Extensive selfing in an endangered population of Pinus parviflora var. parviflora (Pinaceae) in the Boso Hills, Japan

Wind-pollinated trees such as conifers have been considered relatively resistant to the deleterious effects of fragmentation and low density by virtue of their abundant production of airborne pollen. However, some studies indicated that population density affects the mating system even in wind-pollinated trees. In the present study, we examined the mating system and genetic structure of an endangered population (Boso) of Pinus parviflora var. parviflora, whose size has fallen by 80 % in only the last quarter century. Genetic analyses using four nuclear microsatellite markers showed that the population was in Hardy–Weinberg equilibrium and harbored a high level of genetic variation comparable to that of a larger population. Additionally, only weak genetic structure was detected among local patches. On the other hand, the outcrossing rate of seeds in the Boso population was much lower (0.277 from MLTR and 0.297 from paternity analysis) than that found in a larger population of the species (0.778 from MLTR). The outcrossing rate for each mother tree was found to correlate positively with local density around the tree in the Boso population. This suggests that the extensive selfing would be a direct result of pollen limitation caused by the extremely low density of the population: an average of 0.201 trees per hectare for local patches. Because the adult population was in HWE, it is likely that the selfed seeds produced at present cannot contribute to population regeneration. The present study suggests that low population density would increase extinction risk even for wind-pollinated conifer populations.