シオギクとスプレーギクとの交雑実生の染色体数変異に関する研究

Abstract

Chromosome numbers were counted in three individuals of Dendranthema shiwogiku, 16 cultivars of spray type chrysanthemum (D. x grandiflorum), 40 F1 hybrids between D. shiwogiku and a spray type cultivar, and 68 backcross (BC1) hybrids between F1 hybrid and spray type cultivar.Aneuploids were observed in D. shiwogiku and spray type cultivars. The chromosome numbers of F1 hybrids varied from 2n=60+B to 68; the average was 2n= 63.9 which is close to the heptaploid chromosome number of 63. Chromosome numbers of BC1 hybrids varied widely from 2n =53 to 65; the average number was 2n =59.0 whose ploidy was considered as the intermediate between hexa- and heptaploidy.Aneuploids were also observed in offsprings of each cross, and their average chromosome number was the same as the mean chromosome number of the parents. It seems that the aneuploids in spray type cultivars and heptaploid F1 hybrids are not sterile, and that their chromosomes are divided to two equal parts.Small B chromosomes, lacking the constriction, were observed in two individuals of D. shiwogiku (frequency; 67%), one spray type cultivar (6%), 13 F1 hybrids (33%), and 12 BC1 hybrids (18%). The B chromosomes were equally divided as well as the autosomes.No correlation exists between chromosome number and diameter of the upper flower head in the spray type cultivars, F1 hybrids, and BC1 hybrids, whereas a significant negative correlation between ploidy level and diameter flower head exists.