Comparative venom transcriptomics of centipedes: evolutionary diversification of a key ecological adaptation

尽管大多数门外汉在识别水母、黄蜂、蜘蛛和蝎子等生物是潜在危险的有毒生物方面没有什么问题，但知道大约3300种蜈蚣拥有强大毒液的人要少得多。一对强大的毒爪位于头部后面，它们拥有巨大的毒腺，其中含有复杂的毒液成分鸡尾酒。不幸的是，我们对蜈蚣毒液的组成几乎一无所知，这不仅给我们对一个具有重要生态意义的群体的理解留下了很大的空白，而且也损害了我们对动物界毒液进化的一般理解。这项研究旨在通过对蜈蚣毒液的组成进行第一次广泛而深入的分析来纠正这种无知。该项目将采用遗传学方法，并将描述五种蜈蚣毒液的毒素特征。这五个物种被选为代表蜈蚣的所有主要群体。对于每个物种，将描述多达50万个mRNA序列。这些前体分子是生产毒素蛋白的模板。通过比较这些序列在选定物种中的图谱，我们可以开始解决与毒液和有毒生物体进化有关的重要问题。可以回答的最基本的问题很简单：蜈蚣的毒腺中表达了哪些毒素？这个问题的答案将是回答其他问题的基础。蜈蚣的毒液和其他种类的毒液有很多相同的毒素吗？我们已经从以前的研究中知道，不同的有毒动物群体可以在它们的毒液中招募许多类似的毒素。他们通过提取一个编码正常身体蛋白质的基因，复制它，并在毒液腺体中特异地表达其中一个拷贝来做到这一点。基因序列的变化会引起蛋白质的变化，这会改变蛋白质的功能，使其成为更有效的毒素。然而，初步研究表明蜈蚣毒液可能含有许多毒素(还没有？)在其他群体中也能找到。这项研究将让我们了解蜈蚣中有多少毒液成分是它们所独有的。新数据可以解决的另一个主要问题是蜈蚣物种的多样化和它们的毒素是否齐头并进。通过整合毒素基因和蜈蚣的系谱，我们可以推断蜈蚣进化的特定时期是否也与毒素进化的周期有关。通过结合其他有毒和无毒动物的数据，我们还可以从蜈蚣的毒素基因进化出什么样的基因。由于到目前为止还没有蜈蚣的数据，我们可以广泛地重新评估我们目前对所有动物中毒素进化模式的理解。我们还可以利用新的数据来询问哪些过程在蜈蚣毒液的组成中起到了重要作用。一个可能很重要的因素是蜈蚣吃的不同种类的猎物的范围。与专门研究某一特定猎物的物种相比，对付范围广泛的猎物的物种可能会有更多种类的毒素。通过将毒液成分与它们肠道中识别的猎物的多样性联系起来，我们可以开始回答这个问题。最后，通过观察毒素序列发生了哪些类型的变化，以及在其中哪些部分发生了变化，我们可以推断对形成毒素多样性最重要的选择压力的类型和强度。

Although most lay people have little problem in identifying creatures like jellyfish, wasps, spiders, and scorpions as potentially dangerous venomous organisms, far fewer know that the ca. 3,300 species of centipedes possess potent venoms. A pair of strong venom claws is located just behind the head, and they house large venom glands that contain complex cocktails of venom components. Unfortunately we know almost nothing about the make-up of centipede venom, which leaves not only a large hole in our understanding of an ecologically important group, but it also compromises our general understanding of venom evolution in the animal kingdom. This study aims to remedy this ignorance by performing the first extensive and intensive analysis of the composition of centipede venoms. This project will take a genetic approach, and will characterize the toxin profiles from the venoms of five species of centipedes. These five species have been chosen to represent all major groups of centipedes. For each species, up to half a million mRNA sequences will be characterized. These precursor molecules are the templates for the production of toxin proteins. By comparing the profiles of these sequences across the selected species we can start to address important questions relating to the evolution of venoms and venomous organisms. The most basic question that can be answered is simply: what toxins are expressed in the venom glands of centipedes? The answer to this question will be the basis for answering the other questions. Does centipede venom have many toxins in common with the venoms of other groups? We already know from previous research that different groups of venomous animals can recruit many similar toxins into their venom. They do this by taking a gene coding for a normal body protein, duplicating it, and expressing one of the copies specifically in the venom gland. Changes in the sequence of the gene can create changes in the protein, and this can change the protein's function to be more effective as a toxin. Preliminary work, however, has suggested that centipede venom may contain many toxins not (yet?) found in other groups. This study will allow us to see how many venom components in centipedes are unique to them. Another major question that can be addressed with the new data is whether the diversification of the centipede species and their toxins went hand in hand. By integrating the family trees of the toxin genes and the centipedes we can infer whether particular episodes in the evolution of centipedes are associated with bouts of toxin evolution as well. We can also infer, by incorporating data from other venomous and non-venomous animals, from what kind of genes the toxin genes in centipedes have evolved. Since there were no centipede data available till now, we can broadly reassess our current understanding of the pattern of toxin in evolution across all animals. We can use the new data also to ask what kinds of processes were important in shaping the composition of centipede venom. One factor that is likely to be important is the range of different kinds of prey the centipede eats. A species tackling a broad range of prey may be expected to have a greater diversity of toxins than a species specializing in just a particular prey species. By correlating venom composition with the diversity of prey identified in their guts we can begin to answer this question. Lastly, by looking at what kinds of changes have occurred in the toxin sequences, and in which parts of them, we can infer the types and intensities of selection pressures that were most important in shaping toxin diversity.