Nuclear markers, mitochondrial DNA and male secondary sexual traits variation in a newt hybrid zone (Triturus vulgaris × T. montandoni)

Abstract

The smooth and the Montandon's newts (Triturus vulgaris and T. montandoni) are genetically similar sister species with highly divergent male secondary sexual traits involved in complex courtship behaviour. Their parapatric ranges overlap at moderate elevations in the Carpathian Mountains where they hybridize readily. Here we present a detailed study of genetic and morphological variation in populations from the area of sympatry. Analysis of variation at seven nuclear markers, mtDNA and male sexual secondary traits was complemented with an ecological survey of breeding sites characteristics. Extensive hybridization was revealed with back-cross individuals similar to either parental species predominating among hybrids. The hybrid zone exhibited a mosaic pattern: the genetic composition of the populations was correlated only weakly with their geographical position. No association with habitat type was found. Departures from Hardy-Weinberg proportions, significant linkage disequilibria and bimodal distribution of genotypes suggest strongly that assortative mating is an important factor shaping the genetic composition of hybrid populations. The pattern of cytonuclear disequilibria did not indicate much asymmetry in interspecific matings. Changes in the frequency of nuclear markers were highly concordant, whereas mtDNA showed much wider bidirectional introgression with 14% excess of T. montandoni haplotype. We argue that the mosaic structure of the newt hybrid zone results mainly from stochastic processes related to extinction and recolonization. Microgeographical differences in mtDNA introgression are explained by historical range shifts. Since morphologically intermediate males were underrepresented when compared to hybrid males identified by genetic markers, sexual selection acting against the morphological intermediates is implied. We discuss the implications of these findings in the context of reinforcement of prezygotic isolation in newts.

Molecular and morphological evidence of hybridization between newts Triturus vulgaris and T. montandoni (Caudata: Salamandridae) in Slovakia

“…Experimental hybridization in laboratory also supported fertility of F1 hybrids that were able to produce F2 and backcross generations (Geyer 1953(Geyer , 1954Macgregor et al 1990). Backcrosses similar to either parental species predominate in the T. vulgaris and T. montandoni hybrid zone studied in Poland (Babik et al 2003) and they were also documented in a hybrid population in the Czech Republic (Kotlík & Zavadil 1999). The bimodal geno-typic distribution in Polish populations where T. montandoni and T. vulgaris backcrosses were present resulted most probably from assortative mating (i.e.…”

“…1; Zavadil et al 2003). Hybrids of intermediate phenotype were recorded in Ukraine by Hofmann (1908), Kushniruk (1963) and Bannikov (1969), in Romania by Fuhn (1963) and Fuhn et al (1975), in Poland by SzeligaMierzeyewski & Ulasiewicz (1931), Juszczyk &Świerad (1984, Rafiński (1985Rafiński ( , 1988, Pecio & Rafiński (1985), Rafiński & Pecio (1989) and Babik et al (2003), and in the Czech Republic described by Rehák (1992Rehák ( , 1993, Zwach (1992), Šálek (1993), Kotlík et al (1997), Kotlík & Zavadil (1999) and Zavadil et al (2003Zavadil et al ( , 2004. Besides hybrids in nature, hybrids in captivity were obtained during laboratory experimental crosses (Wolterstorff 1925;Geyer 1953Geyer , 1954Macgregor et al 1990;Cogȃlniceanu 1992Cogȃlniceanu , 1994Michalak & Rafiński 1999).…”

“…Besides hybrids in nature, hybrids in captivity were obtained during laboratory experimental crosses (Wolterstorff 1925;Geyer 1953Geyer , 1954Macgregor et al 1990;Cogȃlniceanu 1992Cogȃlniceanu , 1994Michalak & Rafiński 1999). A detailed survey of T. vulgaris and T. montandoni hybrid zone in Poland exhibits its mosaic pattern, i.e., genetic composition of populations is only weakly associated with their geographic position (Babik et al 2003). Backcross hybrids similar to either parental species predominate in the hybrid zone.…”

“…Backcross hybrids similar to either parental species predominate in the hybrid zone. In mixed populations where both parental species live syntopically, the formation of F1 hybrids is rare due to strong assortative mating (Babik et al 2003).…”

Widespread introgression does not leak into allotopy in a broad sympatric zone

“…Thus, female hybrids are difficult to identify and female backcrosses are almost impossible to distinguish from parental species (Naisbit et al, 2003) by morphological criteria alone. Moreover, in another study of hybridization between the small-bodied newts L. vulgaris and L. montandoni, even introgressed males were often indistinguishable from parental species (Babik et al, 2003).…”