Microsatellite analyses of artificial and spontaneous dogrose hybrids reveal the hybridogenic origin of Rosa micrantha by the contribution of unreduced gametes.

Dogroses are characterized by a unique meiosis system, the so-called canina meiosis, which facilitates sexual reproduction at odd-number ploidy. The mostly pentaploid somatic level of dogroses is restored by a merger of haploid sperm cells and tetraploid egg cells. We analyzed experimental hybrids between different dogrose species using microsatellites to determine pollen-transmitted alleles. This information was used to reconstruct the putative hybridogenic origin of Rosa micrantha and R. dumalis and to estimate the frequency of spontaneous hybridization in a natural population. We found no evidence for the hybrid origin of R. dumalis, but our data suggest that R. micrantha presumably arose by hybridization between R. rubiginosa and R. canina or R. corymbifera. We observed only hexaploid individuals of R. micrantha, thus the establishment of this hybridogenic species was favored when unreduced gametes contributed to their origin. We demonstrate that spontaneous hybrids originated infrequently from the parental species in a natural population, but hybridization was often associated with the formation of unreduced gametes. We postulate that unreduced gametes play a major role in the evolutionary success of dogrose hybrids because they provide highly homologous chromosomes crucial for bivalent formation during canina meiosis and thus ensuring this unique form of sexual reproduction.