Admixture increases performance of an invasive plant beyond first-generation heterosis

1. 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 (F-1), but have not considered further generations. In this paper, we test whether admixture benefits can be maintained in subsequent generations of an invasive plant. 2. 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 F-1 performance. Here, we performed further crosses to create non-admixed progeny, F-1 progeny resulting from within-and between-range admixture, and subsequent F-2 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. 3. We found that non-admixed progeny of M. guttatus were outperformed by admixed progeny (averaged across F-1 and F-2), particularly by progeny from between-ange admixture. However, the benefit of admixture was stronger in F-1 than in F-2 progeny, especially when the F-2 was produced by self-fertilization. The benefit of admixture also depended on the range of origin and the test environment. 4. Synthesis. Our findings indicate that increased performance of admixed F-1 progeny is partly maintained in the F-2 progeny. Admixture might thus significantly boost performance of an invasive plant across multiple generations.