Abstract: The maintenance of species boundaries between widespread and narrow endemic congeneric species in sympatric sites remains a fundamental question in ecology and evolutionary biology. For plants with specialized pollination mechanisms, pre- and postpollination isolation mechanisms likely play distinct roles in reproductive isolation and species integrity. Parnassia (Celastraceae) is characterized by one-by-one stamen movement and has its distribution center in southwest China, where many widespread and local endemic species coexist. To quantify pre- and postpollination barriers and their relative roles in maintaining species boundaries, we conducted field experiments with the widespread Parnassia wightiana Wall. ex Wight & Arn. and the local endemic Parnassia amoena Diels over two separate years at Jinfo Mountain, southwest China. We examined four prepollination barriers (ecogeography, blooming phenology, stamen movement, and pollinator type) and three postpollination barriers (fruit set, seed production, and seed viability). Our findings indicate that prepollination barriers played a more significant role in reproductive isolation than postpollination barriers. For the widely distributed P. wightiana, ecogeographical isolation was the primary barrier, followed by phenology and pollinator type isolation. In the narrow endemic P. amoena, which exhibits slower stamen movement, this feature contributed significantly to isolation, with phenological isolation being the second most important factor. Among postpollination barriers, seed viability was the most significant for both species. Our results indicate that prepollination barriers are the predominant isolation mechanism for these two sympatric Parnassia species, and stamen movement may serve as a novel type of prepollination barrier, particularly for the narrow endemic species.

Key words: pollination isolation, reproductive barrier, species boundary, stamen movement, sympatry.