Assessment of the ability to cross bilberry (Vaccinium myrtillus L.) autotetraploids with highbush blueberry (V. corymbosum L.)

The aim of our research was to obtain autotetraploids of bilberry (Vaccinium myrtillus L.) capable of crossing with cultivated species of highbush blueberry (V. corymbosum L.). Rapid changes in agricultural technology and growing interest in the consumption of blueberry fruits due to their health benefits have directed breeders’ attention to developing improved cultivars. However, highbush blueberry cultivars with purple fruit flesh, containing high anthocyanin content, are not available yet. The fruit flesh of cultivated Vaccinium species is translucent as it does not contain anthocyanins. In contrast, the bilberry is characterized by purple fruit flesh. To introduce new traits, e.g., anthocyanin-containing flesh from wild diploid species of V. myrtillus to V. corymbosum which is tetraploid, it is crucial to double the chromosome number in diploid species. This is necessary to overcome a post zygotic crossing barrier, a strong triploid block, existing within the genus Vaccinium. In previous studies, several autotetraploids were induced in vitro for five genetically diverse bilberry accessions. The obtained tetraploids were multiplied in vitro, rooted and grown ex vitro. The first flowering was observed in 1.5-year-old plants. Bilberry autotetraploids bloomed slightly later and they had less flowers but significantly larger leaves, flowers and pollen tetrads compared to diploids. The germination capacity of pollen tetrads was high in tetraploids (ca. 90%). After pollinating the flowers of three highbush blueberry cultivars with pollen from the bilberry tetraploid plants, a few seedlings were obtained for each of the crosses with the cultivars ‘Bluecrop’, ‘Liberty’, and ‘Northland’. Most of the seedlings from interspecific crosses showed a weaker growth and had fewer and shorter shoots with slightly smaller leaves compared to V. corymbosum seedlings. The hybrid nature of some seedlings was confirmed by ISSR molecular markers. Germination of V. myrtillus pollen grains on stigma pistils of V. corymbosum 24 and 48 h after pollination was generally very poor, but pollen germination in reciprocal crossing was more abundant and fast. Further molecular analyses and phenotypic observations are needed to confirm the hybrid status of seedlings originated from the interspecific crosses of V. corymbosum and V. myrtillus.