TESTS FOR SEXUAL INCOMPATIBILITY BETWEEN TWO NEWT SPECIES, TRITURUS VULGARIS AND TRITURUS MONTANDONI : NO-CHOICE MATING DESIGN

Linkage Map of Lissotriton Newts Provides Insight into the Genetic Basis of Reproductive Isolation

Linkage maps are widely used to investigate structure, function, and evolution of genomes. In speciation research, maps facilitate the study of the genetic architecture of reproductive isolation by allowing identification of genomic regions underlying reduced fitness of hybrids. Here we present a linkage map for European newts of the Lissotriton vulgaris species complex, constructed using two families of F2 L. montandoni × L. vulgaris hybrids. The map consists of 1146 protein-coding genes on 12 linkage groups, equal to the haploid chromosome number, with a total length of 1484 cM (1.29 cM per marker). It is notably shorter than two other maps available for salamanders, but the differences in map length are consistent with cytogenetic estimates of the number of chiasmata per chromosomal arm. Thus, large salamander genomes do not necessarily translate into long linkage maps, as previously suggested. Consequently, salamanders are an excellent model to study evolutionary consequences of recombination rate variation in taxa with large genomes and a similar number of chromosomes. A complex pattern of transmission ratio distortion (TRD) was detected: TRD occurred mostly in one family, in one breeding season, and was clustered in two genomic segments. This is consistent with environment-dependent mortality of individuals carrying L. montandoni alleles in these two segments and suggests a role of TRD blocks in reproductive isolation. The reported linkage map will empower studies on the genomic architecture of divergence and interactions between the genomes of hybridizing newts.

Genomic heterogeneity of historical gene flow between two species of newts inferred from transcriptome data

The role of gene flow in species formation is a major unresolved issue in speciation biology. Progress in this area requires information on the long-term patterns of gene flow between diverging species. Here, we used thousands of single-nucleotide polymorphisms derived from transcriptome resequencing and a method modeling the joint frequency spectrum of these polymorphisms to reconstruct patterns of historical gene flow between two Lissotriton newts: L. vulgaris (Lv) and L. montandoni (Lm). We tested several models of divergence including complete isolation and various scenarios of historical gene flow. The model of secondary contact received the highest support. According to this model, the species split from their common ancestor ca. 5.5 million years (MY) ago, evolved in isolation for ca. 2 MY, and have been exchanging genes for the last 3.5 MY Demographic changes have been inferred in both species, with the current effective population size of ca. 0.7 million in Lv and 0.2 million in Lm. The postdivergence gene flow resulted in two-directional introgression which affected the genomes of both species, but was more pronounced from Lv to Lm. Interestingly, we found evidence for genomic heterogeneity of interspecific gene flow. This study demonstrates the complexity of long-term gene flow between distinct but incompletely reproductively isolated taxa which divergence was initiated millions of years ago.

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

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.