Fertility variation and its effects on gene diversity in forest tree populations

Abstract

Differences in fertility among parents influence progeny relatedness, inbreeding and diversity, they should therefore be evaluated and their impacts mitigated. Flower, pollen, fruit and seed production were used to estimate fertility variation from observations in natural stands, plantations and seed orchards. Fertility variation was also compiled from the literature. Differences in fertility are described by the power function y=x (a≥1), where y is the accumulative parental contribution to the progeny and x the ranked proportion of parents. Fertility variations were also described by the sibling coefficient A, which expresses how parents vary in fertility and the likelihood for sibs to occur compared with the situation when parents contribute equally to the gamete pool. A=a=1 when all individuals in the population have the same fertility, and both parameters increase with unbalanced parental contributions to the progeny. Tree fertility varied widely with some parents over – and others under – represented in the gamete pool. The power function exponent and sibling coefficient were higher than 1 in most populations studied. Fertility variation was higher in stands than in seed orchards, and in both cases only about 15% of observations had A values close to 1. Age and flowering abundance appear to have a great impact on fertility variation, higher A values were observed in young populations and during poor flowering years. The increase on group coancestry and the reduction in gene diversity with increasing differences in fertility among parents was quantified. It was also described how relatedness accumulates over generation shifts as a function of differences in fertility. Making parents contribute as uniformly as possible to the progeny, e.g., by collecting the same amount of seed across the population, reduced relatedness and gene diversity was better preserved. The loss of gene diversity at generation shifts is inversely proportional to the number of parents, and using a large number of parents is a way to preserve gene diversity. Knowing the magnitude of fertility variation, the number of parents can be chosen to obtain the desired gene diversity.