Jackson Daniel J, Wörheide Gert, Degnan Bernard M
School of Integrative Biology, University of Queensland, Brisbane Queensland 4072, Australia.
BMC Evol Biol. 2007 Sep 10;7:160. doi: 10.1186/1471-2148-7-160.
The Mollusca constitute one of the most morphologically and ecologically diverse metazoan phyla, occupying a wide range of marine, terrestrial and freshwater habitats. The evolutionary success of the molluscs can in part be attributed to the evolvability of the external shell. Typically, the shell first forms during embryonic and larval development, changing dramatically in shape, colour and mineralogical composition as development and maturation proceeds. Major developmental transitions in shell morphology often correlate with ecological transitions (e.g. from a planktonic to benthic existence at metamorphosis). While the genes involved in molluscan biomineralisation are beginning to be identified, there is little understanding of how these are developmentally regulated, or if the same genes are operational at different stages of the mollusc's life.
Here we relate the developmental expression of nine genes in the tissue responsible for shell production - the mantle - to ecological transitions that occur during the lifetime of the tropical abalone Haliotis asinina (Vetigastropoda). Four of these genes encode evolutionarily ancient proteins, while four others encode secreted proteins with little or no identity to known proteins. Another gene has been previously described from the mantle of another haliotid vetigastropod. All nine genes display dynamic spatial and temporal expression profiles within the larval shell field and juvenile mantle.
These expression data reflect the regulatory complexity that underlies molluscan shell construction from larval stages to adulthood, and serves to highlight the different ecological demands placed on each stage. The use of both ancient and novel genes in all stages of shell construction also suggest that a core set of shell-making genes was provided by a shared metazoan ancestor, which has been elaborated upon to produce the range of molluscan shell types we see today.
软体动物是形态和生态最为多样的后生动物门类之一，占据了广泛的海洋、陆地和淡水栖息地。软体动物在进化上的成功部分可归因于其外壳的可进化性。通常，外壳在胚胎和幼虫发育期间首先形成，随着发育和成熟过程的推进，其形状、颜色和矿物成分会发生显著变化。外壳形态的主要发育转变通常与生态转变相关（例如在变态时从浮游生活转变为底栖生活）。虽然参与软体动物生物矿化的基因已开始被识别，但对于这些基因如何在发育过程中受到调控，或者相同的基因在软体动物生命的不同阶段是否发挥作用，人们了解甚少。
在这里，我们将负责外壳生成的组织——外套膜中九个基因的发育表达与热带鲍鱼（皱纹盘鲍，腹足纲）一生当中发生的生态转变联系起来。其中四个基因编码进化上古老的蛋白质，另外四个基因编码的分泌蛋白与已知蛋白几乎没有或完全没有同源性。另一个基因先前已在另一种皱纹盘鲍腹足纲动物的外套膜中被描述过。所有九个基因在幼虫外壳区域和幼体外套膜内均显示出动态的时空表达模式。
这些表达数据反映了从幼虫阶段到成年阶段软体动物外壳构建背后的调控复杂性，并突出了每个阶段所面临的不同生态需求。在外壳构建的所有阶段都使用古老和新颖的基因也表明，一组核心的造壳基因是由后生动物的共同祖先提供的，在此基础上进一步发展，从而产生了我们今天所看到的各种软体动物外壳类型。
