In the genus Vaccinium L., there are no fundamental sterility barriers between homoploid members of the same phyletic section (subgenus) (Darrow et al., 1949; Meader and Darrow, 1944; Sharpe, 1953; Sharpe and Darrow, 1959). Diploid, tetraploid, and hexaploid species exist within several sections of the genus. In section Cyanococcus L., which includes the cultivated blueberries, partial to complete self-incompatibility and interfertility between homoploid species has allowed formation of interspecific hybrid swarms (Camp, 1942; Vander Kloet 1983, 1988).
Studies of self-pollination and cross-pollination in several Vaccinium species have provided varying results. In most cases, partial to complete self-incompatibility was present, particularly in wild or “unimproved” plants (Bailey, 1938; Coville, 1921; Meader and Darrow, 1944; Merrill, 1936; Merrill and Johnston, 1940; Morrow, 1943; White and Clark, 1939). In cultivated highbush blueberry (tetraploid V. corymbosum L. and its hybrids), cross-pollination usually produced earlier ripening berries, increased berry size, and higher fruit set. Fewer fully developed seeds were found in self-pollinated berries. Partial to complete self-incompatibility, slower pollen tube growth, or collapse of ovules after fertilization could be factors that produced these results (Morrow, 1943).
Meader and Darrow (1944) studied the crossing behavior of several hexaploid rabbiteye (V. virgatum Aiton) varieties. Fruit set was higher after cross-pollination. Berry weight and seed weight per berry were always higher after cross-pollination than after self-pollination. In the same study, several section Cyanococcus species from different ploidy levels were self- and crosspollinated. Tetraploid forms of V. virgatum were found to be self-sterile but gave a fruit set of 1% to 27% when cross-pollinated with tetraploid V. corymbosum. Vaccinium tenellum Aiton (2x) gave a fruit set below 29% when selfed, but when crossed with a different clone of the same species, had over 80% fruit set. Vaccinium darrowii Camp (2x) set 15% when selfed. Vaccinium darrowii, crossed with other diploid species (V. pallidum Aiton and V. elliottii Chapman), gave a fruit set of 71.5% and 62.2%, respectively. Berries formed after pollinating V. darrowii flowers with V. elliottii pollen ripened earlier than those produced by self-pollination of V. darrowii. However, V. myrsinites (4x) produced two-thirds less fruit set when crossed with other tetraploid species than it did when self-pollinated. Fruit set of self-pollinated V. myrsinites Lam. averaged 79%.
Krebs and Hancock (1990) reported that fruit set was significantly greater after crosspollination of V. corymbosum cultivars. They proposed that variable fertility after self- and cross-pollination in cultivated highbush was produced by early-acting inbreeding depression (seed abortion). They proposed that the degree of self-incompatibility depended on the level of zygotic inbreeding, which depended on the parents that were mated. Reduced self-fertility was attributed to homozygosity for sublethal mutations at loci controlling embryo development or to loss of heterotic interactions at these loci (Hokanson and Hancock, 2000; Krebs and Hancock, 1991).
Diploid V. darrowii has been extensively used in breeding blueberries, but most crosses have involved only one V. darrowii clone, Fla. 4B. Vaccinium darrowii in Florida is quite variable. Lyrene (1986) described three types of V. darrowii in Florida: 1) the Florida panhandle race, a petit form with highly glaucous leaves, which matches Camp's original description of the species (see Camp, 1945); 2) the Ocala Forest race, a tall form with shiny green leaves (glaucous on new growth flushes); and 3) the Istokpoga race, found at the southern end of the species range in the Florida peninsula, a highly variable population with short and tall plants. Introgression from a diploid highbush species (V. fuscatum) was clearly occurring in the Florida peninsula. The use of a wider range of V. darrowii accessions in breeding would provide beneficial diversity in the cultivated gene pool.
Vaccinium arboreum (sparkleberry) is a diploid species that is abundant and widespread in the southeastern United States. Because it is in section Batodendron Marsh. and does not readily make fertile hybrids with cultivated blueberries, which are in section Cyanococcus, it has not been much used in breeding. Recently, genes from V. arboreum have been shown to be useful in breeding (Brooks and Lyrene, 1995; Lyrene, unpublished data), and more information on the pollination biology of this species is needed. The purpose of this research was to determine levels of self-compatibility within the populations of several Vaccinium species and to search for plants with high self-compatibility that could be used in breeding self-compatible southern highbush cultivars.
Chavez, D.J. & Lyrene, P.M. 2009 Interspecific crosses and backcrosses between diploid Vaccinium darrowii and tetraploid Southern Highbush blueberry J. Amer. Soc. Hort. Sci. 134 273 280
Hokanson, K. & Hancock, J. 2000 Early-acting inbreeding depression in three species of Vaccinium (Ericaceae) Sex. Plant Reprod. 13 145 150
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Krebs, S.L. & Hancock, J.F. 1991 Embryonic genetic load in the highbush blueberry, Vaccinium corymbosum (Ericaceae) Amer. J. Bot. 78 1427 1437
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