A streamlined synthetic octoploid system that emphasizes Fragaria vesca as a bridge species
The synthetic octoploid (SO) system is a method of combining diploid, tetraploid and hexaploid 'Fragaria' species into hybrid octoploids for introgression into octoploid strawberry cultivars of 'F'. * 'ananassa'. In this study, (diploid * diploid) and (diploid * hexaploid) crosses were used to create tetraploids followed by (tetraploid * tetraploid) crosses to create SOs from a wide germplasm base. Techniques refined for the SO system included flow cytometry protocols for identification of ploidy levels, 'in vitro' germination and 'in vitro' delivery of colchicine using the dropper method. Of the 36 species combinations investigated, 20 resulted in viable F 1 plants, 13 bloomed and produced seeds, and seven did not bloom. When nine species were intercrossed at the diploid level, the best crossability occurred with 'F. pentaphylla' and 'F. vesca'; each formed fertile F1 hybrids with five other species. No significant differences in interspecific crossability were found among ' F. vesca subspecies bractacea, americana, vesca' and 'var. semperflorens' when pollinated with 'F. nilgerrensis, F. nubicola, F. pentaphylla,' and 'F. viridis'. These hybrids were treated with colchicine to create tetraploids. Tetraploids were also created from 'F. moschata' (6*) hybridized with 'F. viridis ' or 'F. nubicola'. The crossing strategy at the tetraploid level emphasized 'F. vesca ' as a bridge species for 'F. nilgerrensis, F. nubicola, F. pentaphylla', and 'F. viridis'. A collection of 104 tetraploids including ('F. vesca' * 'diploid') hybrids, 'F. vesca' (4*), 'F. orientalis', ('F. nubicola' * 'F. moschata'), (' F. moschata' * 'F. viridis'), ['F. orientalis ' ,('F. moschata' * ' diploid species')] and ('F. viridis' * ' F. nubicola') were intercrossed and doubled with colchicine to create SOs. When the ('F. vesca' * 'diploid species') colchi-tetraploids were intercrossed, only 37% fruit set with an average of three seeds/flower. When these colchiploids were pollinated with tetraploids derived from 'F. orientalis' or ('F. moschata' * 'diploid species'), 52% fruit set and 10 seeds/flower were obtained. A total of 192 SOs were created from 25 combinations of species. Of the 98 SO's planted in 1997, 28 produced fruit in 1998. The seven species successfully incorporated into SOs were 'F. vesca, F. nilgerrensis, F. nubicola, F. pentaphylla, F. viridis, F. orientalis', and ' F. moschata'. The results suggests 'Fragaria' species are closely related and have great potential for breeding at the diploid and tetraploid levels and for introgression into 8 material.