Aneuploid Strawberry (2n = 10x − 3 = 67) Produces Variable Aneuploid Gametes and Fertile Hybrids with Improved Fruit Traits

For Fragaria (strawberry) species, individual chromosome identification is challenging due to the small size of chromosomes; consequently, the chromosome numbers of most cultivars remain unreported. In this study, we investigated the ploidy of 26 strawberry cultivars by counting root-tip chromosomes. Surprisingly, 25 cultivars were octoploid (2n = 8x = 56), whereas only ‘Tokun’ was aneuploid (2n = 10x − 3 = 67). To explore the breeding potential of this aneuploid, we performed reciprocal crosses between ‘Tokun’ and ‘Benihoppe’, a widely cultivated octoploid cultivar. Only the combination with ‘Benihoppe’ as female and ‘Tokun’ as male produced fertile progenies. The progenies had variable chromosome numbers of 61, 62 or 63, meaning that ‘Tokun’ produced aneuploid gametes with varying chromosome numbers ranging from 33 to 35. Some hybrids exhibited superior aroma, firmness, or soluble solids compared with their parents. Specifically, J2138 (2n = 9x − 2 = 61) was selected for its larger single-fruit weight, J2139 (2n = 9x − 2 = 61) and J2153 (2n = 9x − 2 = 61) for their white flesh, and J2151 (2n = 9x − 1 = 62) and J2156 (2n = 9x = 63) for their soluble solids content exceeding that of the higher parent, showing potential value for commercial production and breeding. Strawberry is usually propagated via stolons or through tissue culture; therefore, aneuploids obtained from 8x × aneuploid ‘Tokun’ crosses would be good sources for cultivar breeding.