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Invasive species affect native vegetation through combinations of direct interactions (e.g. competition) and indirect interactions (e.g. altered pollination regimes) mediated through the occupation of shared environments. Hybrid cattails (Typha × glauca) are rapidly displacing their parental species, the native broadleaf cattail (T.latifolia) and the introduced narrow-leaf cattail (T. angustifolia), but the mechanisms involved are poorly understood. We evaluated the potential for direct and indirect interactions among cattails using a series of experiments. First, we evaluated whether seed germination was affected by hybrid leaf litter using a factorial experiment in which we tested for indirect effects of leaf-litter leachate and shading. Second we tested whether these indirect effects of hybrid leaf litter affected seedling performance using a factorial common-garden experiment. Finally, we tested for direct competitive interactions between hybrid and native cattail seedlings. One or more components of hybrid leaf-litter leachate strongly reduced the germination of the parental cattail species, but not that of the hybrid. There was no evidence that direct competitive interactions affected seedling performance. Because regular recruitment by seeds appears to be important in shaping patterns of genetic diversity in natural populations, our findings provide a mechanism for explaining the dominance of hybrid cattails in eastern North America.

Spatial separation between hybridizing taxa can limit hybridization and introgression when mating occurs between near neighbours. Water depth can be an important factor structuring wetland plant communities, which often include cattails (Typha spp., Typhaceae). In eastern North America, native Typha latifolia regularly hybridizes with introduced T. angustifolia to produce the invasive F1 hybrid T. × glauca, which can backcross to parental species. Hybridization in this wind-pollinated species complex may be facilitated by overlapping niches among taxa, but previous studies have yielded contradictory results, possibly because these studies were limited to shallow sites or single locations. We investigated patterns of depth occupancy in ditches (mean depth 3.4 cm) and permanent wetlands (mean depth 25.0 cm) with various mixtures of T. latifolia, T. angustifolia, and T. × glauca across a 1700 km transect from Michigan, USA, to Nova Scotia, Canada. Overall, we found that water depth cannot on its own explain patterns of cattail occupancy. We therefore conclude that niche partitioning by water depth does not promote mating isolation in Typha. Previous findings of habitat segregation by depth may be attributable to a combination of idiosyncratic features of individual sites, multi-dimensional niches, and phenotypic plasticity, which could prevent consistent niche segregation among Typha taxa. A lack of spatial segregation between Typha taxa should promote hybridization and introgression. This, coupled with the competitive superiority of hybrids, appears to be facilitating the displacement of native T. latifolia across a wide geographical range.

Cattails (Typha spp.) are essential components of wetland around the world, although they can also be problematic invaders. In some regions of North America, T. latifolia and T. angustifolia hybridize to produce T. x glauca, which is invading and dominating wetlands around the Laurentian Great Lakes. However, in other regions such as China and the maritime provinces of Canada, hybrids are either non-existent or maintained at very low frequencies, and cattails are not considered invasive. Little is known about cattail hybrids in Europe. We used microsatellite markers to characterize T. latifolia, T. angustifolia, T. laxmannii, and T. domingensis sampled from 63 sites across nine European countries. Hybrids were very uncommon (

Rapid evolution following interspecific hybridization can facilitate biological invasions. Around the Great Lakes in North America, the hybrid cattail Typha × glauca is dominating wetlands and displacing both parental species. We measured water depth and height of T. × glauca and its parental species (Typha angustifolia and Typha latifolia) throughout the growing season at a site near Lake Ontario that harbors both parental species plus hybrids. We found no evidence of niche partitioning by water depth, nor was there evidence that water depth was influencing plant height. At the beginning of the growing season, T. latifolia comprised the tallest plants, but this potential advantage was short-lived, and for most of the growing season, F1 hybrids were taller than all or most other taxa (T. angustifolia, T. latifolia, and advanced-generation/backcrossed hybrids). Heterosis, inferred from height, is therefore evident in F1 hybrids, but not in advanced-generation/backcrossed hybrids. Typha stands often achieve high densities, and the competitive advantage of superior height is likely contributing to the invasive success of T. × glauca F1 hybrids in the Great Lakes region.

