Genetic structure in hybrids and progenitors provides insight into processes underlying an invasive cattail (Typha × glauca) hybrid zone

Abstract

Traditional models of hybrid zones have assumed relatively low hybrid fitness, and thus focussed more on interspecific gene flow than on hybrid dispersal. Therefore, when hybrids have high fitness and the potential for autonomous dispersal, we have limited understanding of whether hybrid dispersal or repeated local hybrid formation is more important for maintaining hybrid zones. The invasive hybrid cattail Typha × glauca occupies an extensive hybrid zone in northeastern North America where it is sympatric with its progenitors T. latifolia and T. angustifolia. We characterized genetic diversity and genetic structure of the three taxa across a broad spatial scale where the maternal parent is relatively rare, and tested the hypothesis that the hybrid shows stronger evidence of gene flow than its progenitor species, particularly among disturbed sites (ditches) compared with established wetlands. Support for this hypothesis would suggest that dispersal, rather than repeated local formation, is more important for maintaining hybrid zones. Within each taxon, genetic differentiation among ditches was comparable to that among wetlands, although clonal richness was consistently greater in ditches, suggesting more frequent seed establishment. Genetic structure across sites was more pronounced in the hybrid compared with either progenitor species. Overall, our data reflect relatively low gene flow in hybrids, and suggest that hybrids are more likely to be created in situ than to be introduced from other sites. Despite the high fitness of invasive T. × glauca and its potential for autonomy, local processes appear more important than dispersal in maintaining this hybrid zone.

Fig. 1. Locations and compositions of Typha stands used in this study.

Sizes of circles indicate different site types rather than reflecting sample size. Wetland and ditch sites that represent one sampling region are connected by a solid black line, and paired wetland and ditch sites are located within 20 km of each other.

Fig. 2. Bar plots from FLOCK output.

Each bar is a multilocus genotype that is partitioned into k-colored segments that represent the proportion of membership in each cluster. a T. angustifolia arranged by site and b T. × glauca arranged by site. Color coding in (a) and (b) are independent of each other. Sites are arranged from west to east. Site codes ending in ‘D’ represent ditches and those ending in ‘W’ represent wetlands.