Study on forest tree breeding for male sterility and low pollen allergen of Cryptomeria japonica D. Don: Pollinosis measures by breeding method for reduction of C. japonica pollen allergen in the air

Sugi, Cryptomeria japonica D. Don, is one of the most commercially important conifers in Japan. However, C. japonica pollinosis, caused by Cry j 1 and Cry j 2 proteins in C. japonica pollen, has become a serious allergic disease. Therefore, an important issue is to reduce the amount of airborne pollen by breeding. Male-sterile tree of C. japonica was found in Toyama prefecture, Japan. The tree was 26 years old in 2006. There was no difference between the male-sterile tree and normal trees in size and shape of the male flower. However, the microspores collapsed after separating from pollen tetrads, resulting in total male sterility. In this case, male sterility is controlled by a recessive allele at a single locus and is expressed only in homozygotes (aa). To find plus tree clones which are heterozygous for a male-sterility gene (Aa), 64 clones were crossed with a homozygous male-sterile tree (aa). Fsub(1) seedlings obtained from the crossing between the male-sterile mother tree and a plus tree clone, Ohara 13, produced 64 male-sterile individuals and 52 fertile individuals. The segregation ratio fitted the expected 1:1 ratio according to a chi-square test. These results clearly demonstrate that the Ohara 13 clone is heterozygous for a male-sterility gene. It is possible to create superior male-sterile trees by crossing with plus trees possessing the male-sterility gene. Cry j 1 content of 420 plus tree clones showed variation from 0.38 to 10.23 pg per pollen grain and 7 clones had less than one pg per pollen grain. The heritability of Cry j 1 estimated by parent-offspring regression was very high at 1.0. From these results, it was suggested that a large number of seedling with low allergens could be created by crossing the 7 trees selected in this study. The miniature seed orchard of Cryptomeria japonica trees with a heterozygous male-sterility gene (Aa) was established in the outdoors. From the result of frequency of open-pollinated male-sterile seedlings (aa) in the seed orchard, it was presumed that pollen contamination rate was about 62%. The glasshouse miniature seed orchard was established to prevent pollen contamination. The germination frequency of seeds produced in the glasshouse was almost the same as that of seeds produced in outdoor seed orchards. So, glasshouse miniature seed orchard with low allergens becomes an effective for pollinosis measure. The use of superior male sterile cuttings and low allergens seedlings will reduce pollen production while maintaining the yield of high-quality timber in the future.