Characteristics of F1 Progenies Obtained by Crossing a Small-flowered Tetraploid Garden Chrysanthemum, 'YS' (2n=36), with Hexaploid Chrysanthemum Cultivars (2n=54)

To elucidate the usefulness of the small-flowered chrysanthemum, 'YS', which has the smallest chromosome number (2n=36) among parental breeding materials, the characteristics of progenies derived from 'YS' crosses with hexaploid (2n=54) cultivars were investigated. Our histological observations on the developmental process of the anther showed that the tapetum did not dehisce at the mature pollen stage. Therefore, we regard 'YS' as one example of male sterility. Cross pollinations between 'YS' or its polyploid (2n=72) with three hexaploid cultivars (2n=54) yielded a number of seeds per inflorescence, but their percentage of germination differed with cross combinations. Examinations of the F1 progenies revealed that most progenies grew vigorously, and their mean height exceeded the intermediate value of their parents. Flower types that were classified according to the morphology of the capitulum showed that the progenies between 'YS' (petaloidy-type) × three cultivars (nonpetaloidy-type) were characteristic of the male parent type. The diameters of inflorescences in the progenies derived from 'YS' (small-type) × three cultivars (medium-type) were about 10% smaller than the intermediate value of their parents, whereas their pollen fertilities were: 'YS' (5.3%), its polyploid (43%), and the three cultivars (78.6- 83.1%), similar to their male parents. The progenies derived from 'YS' (male-sterile) × three cultivars (male-fertile) were all male-fertile. The chromosome numbers in the progenies between 'YS' (2n=36) × three cultivars (2n=54) were most frequently pentaploid (2n=45), intermediate between the parents. The stability of the heritable characteristics of progenies over four years showed that although it differed between years and crosses, variable characteristics were plant height, plant form and flowering times, whereas the stable characteristics were their growth appearance, diameter of inflorescence and flower types. The most stable characteristic was flower color. From this study, we considered that 'YS' poses no problem to use as a female parent in crosses between cultivars whose chromosome numbers differ significantly.

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