Living on the edge: genetic variability and range dynamics of the American dog tick

每个物种都有独特的地理分布。一些物种的活动范围扩大是为了应对气候变化。如果物种是重要的食物来源、具有文化意义、濒临灭绝的关键物种、入侵物种或影响野生动物、家畜和人类健康的寄生虫或微生物，那么详细了解整个景观的物种范围和丰富程度就尤为重要。例如，蜱传病原体给全世界的养牛者造成了重大的经济损失。了解是什么因素限制了物种的范围仍然是生态学和进化生物学的一个关键问题。我的长期研究目标是了解生态和进化因素如何影响寄生虫的分布和丰度，并揭示寄生和内共生作为基本生态相互作用和选择驱动因素的力量。中心-外围假说（CPH）假设，在范围边缘的个体经历了更恶劣的生态条件，并且比那些在更“核心”种群中的个体适应性更低。范围边缘的种群较小，遗传多样性较低，并且在超过分布极限的栖息地中应对环境条件的适应潜力降低。尽管CPH对物种分布的限制提供了一个一般性的解释，但它目前的形式并不代表一个普遍的规则。此外，很少有研究检查CPH是否适用于占所有动物30-50%的寄生虫。在接下来的五年里，我们将使用一种独特的组合方法（实验室和分子工具的现场实验），通过检查三个相互关联的目标，揭示遗传变异和物种相互作用如何影响蜱虫的分布和丰度：(1)比较了“核心”种群与“边缘”种群的遗传多样性和差异；(2)研究了掠食性蚂蚁对蜱生存的影响；(3)阐明了蜱的生存是否受到细菌内共生体的影响。第三个目标是独特的，因为内共生体对其蜱宿主生存的影响在很大程度上被忽视了。美洲狗蜱是本研究的重点生物，因为它在北美有广泛的地理范围，在萨斯喀彻温省向北和向西扩展。进一步了解是什么限制了D. variabilis的范围扩展，对于其他研究人员制定有效的策略来控制该物种，以及目前正在扩大其范围到加拿大的其他寄生节肢动物媒介及其病原体也很重要。我的研究项目还有助于培养研究生和本科生，以及加拿大和世界其他地区未来的研究领导者，他们将有能力应对寄生节肢动物及其病原微生物日益增加的经济和社会负担。

Each species has a unique geographical distribution. Range expansion by some species is in response to climate change. Detailed knowledge of the range and abundance of species across the landscape is particularly important if they are a vital food source, of cultural significance, keystone species at risk of extinction, invasive species, or parasites or microbes that affect the health of wildlife, domestic animals and humans. Tick-borne pathogens, for example, cause significant economic losses for cattle producers worldwide. Understanding what factors restrict the ranges of species remains a key issue in ecology and evolutionary biology. My long-term research goals are to understand how ecological and evolutionary factors influence the distribution and abundance of parasites, and to unveil the strength of parasitism and endosymbiosis as fundamental ecological interactions and drivers of selection. The centre-periphery hypothesis (CPH) postulates that individuals at the range edge experience harsher ecological conditions and have reduced fitness than those in more `core' populations. Populations at range edges are smaller, genetically less diverse and have reduced adaptive potential to cope with environmental conditions in habitats beyond the distributional limits. Although the CPH provides a general explanation as to what limits species distributions, its does not in its current form, represent a universal rule. Also, few studies have examined if the CPH is applicable to parasites, which comprise 30-50% of all animals. Over the next five years, we will use a unique combined approach (laboratory and field experiments with molecular tools) to uncover how genetic variation and species interactions influence tick distribution and abundance by examining three interrelated objectives: (1) compare the genetic diversity and divergence of tick populations near the range edges with more `core' populations, (2) examine the effect of predatory ants on tick survival and (3) elucidate if tick survival is influenced by bacterial endosymbionts. The third objective is unique, in that, the effect of endosymbionts on the survival of their tick hosts has largely been overlooked. The American dog tick (Dermacentor variabilis) is the focal organism of this study because it has a broad geographic range in North America that is expanding northwards and westwards in Saskatchewan. Advancing our understanding of what limits range expansion of D. variabilis will also be of importance for other researchers in the development of effective strategies to control this species, and other parasitic arthropod vectors and their pathogens that are currently expanding their ranges into Canada. My research program also facilitates the training of graduate and undergraduate students, future research leaders in Canada and other parts of the world, who will be well equipped to deal with the increasing economic and social burden of parasitic arthropods and their pathogenic microbes.