HYBRIDIZATION OFBOMBINA BOMBINAANDB. VARIEGATA(ANURA, DISCOGLOSSIDAE) AT A SHARP ECOTONE IN WESTERN UKRAINE: COMPARISONS ACROSS TRANSECTS AND OVER TIME

Abstract

Bombina bombina and B. variegata are two anciently diverged toad taxa that have adapted to different breeding habitats yet hybridize freely in zones of overlap where their parapatric distributions meet. Here, we report on a joint genetic and ecological analysis of a hybrid zone in the vicinity of Stryi in western Ukraine. We used five unlinked allozyme loci, two nuclear single nucleotide polymorphisms and a mitochondrial DNA haplotype as genetic markers. Parallel allele frequency clines with a sharp central step occur across a sharp ecotone, where transitions in aquatic habitat, elevation, and terrestrial vegetation coincide. The width of the hybrid zone, estimated as the inverse of the maximum gradient in allele frequency, is 2.3 km. This is the smallest of four estimates derived from different clinal transects across Europe. We argue that the narrow cline near Stryi is mainly due to a combination of habitat distribution and habitat preference. Adult toads show a preference for either ponds (B. bombina) or puddles (B. variegata), which is known to affect the distribution of genotypes within the hybrid zones. At Stryi, it should cause a reduction of the dispersal rate across the ecotone and thus narrow the cline. A detailed comparison of all five intensively studied Bombina transects lends support to the hypothesis that habitat distribution plus habitat preference can jointly affect the structure of hybrid zones and, ultimately, the resulting barriers to gene flow between differentiated gene pools. This study also represents a resampling of an area that was last studied more than 70 years ago. Our allele-frequency clines largely coincide with those that were described then on the basis of morphological variation. However, we found asymmetrical introgression of B. variegata genes into B. bombina territory along the bank of a river.

Physiological adaptation along environmental gradients and replicated hybrid zone structure in swordtails (Teleostei:Xiphophorus)

“…Environmental gradients not only provide an initial stimulus for divergence, but may also serve as conduits for secondary contact between related species occurring in distinct environments, thus facilitating hybridization. Consequently, hybrid zones commonly coincide with environmental gradients (Fritsche & Kaltz, 2000; Yanchukov et al. , 2006; Kameyama et al.…”

Strong reproductive barriers in a narrow hybrid zone of West-Mediterranean green toads (Bufo viridissubgroup) with Plio-Pleistocene divergence

“…[16-20]], but very few have combined such estimates with multi-locus transect approaches to infer the time required to reach reproductive isolation in natural contexts [e.g. [8,21,22]].…”

Repeated sampling detects gene flow in a flightless ground beetle in a fragmented landscape

“…Our study is one of the first investigations of the temporal changes in the genetic structure of natural populations of terrestrial animals (Hale et al 2001; Johnson et al 2004; Jehle et al 2005; Yanchukov et al 2006; Habel et al 2011), whereas studies on marine or limnic organisms have frequently been conducted (Jorde and Ryman 1996; Chevolot et al 2008; Demandt 2010). Despite the short timeframe of only 15 to 20 years we found a change in the population genetic structure of our study species.…”