Heterozygosity and fitness: no association in Scots pine.

The association of six quantitative traits related to fitness with heterozygosity at 12 allozyme loci has been examined in three populations of Scots pine, Pinus sylvestris. Because of several characteristics of this organism and of this extensive data set, it appeared that this study would show a positive association between heterozygosity and these traits if indeed heterozygotes had higher values for these quantitative traits. Using several different statistical techniques including analysis of variance, regression with the scaling recommended from the adaptive distance model, and multiple regression, no evidence of an association was found. For example, only between 7 and 8% of the regression tests were significant at the 5% level and half of these showed a positive association and half showed a negative association. Further, the multiple regression analysis explained on average only 5.8% of the variation observed in the six different traits and only 1.5% of this variation was explained by a positive association. Power analysis was carried out (for the first time on these type of data), both for the single locus heterozygous advantage and the association of individual multiple locus heterozygosity and the quantitative traits. For diameter and height, two traits often used in similar studies, the average power to detect a single locus heterozygous advantage of 0.10 was 0.737 and the average power to detect a mean heterozygote advantage of 0.05 per locus for multiple loci was 0.797. As a result of this study and an examination of the published results from other studies, it appears that what positive associations have been observed are probably not, in large part, due to the presence of intrinsic heterozygote advantage.