Hybrid speciation via inheritance of alternate alleles of parental isolating genes

Abstract It is increasingly realized that homoploid hybrid speciation (HHS), involving no change in chromosome number, is an important mechanism of speciation likely to increase in frequency as ecological and geographical barriers between species continue to be disrupted by human activities into the future. HHS requires the establishment of reproductive isolation between a hybrid and its parents. However, the genes and genetic mechanisms involved remain largely unknown at the molecular level. Here we show by multiple approaches that reproductive isolation originated in one homoploid hybrid plant species through the inheritance of alternate alleles at genes determining 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 show that the hybrid species inherited 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 a difference in flowering time and from the other by habitat adaptation (iron tolerance). These findings and results of modeling the process suggest that HHS may occur relatively easily via the inheritance of alternate, parental premating isolating genes and barriers.