Developmental Modules & Evolvability: The Gastropod Foregut as a Model

为什么有些生物群自首次出现以来就发生了爆炸性的多样化，而另一些生物群却在数百万年的时间里毫无变化？这个谜团是进化发育生物学这个新兴领域的核心问题，它不满意传统的观点，即自然选择作用于后代之间随机的、基因指定的变异，提供了物种进化的完整故事。虽然选择显然很重要，进化发育生物学家强调，信息从基因型到表型的转移是通过发育过程发生的，发育组织可以影响作为选择原材料的变异的数量和种类。普遍的假设认为，在被组织成半自主模块的发育系统中，最大程度的“可进化性”将被发现，这些模块可以在不破坏整个复杂结构的发育完整性的情况下衍生出不同的表型。这一假设在概念上很有吸引力，但其有效性取决于找到能够证明高进化率与模块化发展组织之间存在联系的系统。***我和我的学生已经确定了一个有前途的模型系统来研究可进化性和发育模块之间的关系。我们研究腹足类动物（蜗牛）的前肠发育，这些动物在加拿大丰富的海洋生物多样性资源中是多样化和丰富的，尽管热带物种也将包括在我们的研究中。本研究将检验以下假设：1)在腹足类动物的进化史早期，两个前肠模块的发育时间差异有助于促进非摄食幼虫的摄食进化，从而对灭绝和物种形成率产生深远影响；2)两个前肠模块之间的物理分离促进了一大群掠食性蜗牛的高进化能力。这将通过对第二组蜗牛的发育特征进行测试，这些蜗牛可能已经独立地进化出了这一关键的发育创新。3)其中一个前肠模块可能已经被选择并重新利用，以允许被封装的胚胎以护理卵为食，这可能最终使被封装的胚胎在其生活史中失去了自由游动的幼虫，随后重新进化成以水柱内的微藻为食的游动幼虫。本研究将为学生提供宝贵的训练机会，学习当代形态分析方法，同时鼓励他们拓展令人兴奋的进化新观点的概念边界

Why have some groups of organisms diversified explosively since their first appearance, while others have puttered along for millions of years without much change at all? This mystery is a core issue for the emerging field of Evolutionary Developmental Biology, which is dissatisfied with the traditional view that natural selection acting on random, genetically-specified variation among offspring provides the complete story about how species evolve. Although selection is clearly important, evolutionary developmental biologists emphasize that transfer of information from genotype to phenotype occurs via the process of development and developmental organization can influence the amount and kind of variation produced as raw material for selection. The prevailing hypothesis suggests that the greatest amount of `evolvability' will be found among developmental systems organized into semi-autonomous modules that can spin off variant phenotypes without disrupting the developmental integrity of whole complex structures. This hypothesis is conceptually appealing but its validity depends on finding systems in which a link between high rates of evolution and a modular organization of development can be demonstrated.***My students and I have identified a promising model system for investigating the relationship between evolvability and developmental modules. We study the developing foregut in gastropods (snails), which are diverse and abundant within Canada's rich resources in marine biodiversity, although tropical species will also be included in our research. The research will test the following hypotheses: 1) differences in time of development for two foregut modules helped facilitate the evolution of feeding from non-feeding larvae early in the evolutionary history of gastropods, which in turn had a profound effect on extinction and speciation rates, 2) a physical dissociation between two foregut developmental modules has facilitated high capacity for evolvability in a large group of predatory snails, which will be tested by characterizing development in a second group of snails that may have independently evolved this key developmental innovation, and 3) one of the foregut modules may have been co-opted and re-purposed to allow encapsulated embryos to feed on nurse eggs, which may have ultimately allowed encapsulated embryos that have lost free-swimming larvae from their life history to subsequently re-evolve into swimming larvae that feed on microalgae within the water column. This research will provide valuable training opportunities for students in contemporary methods of morphological analysis, while encouraging them to extend the conceptual boundaries of an exciting new view of evolution.**