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，是濒临灭绝的关键物种，入侵物种，或者影响野生动物、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中物种的范围和丰富程度就特别重要。例如，壁虱传播的病原体给世界各地的牛生产者造成了巨大的经济损失。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中的一个关键问题。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)假定，处于范围边缘的个体比更多“核心”种群的个体经历更恶劣的生态条件，并降低适合度。牧场边缘的种群较小，遗传多样性较低，适应超出分布界限的生境的环境条件的能力降低。尽管CPH对什么限制物种分布提供了一般性的解释，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是否适用于寄生虫，寄生虫占所有动物的30%-50%。在接下来的五年里，我们将使用一种独特的组合方法(使用分子工具的实验室和现场实验)，通过检查三个相互关联的目标来揭示遗传变异和物种相互作用如何影响壁虱的分布和丰度：(1)比较分布在范围边缘附近的壁虱种群的遗传多样性和差异性，(2)检查捕食性蚂蚁对壁虱生存的影响，以及(3)阐明壁虱生存是否受到细菌内共生菌的影响。第三个目标是独特的，因为内共生菌对其宿主的生存的影响在很大程度上被忽视了。美国狗虱是这项研究的重点生物，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变形虫的范围扩大，对于其他研究人员开发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将具有重要意义。我的研究计划还有助于培养研究生和本科生，加拿大和世界其他地区未来的研究带头人，他们将做好准备，应对寄生节肢动物及其致病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.