What sampling is needed for reliable estimations of genetic diversity in Fraxinus excelsior L. (Oleaceae)?

Sample size is a critical issue for genetic diversity studies and conservation programs. However, sample size evaluation requires previous knowledge of allele frequencies estimated with precision and this is not often the case. Here, we evaluated sample size requirements for accurate genetic diversity in adult trees and family arrays in a 12 ha plot of Fraxinus excelsior L. (Oleaceae) in a community forest in central France. Data consisted of 579 adult trees and 480 offspring from 24 families genotyped at four nuclear microsatellites. Mean square errors (MSE) estimates performed on Monte Carlo simulations of resampled data indicated that several adult individuals (> 300) are necessary for accurate measures of allele richness. However, expected heterozygosity requires smaller samples (< 30). Seeds captured about 90% of adult allelic diversity requiring a sampling effort roughly 50% larger than that of adult trees (480 seeds vs. 300 adults) suggesting that seed sampling is heavily penalized for allele counts. Nevertheless, gene diversity of seeds was essentially identical to that of the adult population. Extrapolation of these results to other ash tree populations appears feasible because of similar levels of diversity reported in the literature but it is not granted for species with significant selfing or high genetic structure.