Admixture increases performance of an invasive plant beyond first-generation heterosis.
Through its potential to enhance progeny performance, admixture (between-population crossing) may promote invasiveness of alien plants. The few studies that tested this idea found evidence for heterosis (positive effects of admixture) in the first generation (F1), but have not considered further generations. In this paper, we test whether admixture benefits can be maintained in subsequent generations of an invasive plant. We follow up on a previous study, in which we made crosses between plants of Mimulus guttatus from native- (western North America) and invaded-range populations (New Zealand and Scotland), and showed that admixture increases F1 performance. Here, we performed further crosses to create non-admixed progeny, F1 progeny resulting from within- and between-range admixture, and subsequent F2 progeny both through outcrossing and through self-fertilization. As heterosis, out- and inbreeding depression may depend on the environment, we assessed progeny performance under benign (well-watered) and drought-stress conditions in a greenhouse experiment. We found that non-admixed progeny of M. guttatus were outperformed by admixed progeny (averaged across F1 and F2), particularly by progeny from between-range admixture. However, the benefit of admixture was stronger in F1 than in F2 progeny, especially when the F2 was produced by self-fertilization. The benefit of admixture also depended on the range of origin and the test environment. Synthesis. Our findings indicate that increased performance of admixed F1 progeny is partly maintained in the F2 progeny. Admixture might thus significantly boost performance of an invasive plant across multiple generations.