In Fertile Intersectional F1 Hybrids of 4x Andean Blueberry (Vaccinium meridionale) and 4x American Cranberry (Vaccinium macrocarpon)1 , authors Mark Ehlenfeldt, James Polashock, Nicholi Vorsa, Juan Zalapa, Fernando de la Torre, and James Luteyn announce the successful breeding of a “Cran-Blue.”
One parent is our familiar Vaccinium macrocarpon (a 4x variety), and the other parent is Vaccinium meridionale—a tetraploid blueberry native to mountainous locations in Jamaica, Colombia, and Venezuela. This father was chosen because its concentrated spring flowering period and its growth habit are desirable traits for mechanical harvest.
The first generation of macrocarpon x meridionale had flowers and fruits that were between the two parents, and grew vigorously. Fertility of the flowers’ male and female parts were tested, and acceptable results were found—this is exciting. When breeding across species, infertile crosses are not too difficult to obtain, but fertile crosses are noteworthy. Fertility tests conducted using the macrocarpon x meridionale cross as a male parent suggest that this cross may be able to used in conventional cranberry breeding without much difficulty.
These crosses are promising to add into the cranberry gene pool—increased flower number and fruit set will become available. Modifying fruit quality (for example, introducing anthocyanins into the fruit flesh as well as the skin) based on these traits may also be possible.
The work of breeding crosses relies on manual manipulation—pollen is collected using paintbrushes, and refrigerated & desiccated if storage is required. Graphite tipped pencils are then dipped into pollen, and pollen is applied to the desired stigmas. A fully insect-free greenhouse is required for this work.
To avoid incorrect chromosome outcomes, the F1 hybrids’ ploidy levels were checked using flow cytometry.
To evaluate female fertility, pollination and fruit set records were kept. Fruit were then collected when they were ripe. Fruit size was measured and seed was extracted, and then seeds were classified as good, good/fair, fair, fair/poor, poor, or aborted. Those rated from “good” to “fair” were judged likely to be capable of germinating.
Seeds were then germinated in a greenhouse, and after developing 3 true leaves, transferred into 36-cell flats to grow. In their second season, the primary hybrids were transplanted to 3-liter pots.
Male fertility was tested by a staining and visual evaluation. Pollen samples were rated from “very good” (almost all tetrads), “good” (tetrads and triads), “fair” (mostly triads), and “poor” (mostly aborted grains).
The crosses in this study were highly successful for an intersectional cross, with 20.7 seeds per fruit. 500+ young and vigorous hybrids are growing in the greenhouse from this cross.
These hybrids show an “intermediate” growth habit between blueberry and cranberry, as seen in Figures 1 and 2. Plants grown in the greenhouse grow with a vining habit, but plants grown outdoors are growing more upright and display some branching. Unlike cranberry, these hybrids have not developed a red/purple leaf color upon entering dormancy—and in fact no specific “dormancy” was observed. Leaves were retained and growth appeared to end as days got shorter and cooler. In some cases, prior-year leaves were not lost until new growth began in the spring.
Flowers on the crosses have been observed across the full spectrum of similarity to the mother and the father of the cross, but most were more similar to meridionale than macrocarpon. Fruit, as well, was variable in shape—meaning that future research can be assisted by selecting those fruits which have the most desirable traits with which to expand the available gene pool. Color variation was observed: from red blush over an apple-green base, to cranberry red, to a deep red-purple. Some crosses displayed flesh pigmentation in very ripe fruit.
Many variations in fruit structure were expressed as well—most common was a fruit with internal locules similar to the cranberry, but with locules much smaller.
Cooperatively, hybrids were shared among New Jersey and Wisconsin breeding facilities. In New Jersey, many hybrids were back-crossed with macrocarpon; in Wisconsin, many hybrids were selfed. A diverse complement of genetics will be developed in this way.
These successful fertile crosses are a milestone in Vaccinium breeding. The introduction of new germplasm into the accessible gene pool will allow for the introduction of many desirable traits into cranberry breeding programs.
1 Fertile Intersectional F1 Hybrids of 4x Andean Blueberry (Vaccinium meridionale) and 4x American Cranberry (Vaccinium macrocarpon) Mark K. Ehlenfeldt and James J. Polashock, Nicholi Vorsa, Juan Zalapa, Fernando de la Torre, James L. Luteyn https://doi.org/10.21273/HORTSCI16824-22
2 Ehlenfeldt MK, Ballington JR. 2017. Prolific triploid production in intersectional crosses of 4x Vaccinium corymbodendron Dunal (section Pyxothamnus) by 2x section Cyanococcus species. Euphytica. 213:238. https://doi.org/10.1007/ s10681-017-2027-9.
3 Ehlenfeldt MK, Luteyn JL. 2021. Fertile intersectional F1 hybrids of 4x Vaccinium meridionale (section Pyxothamnus) and highbush blueberry, V. corymbosum (section Cyanococcus). HortScience. 56:318–323. https://doi.org/10.21273/ HORTSCI15523-20.
4 Ehlenfeldt MK, Polashock JJ, Ballington JR. 2018. Vaccinium corymbodendron Dunal as a bridge between taxonomic sections and ploidies in Vaccinium: A work in progress. North American Blueberry Research and Extension Workers Conference. https://digitalcommons.library. umaine.edu/nabrew2018/proceedingpapers/ proceedingpapers/15.This article was posted in Cranberry and tagged Allison Jonjak, American Cranberry Hybrids, CCMJ, Cranberries, cranberry hybrids, Fernando De La Torre, Fertile Andean Blueberry, hybrids, Juan Zalapa, Mark Ehlenfeldt.