Hybridization and invasion: one of North America's most devastating invasive plants shows evidence for a history of interspecific hybridization

Abstract

Hybridization has been hypothesized to influence invasion through the generation of novel phenotypes and/or increased levels of genetic variance. Based on morphology, hybrids between diffuse knapweed and spotted knapweed, two invasive plants in North America, are present in the invaded range. Some individuals within most diffuse knapweed sites in North America exhibit intermediate diffuse × spotted floral morphology. We examined hybridization at the molecular level, using amplified fragment length polymorphisms. Approximately a quarter of the assayed North American diffuse knapweed individuals exhibited evidence of introgression from spotted knapweed. However, plants with intermediate morphology did not show evidence of mixed ancestry more often than the plants with typical diffuse knapweed morphology. The high proportion of hybrid individuals in North American diffuse knapweed sites found here, combined with evidence from recent studies, suggests that diffuse knapweed was likely introduced with admixed individuals, and the hybrids are not newly created postintroduction. A century of backcrossing with diffuse knapweed has likely decoupled the relationship between morphology and admixture at the molecular level. In contrast to the scenario encountered in North America, in the native range where diploid diffuse and spotted knapweed overlap, hybrid swarms are common. In such sites, the floral phenotype aligns more closely with the genotype.

Keywords: STRUCTURE; amplified fragment length polymorphism; biological invasion; diffuse knapweed; hybridization; spotted knapweed.

Figure 1

European site locations for diffuse knapweed (2n), spotted knapweed (2n), and active hybrid swarms in Ukraine (i.e. both parent species and a morphological gradient of hybrids) used in AFLP analyses.

Figure 2

North American site locations for diffuse knapweed + hybrid-like plants and meadow knapweed used in AFLP analyses. All diffuse knapweed sites contained hybrid-like plants at varying frequencies.

Figure 3

Log-likelihood probabilities of the number of clusters (K) for four independent series of K = 2 through 10 estimated using STRUCTURE v. 2.2 with admixture (Pritchard et al. 2000; Falush et al. 2007). The value ln P(D) is the probability that K is the correct number of clusters given the data. The larger, or less negative, the log likelihood value, the better the K fits the data.

Figure 4

Bayesian assignment probabilities with admixture and (A) K = 3 and (B) K = 2 [STRUCTURE v. 2.2; Pritchard et al. (2000); Falush et al. (2007)]. Each vertical bar represents one individual. The black, grey, and white coloring represents the posterior mean proportion of ancestry from diffuse knapweed (2n), spotted knapweed (2n), and in (A) meadow knapweed, respectively. *population of origin genome probability does not include one, indicating hybridization. EU, Europe; NA, North America; EU hybrid, individuals from spotted × diffuse hybrid swarms in the Ukraine; MK, meadow knapweed (out-group). Based on morphology, diffuse and hybrid-like plants were included in approximately equal numbers from North America sites (see text).

Figure 5

A dendrogram from hierarchical cluster analysis (Ward's method) of diffuse-like and hybrid-like plants surveyed across nine diffuse knapweed sites in western North America in 2005. Five morphological floral characters were analyzed. The top cluster includes plants with typical diffuse knapweed morphology, while the bottom cluster includes plants with hybrid morphology. The numbers to the left of the branches are the genome proportions associated with the diffuse knapweed group [(K = 2 with admixture, STRUCTURE v. 2.2; Pritchard et al. (2000); Falush et al. (2007)]. *population of origin genome probability interval does not include one, indicating interspecific hybridization. See text for details.

Figure 6

The posterior mean proportion of ancestry associated with the diffuse knapweed cluster, as calculated by STRUCTURE v. 2.2 [Pritchard et al. (2000); Falush et al. (2007)]. EU, Europe; NA, North America; UA, Ukraine; DK, diffuse knapweed; H, hybrid; SK, spotted knapweed. Based on morphology, the European diffuse and spotted knapweed sites did not contain any hybrid-like plants. Diffuse and hybrid-like plants from North America were combined into one group for this analysis because they had identical posterior mean proportion of ancestry values from the diffuse knapweed cluster (see text). Individuals from Ukraine came from apparently active hybrid swarms, and the morphological grouping of an individual plant from these sites as spotted, diffuse, or hybrid was done by hierarchical cluster analysis (Blair and Hufbauer 2009). Values represent mean ± 1 SE. Different letters denote significantly different means (Tukey's test P < 0.05).