后共同祖先中存在的幼虫状幼虫的假说。我们认为，尽管自由游动的无节幼体幼虫减少，但幼虫肌肉模式形成的大部分发育序列在真软甲类谱系中得以保留，但在等足类群中严重减少。
