Developmental Modules & Evolvability: The Gastropod Foregut as a Model

为什么一些生物体群体从它们第一次出现以来就出现了爆炸性的多样性，而另一些生物体群体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新兴领域的一个核心问题，该领域不满足于传统观点，即自然选择作用于后代之间的随机、基因指定的变异，提供了物种如何进化的完整故事。尽管选择显然很重要，但进化发展生物学家强调，信息从基因型到表型的转移发生在发育和发育组织的过程中，可以影响作为选择原料产生的变异的数量和种类。普遍的假说认为，在被组织成半自主模块的发育系统中，将发现最大程度的“进化性”，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分离出不同的表型。这一假设在概念上很吸引人，但它的有效性取决于找到能够证明高进化率和模块化发展组织之间联系的系统。*我的学生和我已经确定了一个很有前途的模型系统，用于研究进化性和发展模块之间的关系。我们研究了腹足类(蜗牛)的发育前景，尽管热带物种也将被包括在我们的研究中，但在加拿大丰富的海洋生物多样性资源中，腹足类(蜗牛)是多样化和丰富的。该研究将检验以下假设：1)在腹足类动物进化史的早期，两个前肠发育模块的发育时间的差异有助于促进从非摄食幼虫取食的进化，这反过来对灭绝和物种形成率产生了深远的影响；2)两个前肠发育模块之间的物理分离促进了一大群捕食性蜗牛的高进化能力，这将通过表征可能独立进化出这一关键发育创新的第二组蜗牛的发育来进行测试；以及3)前述模块之一可能已经被增选和重新利用，以允许被封装的胚胎以哺乳卵为食，这可能最终允许从生活史上失去自由游泳幼虫的封装胚胎随后重新进化为以水柱内微藻为食的游动幼虫。这项研究将为学生提供宝贵的当代形态分析方法的培训机会，同时鼓励他们扩展令人兴奋的新进化观的概念边界。

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.**