Jirikowski Günther Joseph, Wolff Carsten, Richter Stefan
Institut für Biowissenschaften, Allgemeine und Spezielle Zoologie, Universität Rostock, Universitätsplatz 2, 18055 Rostock, Germany.
Institut für Biologie, Vergleichende Zoologie, Humboldt-Universität zu Berlin, Philippstr. 13, Haus 2, 10115 Berlin, Germany.
Evodevo. 2015 Mar 11;6:4. doi: 10.1186/2041-9139-6-4. eCollection 2015.
Within Malacostraca (Crustacea), direct development and development through diverse forms of larvae are found. Recent investigations suggest that larva-related developmental features have undergone heterochronic evolution in Malacostraca. In the light of current phylogenetic hypotheses, the free-swimming nauplius larva was lost in the lineage leading to Malacostraca and evolved convergently in the malacostracan groups Dendrobranchiata and Euphausiacea. Here we reconstruct the evolutionary history of eumalacostracan (Malacostraca without Phyllocarida) development with regard to early appendage morphogenesis, muscle and central nervous system development, and determine the heterochronic transformations involved in changes of ontogenetic mode.
Timing of 33 developmental events from the different tissues was analyzed for six eumalacostracan species (material for Euphausiacea was not available) and one outgroup, using a modified version of Parsimov-based genetic inference (PGi). Our results confirm previous suggestions that the event sequence of nauplius larva development is partly retained in embryogenesis of those species which do not develop such a larva. The ontogenetic mode involving a nauplius larva was likely replaced by direct development in the malacostracan stem lineage. Secondary evolution of the nauplius larva of Dendrobranchiata from this ancestral condition, involved only a very small number of heterochronies, despite the drastic change of life history. In the lineage leading to Peracarida, timing patterns of nauplius-related development were lost. Throughout eumalacostracan evolution, events related to epidermal and neural tissue development were clearly less affected by heterochrony than events related to muscle development.
Weak integration between mesodermal and ectodermal development may have allowed timing in muscle formation to be altered independently of ectodermal development. We conclude that heterochrony in muscle development played a crucial role in evolutionary loss and secondary evolution of a nauplius larva in Malacostraca.
在软甲纲（甲壳纲）中，存在直接发育以及通过多种幼虫形式发育的情况。最近的研究表明，与幼虫相关的发育特征在软甲纲中经历了异时进化。根据当前的系统发育假说，自由游动的无节幼虫在通向软甲纲的谱系中消失，并在软甲纲的枝鳃亚目和磷虾亚目中趋同进化。在此，我们针对真软甲亚纲（不包括叶虾亚目的软甲纲）的发育进化史进行重建，涉及早期附肢形态发生、肌肉和中枢神经系统发育，并确定个体发育模式变化中所涉及的异时转变。
使用基于Parsimov的遗传推断（PGi）的改进版本，对六个真软甲亚纲物种（无法获取磷虾亚目的材料）和一个外类群不同组织的33个发育事件的时间进行了分析。我们的结果证实了先前的推测，即无节幼虫发育的事件序列在那些不发育此类幼虫的物种的胚胎发生中部分得以保留。涉及无节幼虫的个体发育模式可能在软甲纲的干群谱系中被直接发育所取代。枝鳃亚目的无节幼虫从这种祖先状态的次生进化，尽管生活史发生了剧烈变化，但仅涉及极少数异时事件。在通向囊虾总目的谱系中，与无节幼虫相关的发育时间模式消失了。在整个真软甲亚纲的进化过程中，与表皮和神经组织发育相关的事件明显比与肌肉发育相关的事件受异时性的影响更小。
中胚层和外胚层发育之间的弱整合可能使得肌肉形成的时间能够独立于外胚层发育而改变。我们得出结论，肌肉发育中的异时性在软甲纲无节幼虫的进化丧失和次生进化中起到了关键作用。
