Modes of reproduction in three invasive milkweeds are consistent with Baker's Rule.
According to Baker's Rule, uniparental reproduction is associated with colonizing plants because it provides a means for population establishment following single long-distance dispersal events. There is, however, limited evidence for the applicability of Baker's Rule to invasive plants. We determined the breeding systems of three invasive milkweed species-Asclepias curassavica, Gomphocarpus fruticosus and G. physocarpus-in their invaded range in south-east Queensland, Australia. Although dependent on pollinators for reproduction, hand-pollinations revealed that all three species are self-compatible which is consistent with Baker's Rule and notable because milkweeds are generally self-incompatible. In progeny performance trials, seedlings from self-pollinations generally did not perform as well as those from cross-pollinations, but the differences were minor. Evidence for self-compatibility in G. physocarpus is a particularly noteworthy feature of this study, as this species has been reported to be self-incompatible in its native range and may thus have evolved self-compatibility during the invasion process. Furthermore, potential for hybridization between the two Gomphocarpus species was observed. Hybridization may have assisted the invasion of these species through providing additional sources of pollen and/or broadening genetic variation. Our study adds to the growing evidence that breeding systems are significant for the process of plant invasion.