Inferring the degree of incipient speciation in secondary contact zones of closely related lineages of Palearctic green toads (Bufo viridis subgroup)

Abstract

Reproductive isolation between lineages is expected to accumulate with divergence time, but the time taken to speciate may strongly vary between different groups of organisms. In anuran amphibians, laboratory crosses can still produce viable hybrid offspring >20 My after separation, but the speed of speciation in closely related anuran lineages under natural conditions is poorly studied. Palearctic green toads (Bufo viridis subgroup) offer an excellent system to address this question, comprising several lineages that arose at different times and form secondary contact zones. Using mitochondrial and nuclear markers, we previously demonstrated that in Sicily, B. siculus and B. balearicus developed advanced reproductive isolation after Plio-Pleistocene divergence (2.6 My, 3.3-1.9), with limited historic mtDNA introgression, scarce nuclear admixture, but low, if any, current gene flow. Here, we study genetic interactions between younger lineages of early Pleistocene divergence (1.9 My, 2.5-1.3) in northeastern Italy (B. balearicus, B. viridis). We find significantly more, asymmetric nuclear and wider, differential mtDNA introgression. The population structure seems to be molded by geographic distance and barriers (rivers), much more than by intrinsic genomic incompatibilities. These differences of hybridization between zones may be partly explained by differences in the duration of previous isolation. Scattered research on other anurans suggests that wide hybrid zones with strong introgression may develop when secondary contacts occur <2 My after divergence, whereas narrower zones with restricted gene flow form when divergence exceeds 3 My. Our study strengthens support for this rule of thumb by comparing lineages with different divergence times within the same radiation.

Coupling, Reinforcement, and Speciation

“…Comparisons among examples with different overall barriers and different combinations of barrier effects, as well as comparative analyses of replicated and independent events of contact in a given system can also help this reconstruction (e.g., Nadeau et al 2014;Vijay et al 2017). In general, it will be important to identify more systems offering multiple hybrid zones to compare situations where coupling may not be at the same stage across the distribution range (e.g., in common voles [Beysard and Heckel 2014], Hyla frogs [Dufresnes et al 2015], or green toads [Dufresnes et al 2014]). Combining genomic analysis of such systems with analysis of phenotypes and the types of barrier effects (one allele vs. two allele) is the most likely route toward understanding of coupling processes.…”

Polyploidy in Amphibia

“…There are several cases of range overlap and interactions through hybridization and polyploidization [Colliard et al, 2010;Dufresnes et al, 2014]. This radiation includes bisexually reproducing diploid (2n = 22), triploid (3n = 33) and tetraploid (4n = 44) species, all of which occur in Central Asia .…”

Sex Chromosome Conservation, <b><i>DMRT1</i></b> Phylogeny and Gonad Morphology in Diploid Palearctic Green Toads (<b><i>Bufo viridis </i></b>Subgroup)

“…Variation in XY-recombination rates was previously detected in tree frogs, in which Central European Hyla males [Berset-Brändli et al, 2008;Stöck et al, 2011a] exhibited no XY recombination in pedigrees, while southern European H. arborea males showed some degree of XY recombination [Dufresnes et al, 2014b].…”