Provenance hybridization in a diallel mating scheme of maritime pine (Pinus pinaster). II. Heterosis

Maritime pine (Pinus pinaster Ait.) is a species characterized by a discont inuous natural range with populations as a rule of small size and growing i n heterogeneous environments. Ten provenances belonging to the three biosys tematic groups of the species (Atlantic, Mediterranean, and North African) were crossed using a diallel scheme. Parental provenances and their hybrids were tested and measured for height growth at age 1, 2, 3, 8 and 13, as we ll as for insect resistance and stem crookedness in a trial in southwestern France. The results show a decrease in the variance of heterosis for heigh t growth with age. Heterosis measured as the relative superiority of the hy brid to the midparent varies from -10% to 42% at age 1 and between 2 and 20 % at age 13. There is no age effect on the mean value of heterosis, which a mounts to 9.7% at age 1 and 8.4% at age 13. The value of heterosis was then compared with the genetic distance of parental provenances measured with v arious traits and markers (phenotypic traits, terpenes, isozymes, denaturat ed proteins). There was a weak but significant positive correlation between genetic divergence based on markers and heterosis for height at all ages. These relations were confirmed when the level of heterosis was compared for hybrids clustered according to the subdivision of the species in three maj or groups. The results are in agreement with theoretical analysis of hetero sis and suggest that there is a disequilibrium between loci controlling hei ght growth and neutral loci at the between population level. No significant relationship between levels of heterosis and stem crookedness or insect re sistance was detected.