Self-fertilization does not lead to inbreeding depression in Typha parent species or hybrids

Some of the most impactful invasive plants are hybrids that exhibit heterosis and outperform their parent species. Heterosis can result from multiple genetic processes, and may also be more likely when parental populations are inbred. However, although outcrossing between relatives and self-fertilization both occur in many widespread plants, no study to our knowledge has investigated whether inbreeding in parental populations could help to explain heterosis in hybrid plants that have displaced their parent species. In the wetlands of southeastern Canada there is a widespread Typha (cattail) hybrid zone in which native T. latifolia (broad-leafed cattail) interbreeds with introduced T. angustifolia (narrow-leafed cattail) to produce the invasive hybrid T. × glauca. Typha reproduce through self-fertilization, outcrossing, and clonal propagation. Heterosis has been identified in T. × glauca by comparing proxy fitness measures between hybrids and parent species, but these studies did not consider the potential importance of inbreeding in parental populations. Because F1 hybrids have higher heterozygosity than their parent species, the self-fertilized offspring of hybrids should have higher heterozygosity than the self-fertilized offspring of parent species; the latter should therefore be more inbred, and potentially more susceptible to inbreeding depression (ID). We tested the hypothesis that self-fertilization leads to greater ID in the offspring of T. latifolia and T. angustifolia compared to the offspring of F1 T. × glauca. We conducted common-garden and wetland experiments using seeds from hand-pollinated plants sourced from natural populations, and quantified several fitness-related metrics in the offspring of self-fertilized versus outcrossed parent species and hybrids. Our experiments provided no evidence that inbreeding leads to ID in self-fertilized T. angustifolia, T. latifolia or T. × glauca in either a common garden or a natural wetland, and thus show that heterosis in a widespread invasive hybrid does not rely on comparisons with inbred parents.