Hybridization of highbush blueberry (Vaccinium corymbosum) in section Cyanococcus with two Vaccinium section Bracteata species native to subtropical Asia

The heat- and drought-sensitive highbush blueberry (Vaccinium corymbosum, section Cyanococcus; hereafter referred to as cor), is one of the main cultivated species of blueberry. In contrast, Vaccinium boninense (bon) and Vaccinium wrightii (wri), in the Bracteata section, are native to subtropical Asia, including Japan. The development of new blueberry cultivars combining the qualities of these species is of considerable interest at present. However, it is difficult to obtain hybrids between species from different sections or with different ploidy levels. Furthermore, our earlier attempt to cross cor with bon and wri did not produce any hybrids (Miyashita, 2017). We hypothesized that the rate of success of interspecific hybridization in Vaccinium is reduced by emasculation, which is commonly performed before pollination. Therefore, the present study aimed to evaluate the crossability between cor and the diploid wild species (bon and wri) and to produce new intersectional hybrids. We made 15 combination crosses between six types of Vaccinium plants, including cor, bon, and wri (five species and an interspecific hybrid), with two crossing methods (emasculation and non-emasculation) for each combination included prior to pollination. The hybridity of the resulting seedlings was identified by polymorphism analysis using simple sequence repeat markers for blueberry. Hybrids were obtained from 10 of the 15 combinations. All these combinations were of the cross type such that both parental section and ploidy level were same, or that either section or ploidy level were different. In seven of the 10 combinations, hybrids were obtained only with the non-emasculation method. However, four combinations of the cross type where both section and ploidy level differed between parents failed to produce any hybrids. These results suggested that intersectional interploid cross type has lower crossability than that of either intersectional or interploid type, and that the non-emasculation method is more effective than the conventional emasculation method to obtain hybrids. However, the non-emasculation crossing did not prevent the production of nonhybrid seedlings, indicating the need to confirm the hybridity of each seedling. Subsequently, six reciprocal cross combinations between cor and the three polyploid or hybrid plants derived from bon and wri were made using the non-emasculation method. This yielded six hybrids from two combinations. To our knowledge, these are the first hybrids derived from cor and two Bracteata species native to subtropical Asia. These hybrids might be useful as breeding materials to enhance heat and drought tolerance in highbush blueberry.