Negative inbreeding effects in tree fruit breeding: self-compatibility transmission in almond

Inbreeding depression has been observed in most fruit trees, negatively affecting the offspring of related parents. This problem is steadily increasing due to the repeated utilization of parents in breeding programmes. In almond, self-compatibility transmission from 'Tuono' to its offspring remains partially unexplained due to deviations from the expected genotype ratios. In order to test if these deviations could be due to inbreeding, the S-genotypes of the seedlings of four almond families, 'Tuono' (S(1)S(f )) x 'Ferragnès' (S(1)S(3)), 'Tuono' (S(1)S(f)) x 'Ferralise' (S(1)S(3)) and reciprocal crosses were studied. The S-genotype determination of each seedling by separation of stylar S-RNases and by S-allele-specific PCR amplification gave identical results. The ratio of S-genotypes of the family 'Tuono' x 'Ferralise' was the one least adjusted to the expected 1:1 ratio, because the number of self-compatible seedlings (S(f)S(3)) was less than a half the number of self-incompatible ones (S(1)S(3)). A mechanism acting against inbreeding would favour cross-breeding in the following generation to increase heterozygosity. This fact stresses the need to avoid crosses between related parents in fruit breeding programmes.