Vaccinium, which includes the commercially important crops blueberries, cranberries, lingonberries, and bilberries, is a large genus, with 150 to 450 species worldwide (Luby et al., 1991). These are grouped into 35 sections (Vander Kloet, 1997). Several Vaccinium sections, including Cyanococcus, Oxycoccus, and Myrtillus, include diploid, tetraploid, and hexaploid species. Highbush blueberry cultivars are tetraploid hybrids involving Vaccinium corymbosum and several other species in section Cyanococcus.
Within section Cyanococcus, all species at a particular ploidy level can be readily hybridized in controlled crosses (Coville, 1927; Darrow and Camp, 1945; Galletta, 1975). F1 hybrids and seedlings from backcrosses and intercrosses are normally vigorous and fully fertile. There is a strong triploid block in Vaccinium, and only a few triploid plants have been reported (Galletta, 1975; Lyrene and Sherman, 1983; Megalos and Ballington, 1988; Vorsa and Ballington, 1991). Most hybrids between diploid and tetraploid Vaccinium species are tetraploid, due to the production of functioning 2n gametes by the diploid (Galletta, 1975).
With so many species and sections, the number of potential intersectional cross combinations in Vaccinium is very large. Relatively few attempted combinations have been reported, and no hybrid seedlings were produced in some reports. Intersectional hybrids obtained by crossing diploid Vaccinium species have usually been sterile or nearly so (Ballington, 1980; Ballington, 2001; Chavez and Lyrene, 2010; Luby et al., 1991; Lyrene, 1991; Lyrene and Ballington, 1986; Ritchie, 1955a). Diploid intersectional hybrids may produce unreduced gametes, which allow them to make fertile tetraploid hybrids when crossed with tetraploid Vaccinium species or hybrids (Ballington, 1980; Brooks and Lyrene, 1998; Lyrene and Ballington, 1986; Zeldin and McCown, 1997).
Tetraploid intersectional hybrids in Vaccinium range from sterile to highly fertile, depending on which species are combined and on the particular hybrid plant within a cross (Lyrene, 2011; Ritchie 1955a, 1955b; Rousi, 1963; Tsuda et al., 2013).
Vaccinium stamineum (section Polycodium; common name deerberry) is a highly polymorphic species with a native range extending from southeastern Ontario, south to central Florida, west to eastern Texas, extreme eastern Oklahoma, and extreme southeastern Kansas (Vander Kloet, 1988). A few isolated populations occur in central Mexico (Vander Kloet, 1988). Although Ashe (1931) divided Polycodium into 6 sections and 21 species, most taxonomists now treat Polycodium as one highly polymorphic species (Baker, 1970; Vander Kloet, 1988). On the coastal plain of the southeastern United States, the deerberry ranges from a subshrub 0.3 m tall, with stems that form large, open colonies, to arborescent shrubs nearly 5.0 m tall (Baker, 1970). In Florida, deerberries commonly grow on infertile, coarse-textured, white sand with little water-holding capacity. Sharpe and Sherman (1971) noted that deerberries are able to produce large berries when growing in the forest on dry, sandy soils. Deerberries in Florida often share xeric sites with turkey oak (Quercus laevis), Florida rosemary (Ceratiola ericoides), garberia (Garberia heterophylla), and other drought-tolerant species. Like other Vaccinium species, deerberries grow only on soils of low pH.
Ballington et al. (1984b) compared seedlings of 11 Vaccinium species native in the southeastern United States in a uniform garden at Castle Hayne, NC, along with four highbush cultivars and four rabbiteye cultivars. Vaccinium stamineum had larger berries, higher percentage soluble solids, and higher berry firmness than any of the 10 section Cyanococcus species in the study and higher firmness than any of the cultivars. Mean soluble solids for 25 deerberry seedlings was 13.3%, compared with a range of 7.9% to 9.0% for four highbush cultivars and 7.8% to 9.8% for four rabbiteye cultivars. Vaccinium stamineum and Vaccinium elliottii seedlings had smaller stem scar diameter than all other populations except Vaccinium myrtilloides. The authors concluded that “Vaccinium stamineum is probably worthy of domestication and improvement on its own merits as a new crop.” Fruit from three genotypes of V. stamineum grown at Jackson Springs, NC, had soluble solid content ranging from 14.7% to 16.8%, which was much higher than strawberries (6.0% to 9.7%) and blueberries (9.0% to 10.2%) (Wang and Ballington, 2006). Total acidity of deerberries ranged from 0.10% to 0.21%, which was lower than strawberries (0.42–0.98) and blueberries (0.38–0.82).
Highbush blueberries develop conspicuous flower buds in autumn. These remain dormant over winter and open to make short, leafless flowering branches in the spring. By contrast, V. stamineum does not form conspicuous flower buds in the fall: it flowers on new leafy shoots produced in the spring.
In flowers of highbush blueberry, the stigma and style are enclosed within the corolla tube until anthesis. At anthesis, the stigma is about even with the distal edge of the corolla tube, and the distal ends of the anthers are 1 to 3 mm inside the end of the corolla tube. In V. stamineum, the corolla tube is very short. The flowers are open in the bud (Camp, 1945) and the styles are visible outside the developing corolla tube long before anthesis. At anthesis, the corollas are short, the tips of the anthers extend 2 to 3 mm beyond the corolla tube, and the styles extend ≈5 mm beyond the end of the corolla tube (Fig. 1).
Ballington (1995) noted that the fruit of wild V. stamineum is quite large for wild Vaccinium, ranging from 5 to 16 mm in diameter, with occasional plants with fruit 19 mm in diameter. The color of mature deerberry fruit varies from plant to plant, ranging from greenish-white, to reddish-black, to dark purple (Ballington, 1995).
