Asymmetrical hybridization between Trillium apetalon and T. tschonoskii for the formation of a hybrid T. miyabeanum (Melanthiaceae)

Trillium apetalon (4×) and T. tschonoskii (4×) hybridize commonly where both species grow sympatrically, leading to the formation of tetraploid T. miyabeanum in Hokkaido, Japan. The present study aimed to determine which isolation factor is responsible for the frequency and asymmetry of hybrid T. miyabeanum formation in a sympatric population of T. apetalon and T. tschonoskii . We examined the contributions and strengths of four reproductive isolation barriers of T. miyabeanum formation: flowering phenology, breeding system, genetic isolation, and hybrid inviability. In addition, we also investigated the effect of flowering phenology on reproductive success (i.e., seed production and outcrossing rates) and outputs (i.e., ovule production) for T. apetalon and T. tschonoskii . We calculated the absolute contribution of each isolation barrier to the total reproductive isolation and found that flowering phenology and differences in breeding systems between the two parental species were more effective when T. apetalon was the maternal parent. Furthermore, hybrids with T. apetalon as the maternal parent had lower viability than those of the reciprocal cross and did not reach the flowering stage. Particularly, absolute contribution of premating isolation, especially by flowering phenology and breeding system, was higher than that of other isolation factors for both crossing directions. For the formation of T. miyabeanum , we concluded that asymmetry of hybridization between T. apetalon and T. tschonoskii would be caused by strong premating isolations. The asymmetry of the isolating barriers may promote T. tschonoskii as the maternal parent of T. miyabeanum .