No abstract

Shoot regeneration from Rubus leaves was obtained on a medium containing MS salts, vitamins and sugars, Staba vitamins, casein hydrolysate (100 mgl-~) and 10 #M thidiazuron. Shoot regeneration from Malus leaves was obtained on N 6 rice anther medium with 5 #M thidiazuron. In vitro pretreatment of source shoots with either colchicine or thidiazuron enhanced the organogenic potential of detached leaves of two Rubus hybrids. The response to colchicine was quadratic and occurred at non-mutagenic concentrations (75-250 #M). The response to thidiazuron was exponential between 0 and 5 #M. When applied as a pretreatment, the effectiveness of several different cytokinins (benzyladenine, thidiazuron, zeatin) at enhancing Malus and Rubus organogenesis was related to the shoot proliferation activity of the cytokinin and to treatment-induced variation in leaf and petiole size.

The potential of using Fragaria vesca L. as a bridge species for interspecific hybridization to F. nilgerrensis Schlect, F. nubicola Lindl., F. pentaphylla Losinsk, and F. viridis Duch. was investigated using a wide germplasm base of 40 F. vesca accessions. This study was successful in producing many hybrids between F. vesca and other diploid species indicating its value as a bridge species. Of the species used as males, F. nubicola, F. pentaphylla, and F. viridis accessions were more successful, averaging 8 to 16 fruit and 16 to 25 seeds/fruit. It was most difficult to obtain hybrids with F. nilgerrensis, which had only three seeds per fruit. Differences among pollen donors were minimal when hybrid seeds were germinated in vitro. For different species combinations, 75% to 99% of seeds had embryos and 77% to 89% of these embryos germinated. The lack of significant differences in crossability variables among the four F. vesca subspecies [i.e., ssp. americana (Porter) Staudt, ssp. bracteata (Heller) Staudt, ssp. vesca L., and ssp. vesca var. semper-florens L.] demonstrated the similarity between these species and the strong potential for gene flow between F. vesca and other diploid species. As European and North American F. vesca subspecies are not sufficiently divergent to differ in interspecific hybridization, F. vesca may be a younger species rather than an older progenitor species.

The crosses Fragaria moschata × F. nubicola, F. moschata × F. viridis, and their reciprocal combinations were done to create tetraploids for eventual introgression into octoploid cultivars of Fragaria × ananassa via the synthetic octoploid system. The combination F. nubicola × F. moschata averaged 3.3 healthy plants/pollination, which was 2.3 times higher than the next best combination, F. moschata × F. viridis. Fragaria viridis × F. moschata crosses averaged only 0.1 healthy plants/pollination and no plants were obtained from F. moschata × F. nubicola hybridizations. The success of the F. nubicola × F. moschata crosses in this study indicated that embryo/endosperm ratios were not as important a factor in producing hybrids with F. moschata × diploids species, as previously suggested.Although 90% of pollinated flowers set fruit with an average of 19 seeds/pollination only 15% of the seeds contained healthy, white embryos and only 38% of these developed into healthy seedlings. The remaining 62% of embryos lacked pigment, had short hypocotyls or arrested development after slight expansion out of the seed coat. Only 39% of the crosses resulted in viable progenies. After one year, 27% of these hybrids bloomed and 71% of the blooming hybrids were tetraploids. All six diploid and six of the seven hexaploid accessions were successfully incorporated into 89 hybrids.