Invasive species are increasingly commonplace as human activities accidentally or intentionally move species or their propagules from place to place. When species are first introduced to a new location they must often contend with new abiotic environments, different species, and a lack of mates. How do introduced species manage to overcome these barriers to their survival and reproduction? In some cases species may be genetically predisposed to their new environment, or possess high levels of plasticity. In other cases newly introduced populations may have to adapt genetically to their new circumstances. In plant species that must cross-pollinate, a lack of mating partners can be a significant barrier to their establishment during introduction. Consequently we might expect introduced populations to have evolved mechanisms that allow them to self fertilize so that they can reproduce in the absence of mates. However, no study has yet identified the evolution of self-compatibility during introduction, despite the fact that this trait can be highly labile in plants. Common ragweed is a highly successful invasive species with a physiological mechanism that prevents self-fertilization. However, we have some evidence to suggest there has been the breakdown of this mechanism in some populations of this species. Using genetic markers and controlled pollinations we will test the hypothesis that recently colonized populations have evolved an increased capacity for self-fertilization to overcome mate limitation inherent in the colonization process.
When invasive species are first introduced to a new region they often encounter novel environments and a lack of suitable mates. Understanding how invasive species manage to overcome these barriers to establish thriving populations is important if we are going to be able to prevent, control, and manage invasions. Evolutionary theory suggests that one way in which some invasive species may become successful in their adopted range is through hybridization; were species once separated geographically have been brought together by human intervention. Hybridization can offer a number of advantages including providing evolutionary novelty that could facilitate adaptation during invasion. The annual plants Cakile maritima and Cakile edentula (both 2n=18) evolved along parallel gradients of great climatic range (over ~30º of latitude) on either side of the North Atlantic in a narrow coastal habitat. They have since undergone human-assisted introductions and subsequent range expansions in multiple regions of the globe (e.g. western North America, Australia, New Zealand, Japan, South America), often co-existing in the same introduced regions. These species differ in their capacity to self-fertilize and ecological modelling suggests that the self-fertilizing species, which can more easily establish new populations due to its mode of reproduction, could also facilitate the introduction of the outcrossing species by providing mating opportunities. Although this novel “piggybacking hypothesis” works in theory, and there is morphological evidence to support hybridization in the introduced range, genetic evidence supporting it has yet to be assessed. To test the feasibility of the piggybacking hypothesis, we will use genomic markers from putatively hybridizing populations in the introduced range and allopatric populations from the native range to identify the signature of past hybridization events between these species.