Pyramiding of male-sterile genes in Cryptomeria japonica is currently being carried out in Niigata prefecture, Japan. This is the first attempt to apply pyramid breeding to forest trees. As the breeding materials for male sterility are limited, special attention must be given to increased genetic relatedness in the process of pyramid breeding to avoid the effects of inbreeding depression as much as possible. In this study, we estimated genetic relatedness based on 246 genome-wide SNP markers for male-sterile individuals in Niigata Prefecture (n = 6) and individuals doubly heterozygous for two male-sterile genes (hereafter referred to as “double-hetero”) produced by marker- assisted selection (n = 124). The pairwise relatedness estimates between male-sterile individuals selected from the same area in Niigata Prefecture were low (−0.01 ± 0.08, mean ± standard deviation), suggesting that there will be almost no negative effects even if the F1 of these male-sterile individuals is used for artificial crossing. On the other hand, the pairwise relatedness between double-hetero individuals in this study was higher than the theoretical relatedness values, as individuals with the slightly higher relatedness were used as parents in artificial crossings. However, there was a large variance in pairwise relatedness for double-hetero individuals. This result suggested that it may be possible to avoid the adverse effects of inbreeding depression by using a pair of double-heteros with lower relatedness for artificial crossing, when we produce a double-homo using the limited breeding materials of male- sterile individuals. It will also be important to continue additional selection of new breeding material for male sterility.
