Profound genetic divergence and asymmetric parental genome contributions as hallmarks of hybrid speciation in polyploid toads

The evolutionary causes and consequences of allopolyploidization, an exceptional pathway to instant hybrid speciation, are poorly investigated in animals. In particular, when and why hybrid polyploids versus diploids are produced, and constraints on sources of paternal and maternal ancestors, remain underexplored. Using the Palearctic green toad radiation (including bisexually reproducing species of three ploidy levels) as model, we generate a range-wide multi-locus phylogeny of 15 taxa and present four new insights: (i) At least five (up to seven) distinct allo-triploid and allo-tetraploid taxa have evolved in the Pleistocene; (ii) All maternal and paternal ancestors of hybrid polyploids stem from two deeply diverged nuclear clades (6 Mya, 3.1-9.6 Mya), with distinctly greater divergence than the parental species of diploid hybrids found at secondary contact zones; (iii) Allotriploid taxa possess two conspecific genomes and a deeply-diverged allospecific one, suggesting that genomic im...