Hybridization of Bombina bombina and B. variegata (Anura, Discoglossidae) at a sharp ecotone in western Ukraine: Comparisons across transects and over time

Schmalhausen Institute of Zoology, National Academy of Sciences of Ukraine, Kiev.
Evolution (Impact Factor: 4.61). 04/2006; 60(3):583-600. DOI: 10.1554/04-739.1
Source: PubMed


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.

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Available from: Sebastian Hofman, Apr 01, 2015
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    • "Additionally, climate-mediated hybridization might allow us to observe the evolutionary dynamics of hybridization in situ. Currently, empirical studies of hybridization frequently rely on well-established hybrid zones (Rand and Harrison 1989; Yanchukov et al. 2006) or experimental crosses between hybridizing lineages (Rieseberg et al. 2003; Donovan et al. 2009). Climate change will provide an opportunity to identify new sites where secondary contact is occurring and to examine the consequences of natural hybridization over time. "
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    • "Alternatively, hybrid zones can be mosaic, with parental forms patchily distributed across heterogeneous habitat and hybridization occurring across patch boundaries or in intermediate habitats (Harrison 1986; Harrison and Rand 1989; Rand and Harrison 1989; Ross and Harrison 2002; but see Searle 1993). In a heterogeneous landscape, hybrid zones may exhibit a mix of different dynamics (e.g., Bombina hybrid zone; Szymura and Barton 1991; Vines et al. 2003; Yanchukov et al. 2006), and as a consequence, reproductive barriers and patterns of introgression may vary geographically (Teeter et al. 2010). Therefore, characterizing multiple transects or regions is essential for understanding both hybrid zone history and hybrid zone dynamics. "
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    ABSTRACT: Hybrid zones provide insight into the nature of species boundaries and the evolution of barriers to gene exchange. Characterizing multiple regions within hybrid zones is essential for understanding both their history and current dynamics. Here, we describe a previously uncharacterized region of a well-studied hybrid zone between two species of field crickets, Gryllus pennsylvanicus and G. firmus. We use a combination of mitochondrial DNA sequencing, morphological data, and modeling of environmental variables to identify the ecological factors structuring the hybrid zone and define patterns of hybridization and introgression. We find an association between species distribution and natural habitat; Gryllus pennsylvanicus occupies natural habitat along forest edges and natural clearings, whereas G. firmus occupies more disturbed areas in agricultural and suburban environments. Hybridization and introgression occur across patch boundaries; there is evidence of substantial admixture both in morphological characters and mtDNA, over a broad geographic area. Nonetheless, the distribution of morphological types is bimodal. Given that F1 hybrids are viable and fertile in the lab, this suggests that strong pre-zygotic barriers are operating in this portion of the hybrid zone.
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    • "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., 2008), and two types in particular, tension zones and zones of bounded hybrid superiority, tend to occur along gradients in biotic and abiotic variables (Good et al., 2000; Gay et al., 2008; Ruegg, 2008). The longstanding belief that hybrid zones are evolutionary sinks that inhibit diversification (Mayr, 1963; Wagner, 1970) has been subtly perpetuated due to a focus on tension zones, in which reduced hybrid fitness is balanced by inward dispersal and continual hybridization of parentals (reviewed in Barton & Hewitt, 1985). "
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