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

Abstract

BackgroundOne key question in evolutionary biology deals with the mode and rate at which reproductive isolation accumulates during allopatric speciation. Little is known about secondary contacts of recently diverged anuran species. Here we conduct a multi-locus field study to investigate a contact zone between two lineages of green toads with an estimated divergence time of 2.7 My, and report results from preliminary experimental crosses.ResultsThe Sicilian endemic Bufo siculus and the Italian mainland-origin B. balearicus form a narrow hybrid zone east of Mt. Etna. Despite bidirectional mtDNA introgression over a ca. 40 km North-South cline, no F1 hybrids could be found, and nuclear genomes display almost no admixture. Populations from each side of the contact zone showed depressed genetic diversity and very strong differentiation (FST = 0.52). Preliminary experimental crosses point to a slightly reduced fitness in F1 hybrids, a strong hybrid breakdown in backcrossed offspring (F1 x parental, with very few reaching metamorphosis) and a complete and early mortality in F2 (F1 x F1).ConclusionGenetic patterns at the contact zone are molded by drift and selection. Local effective sizes are reduced by the geography and history of the contact zone, B. balearicus populations being at the front wave of a recent expansion (late Pleistocene). Selection against hybrids likely results from intrinsic genomic causes (disruption of coadapted sets of genes in backcrosses and F2-hybrids), possibly reinforced by local adaptation (the ranges of the two taxa roughly coincide with the borders of semiarid and arid climates). The absence of F1 in the field might be due to premating isolation mechanisms. Our results, show that these lineages have evolved almost complete reproductive isolation after some 2.7 My of divergence, contrasting sharply with evidence from laboratory experiments that some anuran species may still produce viable F1 offspring after > 20 My of divergence.

The biogeography of mitochondrial and nuclear discordance in animals

“…Tamiasciurus squirrels; Chavez et al 2011). Some authors suggest that this pattern may be due to the fact that, in those cases, mtDNA harbours fewer genetic incompatibilities as compared to the nuclear genome, and thus, there are fewer barriers to introgression (Colliard et al 2010). Fig.…”

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 .…”

DNA variations of the green toad Pseudepidalea viridis (syn. Bufo viridis) from various habitats

“…Genetic variability and the geographical distribution of P. viridis were intensively studied in Israel and in other parts of the world (Liu et al 2000;Stock et al 2005Stock et al , 2006Stock et al , 2008Batista et al 2006;Colliard et al 2010). Batista et al (2006) analyzed 35 individuals from widespread populations for partial 12S and 16S ribosomal (r)RNA mtDNA.…”