Environmental DNA (eDNA) has recently been employed as a surveillance tool for invasive species, although its use has so far been limited to animals. We investigated the possibility of using eDNA to monitor aquatic invasive plants by assessing the potential utility of three regions of the chloroplast genome- matK, rbcL, and trnH–psbA- in ten aquatic invasive plant species. Of the three regions, matK was most likely to harbor putatively species-specific nucleotides in candidate primer-binding sites, and the potentially diagnostic primers that we designed for this region were least likely to amplify DNA from non-target species. A lack of species-specific nucleotides precluded the design of appropriate rbcL primers in most of our species, and amplification of non-target DNA often precluded the use of trnH–psbA markers. We, therefore, suggest that of the three regions, matK is the most useful starting point for designing eDNA markers for aquatic plants. We also demonstrated that these markers can identify species from eDNA that has been extracted from water samples. Our results should be useful to the ongoing development of eDNA as a tool for the early detection and ongoing surveillance of aquatic invasive plants.

Cattails ( Typha spp.) in northeastern North American wetlands are increasingly dominant, partly because of hybridization between Typha latifolia and Typha angustifolia which results in the widespread and successful hybrid T. × glauca . Both parental species are protogynous, and previous studies have shown that T. angustifolia flowers earlier than T. latifolia. Nevertheless, experimental studies have shown that hybridization is overwhelmingly asymmetric, involving T. angustifolia female flowers and T. latifolia pollen which should have minimal overlap in the times at which they are produced. In this study we surveyed two Typha stands in eastern North America, and at both sites found substantial overlap in the times at which the parental species and the hybrids shed pollen and presumably are receptive to fertilization; this means that flowering time is not a prezygotic barrier to either the formation of hybrids or their subsequent backcrossing to parental species. Consistent with experimental studies, all hybrids had T. angustifolia as their maternal parent. Our results suggest that reproductive barriers between Typha spp. are evolving.

A recent increase in the abundance of cattails (Typha spp.) in North American wetlands has been anecdotally linked with hybridization between Typha latifolia and Typha angustifolia. In this study, we used molecular genetic markers (microsatellites) to investigate whether the hybrid lineage (Typha × glauca) is restricted to The Great Lakes region, or exists across a much broader spatial scale. We also investigated the possibility of backcrossing and genetic introgression in natural populations. Parental species could be distinguished from one another based on the distribution of alleles at six microsatellite loci. Species identification based on genetic data corresponded well with species identifications based on leaf width, a key morphological trait that can distinguish the two parental species. We found that hybrids occur in Ontario, Quebec, New Brunswick, and Nova Scotia, but we did not detect hybrids in Maine. F1s are more abundant than backcrossed or intercrossed hybrids, although we also found evidence of backcrossing, particularly in Ontario. This indicates that hybrids are fertile, and are therefore potential conduits of gene flow between the parental species. Further work is needed to determine whether T. × glauca is particularly successful in the Great Lakes region relative to other areas in which the two parental species co-exist, and to assess whether introgression may lead to increased invasiveness in the species complex.

Interspecific hybridization is widespread in plants and is an important evolutionary process. Hybrids may be fitter than their parental species, at least under some environmental conditions, and this may lead to partitioning of taxa by habitat. In eastern Canada, two cattail species ( Typha latifolia and Typha angustifolia ) and their hybrid ( Typha x glauca ) are known to have become increasingly widespread in recent decades, although their habitat preferences and patterns of co-existence at the local scale are not well known. We quantified the occurrence of these three taxa in three different habitat types (high traffic, low traffic, and ‘natural’) at 40 different sites along a sampling route of approximately 2000 km in eastern Canada. There were no significant overall associations between habitat type and taxon, although intraspecific comparisons among sites showed that the hybrid was most likely to grow in high traffic (highly disturbed) sites. In addition, pairwise comparisons revealed significant independence of T. latifolia and T. angustifolia , although the hybrid was equally likely to co-exist with either of its parental species. The presence of the three taxa in several habitats, including highly disturbed roadside areas, is consistent with their increasingly invasive tendencies.