Developmental Modules & Evolvability: The Gastropod Foregut as a Model

为什么有些生物群体从第一次出现起就发生了爆炸性的变化，而另一些生物群体却沿着了数百万年，没有发生太大的变化？这个谜团是新兴的进化发育生物学领域的核心问题，它不满意传统观点，即自然选择作用于后代中随机的遗传特异性变异，提供了物种如何进化的完整故事。虽然选择显然是重要的，进化发育生物学家强调，信息从基因型到表型的转移发生在发育过程中，发育组织可以影响作为选择原材料产生的变异的数量和种类。流行的假说认为，最大量的“进化性”将被发现在发展系统中组织成半自主模块，可以剥离出不同的表型，而不破坏整个复杂结构的发展完整性。这一假说在概念上很有吸引力，但其有效性取决于找到一个系统，在该系统中，可以证明高进化率和发展的模块化组织之间的联系。 我和我的学生已经确定了一个有前途的模型系统，用于调查进化性和发展模块之间的关系。我们研究腹足类（蜗牛），这是加拿大丰富的海洋生物多样性资源的多样性和丰富的发展中的前肠，虽然热带物种也将包括在我们的研究。研究将检验以下假设：1）两个前肠模块发育时间的差异有助于促进腹足类进化史早期从非摄食幼虫进化为摄食，这反过来又对灭绝和物种形成率产生深远影响，2）两个前肠发育模块之间的物理分离促进了大群捕食性蜗牛的高进化能力，这将通过表征第二组蜗牛的发育来测试，所述第二组蜗牛可能已经独立地进化了这种关键的发育创新，以及3）前肠模块之一可能已经被增选并重新利用以允许封装的胚胎以保育卵为食，这可能最终使那些在生活史中失去了自由游动幼虫的包囊胚胎，进化成游泳的幼虫，以水柱中的微藻为食。这项研究将为学生提供宝贵的培训机会，在形态分析的当代方法，同时鼓励他们扩展一个令人兴奋的新的进化观的概念界限。

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.