Ballinger et al. (1982) found that the anthocyanins of deerberries resemble those of Vaccinium section Oxycoccus, which includes cranberries. Ballington et al. (1988) found that the anthocyanins of deerberries, cranberries (Vaccinium oxycoccus), lingonberries (Vaccinium vitis-idaea, section Vitis-idaea), and Vaccinium parvifolium (section Myrtillus) were similar. Vaccinium stamineum had only three distinct types of anthocyanins. By contrast, sparkleberry (Vaccinium arboreum, section Batodendron) had at least 12 distinct types, and its array of anthocyanins resembled highbush blueberry much more closely than did V. stamineum (Ballinger et al., 1982). Based on anthocyanin data, deerberries appeared to be less closely related to highbush blueberries than the sparkleberries.
Vaccinium stamineum fruit typically have some bitterness in the skin (Ballington, 1995), but the levels of bitterness and sweetness are quite variable. Some purple-fruited plants in the sandhill region of South Carolina, a strip of ancient beach dunes that divides the Piedmont from the Coastal Plain, were only slightly bitter, and some white-fruited forms were completely free of bitterness and had a pleasant “peary” flavor. Ballington (1995) quoted Steyermark (1963) as saying that the fruit of V. stamineum served cold after being cooked has a flavor suggesting cranberry and gooseberry sauce combined with grapefruit marmalade. Ballinger et al. (1981) stated that deerberry fruit resembles cranberries in fresh market and culinary qualities as well as in anthocyanin and flavonol content. Uttal (1987) described the deerberries of Virginia as “sour, bitter, or mildly sweet, the best acceptable for preserves.”
Several characteristics have prevented the domestication of the deerberry as a crop plant. The plants are very difficult to propagate by stem or root cuttings, making it hard to clone superior plants for breeding or for gardening. The fruit of most plants is bitter, and from part of the geographic range, astringent. The berries on most bushes fall from the plant with pedicel attached shortly after ripening. In the forest, at the peak date of deerberry ripening, it can be hard to find plants with a large crop of mature fruit still hanging on the bush ready to harvest. Individual plants vary widely in their tendency for quick abscission of ripe berries (shattering). Ballington et al. (1984b) found non-shattering genotypes from South Carolina and Georgia, and non-shattering or delayed-shattering genotypes are not uncommon in the sandhills of north Florida.
Longley (1927) and Coville (1927) found the chromosome number of V. stamineum to be 2n = 2x = 24. A subsequent count by McDaniel (1962) and 26 counts by Baker (1970) have all been diploid. Despite the wide range and morphological diversity of V. stamineum, tetraploid plants have not been reported from the wild.
Coville (1937) reported that diploid species in section Cyanococcus could be crossed with V. stamineum. He obtained viable hybrids between V. stamineum and V. myrtilloides, a Cyanococcus diploid (Darrow and Camp (1945). From two deerberry × blueberry crosses made in 1945, Darrow (1947) obtained 70 seeds, but he gave no details as to which blueberries were used in the crosses or whether the seeds were germinated. Meader obtained hybrids by crossing diploid lingonberry (V. vitis-idaea, section Vitis-idaea) with V. stamineum (Ballington, 2001), but the hybrids were not vigorous (Kim Hummer, personal communication). Sharpe and Sherman (1971), from their 1970 crosses, produced what they believed based on vegetative morphology to be hybrids between section Cyanococcus and V. stamineum, but they were awaiting flowers and fruit to confirm the hybridity of the plants, and gave no subsequent reports.
Ballington (1980) obtained hybrid seedlings by pollinating flowers of four diploid Cyanococcus species (Vaccinium atrococcum, Vaccinium caesariense, Vaccinium darrowii, and Vaccinium tenellum) with pollen from V. stamineum from central South Carolina. Except for V. stamineum × V. caesariense, in which 50 pollinated flowers gave two seedlings, the reciprocal crosses failed. Vaccinium darrowii × V. stamineum diploid hybrids yielded F1 progeny sufficiently fertile to produce F2 and BC1 progeny (Ballington, 1980), and partially fertile diploid hybrids were also obtained from the cross V. tenellum × V. stamineum (Ballington, 2001). In general, however, diploid hybrids between section Cyanococcus species and section Polycodium species have had very low fertility.
In 1980, three diploid Cyanococcus species (V. darrowii, Vaccinium fuscatum, and V. elliottii) were crossed as females with V. stamineum in Florida (P.M. Lyrene, unpublished data). Seedlings were obtained from each of the crosses. A few dozen of the most vigorous hybrids were maintained in a field nursery for several years, but the plants had low vigor. A few eventually flowered, but produced no berries. Diploid hybrids between V. darrowii and V. arboreum (section Batodendron), planted in the same field, were far more vigorous and produced a small amount of viable seed after open pollination in the presence of tetraploid highbush cultivars (Lyrene, 1991).
The purpose of this paper is to report the results of crosses and backcrosses involving highbush blueberry cultivars and tetraploid V. stamineum plants selected after colchicine treatment. The long-range goal of the project is to enable gene flow from cultivated highbush blueberries into V. stamineum and from V. stamineum into highbush. Genes from highbush may enable the production of V. stamineum populations that root readily from softwood cuttings, lack bitterness in the berry, and retain the ripe berries on the bush through harvest. Genes from V. stamineum might allow production of highbush blueberries having upland soil adaptation, open flower clusters, and berries that ripen late, and have purple flesh, increased firmness, higher soluble solids, and exotic flavors.
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