Abstract:

Reproductive isolation is essential for sympatric populations of closely related species to maintain species integrity and to prevent genetic introgression caused by hybridization. Primulina is the largest genus of Gesneriaceae in China, with a high degree of species diversity and endemism. Most species of the genus are karst habitat specialists (i.e. calciphiles), and many closely related species show a sympatric distribution in karst landscapes. To better understand the mechanism of sympatry in Primulina, post-pollination reproductive isolation, including pollen competition, fruit set, seed mass, seed germination, and pollen viability, was investigated in two closely related species, P. eburnea and P. mabaensis. Results indicated that the total post-pollination isolation strength for P. eburnea and P. mabaensis was 0.09 and 0.13, respectively, which were not strong enough to prevent hybridization completely. The strength of reproductive isolation from pollen competition and seed germination of P. eburnea and P. mabaensis was negative, suggesting facilitation for gene flow between species; while the strength of the fruit set, seed mass, and pollen viability showed a weak role in preventing interspecies hybridization. However, the two species are able to maintain their integrity well, as rare hybrid individuals are found in nature, suggesting that the existence of pre-pollination isolation mechanisms may play a more important role in maintaining species boundaries in these two species.

Key words: karst plant, Primulina, sympatric populations, reproductive isolation, hybridization, pollen competition