A vertebrate reproductive system involving three ploidy levels: Hybrid origin of triploids in a contact zone of diploid and tetraploid palearctic green toads (bufo viridis subgroup)

The rise and consequences of polyploidy in vertebrates, whose origin was associated with genome duplications, may be best studied in natural diploid and polyploid populations. In a diploid/tetraploid (2 n/4 n) geographic contact zone of Palearctic green toads in northern Kyrgyzstan, we examine 4 ns and triploids (3 n) of unknown genetic composition and origins. Using mitochondrial and nuclear sequence, and nuclear microsatellite markers in 84 individuals, we show that 4n (Bufo pewzowi) are allopolyploids, with a geographically proximate 2 n species (B. turanensis) being their maternal ancestor and their paternal ancestor as yet unidentified. Local 3 n forms arise through hybridization. Adult 3 n mature males (B. turanensis mtDNA) have 2 n mothers and 4 n fathers, but seem distinguishable by nuclear profiles from partly aneuploid 3 n tadpoles (with B. pewzowi mtDNA). These observations suggest multiple pathways to the formation of triploids in the contact zone, involving both reciprocal origins. To explain the phenomena in the system, we favor a hypothesis where 3 n males (with B. turanensis mtDNA) backcross with 4 n and 2 n females. Together with previous studies of a separately evolved, sexually reproducing 3 n lineage, these observations reveal complex reproductive interactions among toads of different ploidy levels and multiple pathways to the evolution of polyploid lineages.