Origin of Polyploidy: Results of Laboratory Crosses Between Green Toad (Bufotes) Diploid Species

Unreduced gametes contribute to the emergence of polyploid plant and animal species. Interspecific diploid and polyploid hybrids can utilize clonal or hemiclonal reproduction methods, in which one or all parental genomes are transmitted to offspring without recombination. To elucidate mechanisms supporting the coexistence of parental species and polyploid hybrids, we conducted artificial crossing experiments between two diploid species of green toads. We determined the ploidy levels of all hybrids resulting from the crosses. The offspring included di-, tri-, tetra-, penta-, and hexaploid individuals. In the first generation, all hybrids were generally diploid. In the second generation, many polyploid individuals appeared in the offspring. Hybrids (even in a single cross) produced tadpoles with different ploidy levels due to gametes with different genomic compositions. Males of triploid hybrids produced predominantly haploid gametes, while females produced diploid ones. We propose a model for the origin of polyploid species in the genus Bufotes.