Natural hybridization between Iris minutoaurea Makino and Iris odaesanensis Y. N. Lee in Korea: evidence from cytological traits

Interspecific hybridization resulting in diploid-homoploid hybrids is relatively rare in natural populations. The Iris genus, which comprises numerous economically important species, is a taxonomically complex group in which polyploidization and hybridization frequently contribute to taxon diversification and speciation in natural populations. In Korea, populations of the diploid Iris odaesanensis (2n = 28) and Iris minutoaurea (2n = 22) come into contact with each other, leading to homoploid hybrids, as recently demonstrated by molecular phylogenetic and morphological surveys. In this study, we aimed to confirm whether the chromosome number, genome size, and pollen viability of the putative hybrids corresponded to the hybrid nature of their diploid parental species. As previously demonstrated, the hybrids exhibited intermediate tepal color traits between the parental taxa. Iris minutoaurea, I. odaesanensis, and the putative hybrids were consistently diploid (2n = 22, 28, and 25, respectively), confirming that homoploid hybridization mainly occurred in natural populations. The genome size of the putative hybrids (mean: 3.84 pg, range: 3.80–3.86 pg) was additive when compared with those of the parental diploid species (i.e., 3.72 pg in I. odaesanensis and 3.95 pg in I. minutoaurea). No fertile pollen grains were found in the putative hybrids, which may have prevented the establishment of hybrid lineages and backcrosses with the parental species I. odaesanensis or I. minutoaurea. Together, these data confirm the existence of natural homoploid Iris hybrid populations in Korea and shed light on the dynamics of interspecific hybridization in the Iris genus.