Hybrid speciation via inheritance of alternate alleles of parental isolating genes.

It is increasingly realized that homoploid hybrid speciation (HHS), which involves no change in chromosome number, is an important mechanism of speciation. HHS will likely increase in frequency as ecological and geographical barriers between species are continuing to be disrupted by human activities. HHS requires the establishment of reproductive isolation between a hybrid and its parents, but the underlying genes and genetic mechanisms remain largely unknown. In this study, we reveal by integrated approaches that reproductive isolation originates in one homoploid hybrid plant species through the inheritance of alternate alleles at genes that determine parental premating isolation. The parent species of this hybrid species are reproductively isolated by differences in flowering time and survivorship on soils containing high concentrations of iron. We found that the hybrid species inherits alleles of parental isolating major genes related to flowering time from one parent and alleles of major genes related to iron tolerance from the other parent. In this way, it became reproductively isolated from one parent by the difference in flowering time and from the other by habitat adaptation (iron tolerance). These findings and further modeling results suggest that HHS may occur relatively easily via the inheritance of alternate parental premating isolating genes and barriers.