Multi-trait selection in a <i>Pinus pinaster</i> Aiton breeding program: developing a selection index dealing with genotype by environment interaction in volume

Abstract Currently, the Galician breeding program for Pinus pinaster is focused on advancing to a second generation of the breeding population through multi-trait selection. In order to achieve a successful selection, we studied the genotype by environment interaction (G × E) for volume as part of developing a selection index. Therefore, we measured the volume, stem form, branch angle, and length between whorls in seven open-pollinated, 12 years old progeny trials. Furthermore, in three of these trials, we measured the modulus of elasticity and basic density. We also studied the resistance of families against Bursaphelenchus xylophilus in two artificial inoculation experiments in a greenhouse at age 2. A primary driver of the observed G × E interaction variance was one trial that was quite distinct for summer temperature and dry season length. Across the other six trials, only a few families were unstable, showing a high phenotypic plasticity. In general, these families performed at average to above average levels across most sites but showed very high performance in specific trials. Thus, the presence of genotype by environment interaction does not impose a limitation for considering volume as a selection criterion. Finally, a very appropriate selection index based on the growing demand for high-quality wood for construction, combined with a reduced risk of pine wilt disease damage, was the one that includes resistance to B. xylophilus, stem form, volume and modulus of elasticity. Genetic gains were further increased by forward selection. Our results support the implementation of a multi-trait selection index to achieve genetic improvements in key traits, considering unstable families with high volume performance in specific environments.