Variable and Asymmetric Introgression in a Hybrid Zone in the Toads, Bufo americanus and Bufo fowleri

The structure and dynamic history of a mosaic hybrid zone are investigated using an analysis of morphometric variation to determine the extent of interbreeding between the species involved in different regions. The hybrid zone between the toads Bufo americanus and Bufo fowleri is geographically widespread. Natural hybrids occur in Ontario at Long Point but not along the nearby Niagara peninsula although the species are sympatric in both places. Discriminant analysis of variation in 27 morphological characters from 286 preserved museum specimens of male toads and putative hybrids was compared with previous allozyme genetic characterization of the same specimens. Reference samples from allopatric populations were used for comparison. Bufo americanus from Long Point were significantly different from other samples of the same species, demonstrating introgression in the direction of sympatric B. fowleri. Long-term hybridization in this region has been confined to Long Point despite sympatry elsewhere. Thus, the hybrid zone between these toads does not conform to gradient or “tension zone” models. Rather, it is a mosaic in that hybrids appear at some times and in some places but not in others and it conforms to the concept of a “localized sympatric” hybrid zone.

Fig. 1.

Eastern Lake Erie and adjacent southern Ontario showing sample localities of toads. The northern limit of Bufo fowleri is along the north shore of Lake Erie. Open circles indicate samples of Bufo americanus, closed circles indicate samples of B. fowleri, the grey circle at Long Point indicates the collection of hybrid individuals

Fig. 2.

Distributions of univariate discriminant function (df) scores for toads from the Niagara Peninsula, Long Point, and reference samples. Hybridization and introgression at Long Point has evidently shifted the morphologies of Bufo americanus (black bars) and Bufo fowleri (white bars) closer to each other compared to toads from the Niagara Peninsula such that the distributions df scores overlap. In contrast, no hybrids have been detected along the Niagara shoreline and there is no overlap in distributions of df scores of sampled toads

Fig. 3.

Correlation of morphological univariate (df) scores versus genetic hybrid index (B) scores (Green, 1984) for all suspected hybrids (r = 0.778)

Fig. 4.

Discriminant function plots (Factor 1 and Factor 2) of multiple discriminant function analyses for populations of Bufo americanus and Bufo fowleri and their hybrids. Ninety-five percent centroids are plotted. The samples of B. americanus are not significantly different from each other nor is there significant difference among them in their morphological distance from B. fowleri. However, the Long Point sample is closest to the reference sample of B. fowleri. The distribution of the hybrids overlaps more extensively with B. americanus than with B. fowleri, but there is no significant difference between the average distance between the hybrids and either species