A Comprehensive Research Framework for Geographic Parthenogenesis in Whiptail Lizards (Genus Aspidoscelis)

One of the most compelling topics in biology has been the ubiquity of sexual reproduction in living organisms. Because the ecological and evolutionary advantages of sex are well founded, those organisms that reproduce asexually remain enigmatic. Parthenogenesis, the clonal reproduction of an all-female species without the need for males, is a relatively common form of asexual reproduction in vertebrates, and has been subject of numerous academic investigations. Many parthenogenic organisms also share aspects of their geographic distributions, such as inhabiting higher latitudes, higher altitudes, islands or island-like habitats, xeric environments, and marginal, disturbed or ecotonal habitats relative to their sexual congeners, a pattern termed geographical parthenogenesis (Vandel, 1928). This has led to the development of numerous hypotheses to account for the geographic distribution and persistence of parthenogenic organisms relative to their sexual relatives. These hypotheses often consider overlapping biological processes, complicating efforts to create a simplified model accounting for parthenogenic reproduction. Instead of treating hypotheses individually, a better approach is to categorize common biological patterns underlying the suite of hypotheses posited in the literature to develop a Comprehensive Research Framework that tests for overall patterns based on their commonalities and differences. In this way, we may tease apart the relative contribution of a particular hypothesis. In this chapter, we review the hypotheses regarding geographic parthenogenesis generated in the literature and emphasize the underlying biological processes. Using these biological processes as our framework, we develop a five-part Comprehensive Research Framework that encompasses the range of biological phenomena acting on parthenogenic organisms: (1) the Population Genetics of Sexual Populations; (2) Hybridity and Heterosis; (3) Clonal Ecological Strategy; (4) Exclusion or Coexistence; and (5) Evolutionary History. In each section, we suggest potential methods and studies that explicitly test biological processes acting at that level, which have the potential to illuminate the biological conditions where parthenogenic reproduction is successful. Using the Comprehensive Research Framework, we conclude with two test studies that each examine the expectations of one of the five parts identified above, using parthenogenic hybrid whiptail lizards (genus Aspidoscelis) as our model species. We explicitly test the Hybridity and Heterosis (Chapter Two) and Clonal Ecological Strategy (Chapter Three) sections, utilizing the methods suggested in the Comprehensive Research Framework. These studies demonstrate the utility of the framework we developed, supporting its use as a road-map for developing further research programs into additional taxa where parthenogenic reproduction occurs