Identification of Tetraploid Hybrids Between Vaccinium arboreum and Vaccinium corymbosum by Flow Cytometry

Wild species are often utilized in horticultural breeding programs as sources of novel and/or increased levels of traits not present in the cultivated germplasm. The University of Florida blueberry breeding program is no exception, with the crosses involving wild Vaccinium species occupying up to 20 percent of the breeding resources each cycle. Recently the focus has been on utilizing Vaccinium arboreum, commonly known as sparkleberry, as a source of several adaptive characteristics of interest for blueberry production, such as tolerance to higher soil pH, a deeper root system, and single trunk architecture. One of the primary barriers to incorporation of wild Vaccinium species in the breeding program is the lack of chromosome number congruence among species. Vaccinium species range from diploid (2n=2x=24) to hexaploid (2n=6x=72), with cultivated southern highbush blueberry (Vaccinium corymbosum) being (2n=4x=48). In some cases, a bridge between ploidal levels occurs naturally, as is the case in diploid Vaccinium darrowii, where a relatively high frequency of unreduced gametes can be utilized to cross directly with tetraploid species. However, in diploid Vaccinium arboreum, unreduced gamete production is rare, and direct hybridization with other diploid Vaccinium species resulted in high levels of sterility. This is presumably due to genome divergence, as V. arboreum is in the Vaccinium section Batodendron, whereas all cultivated blueberries belong to section Cyanococcus. Therefore, V. arboreum seed were treated with colchicine to induce chromosome doubling for the purpose of developing plants that could be directly hybridized with tetraploid V. corymbosum. Putatively tetraploid V. arboreum plants were identified by pollen and stomata size and used to cross with tetraploid V. corymbosum germplasm.  The resulting hybrid selections are presumed to result from an amphidiploid pairing of the genomes of V. arboreum and V. corymbosum .  The objective of this research was to confirm tetraploidy of the hybrids and colchicines-seedlings treated by flow cytometry prior to their utilization in backcross and hybrid breeding methods.