When can chromosomes drive speciation? The peculiar case of the Malagasy tomato frogs (genus Dyscophus)

Except for polyploidy, parthenogenesis and hybridogenesis, the role of chromosome rearrangements in speciation is still highly debated. This paper highlights an unusual karyotype diversification and a putative case of chromosomal speciation in the endemic Malagasy tomato frogs. The tomato frogs include Dyscophus insularis as well as the sister species D. antongilii and D. guineti, which are characterized by a disjunct distribution and a low genetic divergence, probably due to a recent hybridization. Our results show that the three Dyscophus species present 2n = 26 biarmed chromosomes, differing in chromatin pattern. Interestingly, both sexes of D. guineti show a heteromorphic chromosome pair 11, with one element larger (11a) than the other (11b) due to the presence of a large terminal tract on the short arm with an euchromatic band enclosed between two heterochromatic bands. This fixed condition reminds that of some newts of the genus Triturus, but in Dyscophus it probably characterizes the beginning of a cladogenetic process. We hypothesize that in D. guineti the raising of the heteromorphic 11th chromosome pair followed the hybridization with D. antongilii, originating a post-zygotic barrier and representing a case of sympatric or parapatric chromosomal speciation. This speciation mechanism may also have had important biogeographic consequences, shaping the current disjunct distribution of D. antongilii and D. guineti, so far not sufficiently explained