Effects of pollen availability on pollen immigration and pollen donor diversity in riparian dioecious trees (Salix arbutifolia)

Abstract

Reduced pollen availability in a fragmented population may affect pollen immigration and diversity of pollen donors in the population. To test this prediction, tree locations, tree sizes, and nuclear microsatellite genotypes were determined for 182 offspring, their 8 mothers, and 194 males of a wind-pollinated, dioecious Salix arbutifolia Pallas population along a river isolated over 4 km from other populations. The effects of the distance between the mother and male trees and the size of the male trees on pollen dispersal as well as fractional paternity allocation for the offspring and the number of unsampled males were estimated simultaneously. Based on the estimated parameters of pollen dispersal, the availability of pollen from the males along the river was obtained for the individual mother trees. The pollen availability was not correlated with the proportion of offspring sired by immigrating pollen from unsampled males (8%–50%), but was positively correlated with the effective number of pollen donors in sampled males (1–8). The results suggest that the reduced pollen donor diversity owing to low pollen availability increases the fraction of full-sibs in offspring and subsequent reduction in genetic diversity, but that pollen immigration compensates the offspring for the reduced genetic diversity.