Chromosome Doubling in Genetically Diverse Bilberry (<i>Vaccinium myrtillus</i> L.) Accessions and Evaluation of Tetraploids in Terms of Phenotype and Ability to Cross with Highbush Blueberry (<i>V. corymbosum</i> L.)

To expand the gene pool and introduce new traits to the tetraploid cultivars of <i>Vaccinium corymbosum</i> from wild diploid species <i>V. myrtillus</i>, it is necessary to double the chromosome number in diploid species in order to overcome a post zygotic crossing barrier and a strong triploid block, existing within the genus <i>Vaccinium</i>. Five genetically diverse bilberry genotypes were selected from 21 accessions taken from the breeding collection of the National Institute of Horticultural Research (Skierniewice, Poland) for this study. The bilberry genotypes were derived from the Polish locations of Bolimów Landscape Park, Budy Grabskie and forest complex Zwierzyniec (Łódź Province), and habitats in Norway. The selection of genotypes was made based on the analysis of amplified fragment length polymorphism (AFLP-PCR). Analysis of the Jaccard similarity indexes and the UPGMA method revealed that the examined accessions formed two main groups on the dendrogram. The first group consisted of accessions from Norway, while the second group agglomerated Polish accessions. A further two classes were distinguished in the Polish group: the first included accessions from Budy Grabskie and the second from Zwierzyniec, located ca. 9 km from Budy Grabskie. In order to obtain plant material for in vitro polyploidisation, in vitro shoot cultures of the selected accessions were initiated and multiplied. Both antimitotics used, colchicine and APM, induced tetraploids for all of the accessions. The obtained tetraploids were multiplied, rooted ex vitro and grown in a greenhouse and then in a field. The first flowering was observed in 1.5-year-old plants, either diploid or tetraploid. Diploids bloomed slightly earlier and more profusely than tetraploid plants. Compared to diploids, autotetraploids had significantly larger flowers by ca. 64% and larger pollen tetrads by ca. 35%. The germination capacity of pollen tetrads was high in tetraploids (87.8%), although slightly lower than in diploids (94.3%). After pollinating the flowers of three highbush blueberry cultivars with pollen from the bilberry tetraploid accession, J-4-4x, the plants formed fruits, some of which contained properly formed seeds. The effectiveness of interspecific crossing between <i>V. corymbosum</i> and tetraploid <i>V. myrtillus</i>, defined as the percentage of obtained seedlings in relation to the number of pollinated flowers, was highest (53.3%) in the blueberry ‘Liberty’, and lower in ‘Bluecrop’ and ‘Northland’, 14.8% and 10.0%, respectively. Before using the seedlings for further breeding, their hybridity will be confirmed by molecular markers and the phenotype will be evaluated.