Self-compatibility and plant invasiveness: Comparing species in native and invasive ranges

A longstanding hypothesis (“Baker's rule”) is that plant invasiveness is facilitated by floral self compatibility rather than self incompatibility. Extending this idea, invasive species whose individuals vary in degree of self compatibility within the native range might be self compatible in invading or weedy populations, due to natural selection on the mating system. We compared mating system between native and invasive ranges for two major world invasives, one annual (Echium plantagineum) and one perennial (Solanum elaeagnifolium). For an additional annual species (Centaurea solstitialis) we compared non-weedy and weedy populations in the native range. No species was strongly spontaneously self pollinating, but the degree of self compatibility after hand pollination varied dramatically. Both annuals were self incompatible in native or non-weedy populations but self compatible in invasive or weedy ones; the reverse was true for the perennial. Individuals within populations of all three species also varied in their degree of self compatibility, suggesting a basis for natural selection, and populations of the same species sharing a status (native/non-weedy, invading/weedy) varied in average self compatibility. These results support the hypothesis that differential selection of progeny during invasion can result in self-compatible populations derived from ancestrally self-incompatible ones, but that this process may be less important in perennial species, which experience multiple opportunities for sexual reproduction. Overall, however, mating system may not operate alone and its contributions to invasiveness may be conditional on other attributes of a species including physiology, morphology, and life history.