Fertile Intersectional F1 Hybrids of 4x Andean Blueberry (Vaccinium meridionale) and 4x American Cranberry (Vaccinium macrocarpon)

Authors:
Mark K. Ehlenfeldt USDA-ARS, Philip E. Marucci Center for Blueberry and Cranberry Research and Extension, Chatsworth, NJ 08019

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James J. Polashock USDA-ARS, Philip E. Marucci Center for Blueberry and Cranberry Research and Extension, Chatsworth, NJ 08019

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Nicholi Vorsa Rutgers University, Philip E. Marucci Center for Blueberry and Cranberry Research and Extension, Chatsworth, NJ 08019

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Juan Zalapa USDA-ARS, University of Wisconsin, Madison, WI 53705

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Fernando de la Torre University of Wisconsin, Madison, WI 53705

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James L. Luteyn New York Botanical Garden, The Bronx, NY 10458

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Abstract

Vaccinium meridionale (section Pyxothamnus), a tetraploid species native to higher altitude locations in Jamaica, Colombia, and Venezuela, is of interest to Vaccinium breeders for its profuse, concentrated springtime flowering and monopodial plant structure, both of which may be useful in breeding for mechanical harvest. In this study, tetraploid V. meridionale was hybridized successfully as a male with 4x V. macrocarpon (section Oxycoccos, American cranberry). The first-generation hybrids with 4x cranberry were intermediate in morphology and notably vigorous. The 4x F1 hybrids displayed a vining plant structure, increased flower bud numbers, and white campanulate flowers. The F1 hybrids displayed modest fertility as females upon selfing and backcrossing to 4x V. macrocarpon. Evaluations of male fertility found good pollen production and a range of pollen quality ranging from very good to poor. Hybrids functioned well as males in crosses that used US 1930, a V. meridionale–V. vitis-idaea hybrid as the female. The fertility suggests that these hybrids, despite being derived from intersectional crosses, might be used conventionally in cranberry breeding without significant difficulty.

This study follows up three previous publications (Ehlenfeldt and Ballington 2017; Ehlenfeldt and Luteyn 2021; Ehlenfeldt et al. 2018). Two of these publications identified the species material used as V. corymbodendron. That material had been identified tentatively as V. corymbodendron (Dunal 1839) when originally collected and deposited at the U.S. Department of Agriculture–Agricultural Research Service (USDA-ARS) National Clonal Germplasm Repository (Corvallis, OR, USA). Further taxonomic study by one of the current authors (J.L.L.) has determined the original identification to be erroneous and has determined that the material should be identified correctly as V. meridionale (Swartz 1788).

V. meridionale and V. macrocarpon Distributions and Habitats

Ehlenfeldt and Luteyn (2021, p. 318) noted the following facts about V. meridionale:

V. meridionale (section Pyxothamnus) is part of a taxonomic complex that includes V. consanguineum Klotzsch (Costa Rica and adjacent Panama), V. floribundum Kunth (Costa Rica to northern Argentina), and probably also V. puberulum Klotzsch (Venezuelan Guyana Highland) (Luteyn, personal observation).

V. meridionale is known mostly from the Caribbean-facing watershed slopes of northern Venezuela (Coastal Cordillera and Andes), westwards into the Andes of northern Colombia where it is quite common at midelevations, and then disjunct to the Caribbean island of Jamaica from where the type was described (Swartz 1788). It is widespread in its range, but only locally common. Its habitat includes isolated populations in high montane cloud forest to sub-páramo thickets, where it is a shrub up to 3.5 m tall. Its elevational range stretches from ca. 1000–2800 m. Notably, flowering material has been collected (for herbarium specimens) in nearly every month of the year; mature fruit have been collected in Jan–Feb, Jul–Aug, and Nov–Dec.

V. meridionale is characterized by small leaves, the blades of which are pinnately nerved and minutely crenate-serrate margined, and by its racemose inflorescences with small, 4–5-merous, white-to-pink, cylindric to ovoid-cylindric, glabrous flowers, with typically as many as 15–25 flowers per inflorescence. Its fruit is usually spherical, or more rarely, slightly oblong or flattened, and ∼14–20 mm diameter. Fruit develop as dark reddish, to dark maroon-black, to blue-black berries in which the top of the ovary is prominently convex or dome-shaped (Fig. 1A and B). The fruit is very uniform in size when optimally pollinated. The fruit is relatively thick-skinned, and the interior flesh rather pulpy. The thick skins are reflective of their high levels of antioxidants (Gaviria et al. 2009). In very ripe fruit the interior locule surfaces begin to accumulate pigment, suggesting that it might be possible to accentuate flesh pigmentation in the future with carefully planned and selected crosses.

Fig. 1.
Fig. 1.

Flowers of (A) V. meridionale, (B) V. meridionale × 4x V. macrocarpon (cranberry), and (C) V. macrocarpon. Fruit of (D) V. meridionale, (E) V. meridionale × 4x V. macrocarpon (cranberry), and (F) V. macrocarpon.

Citation: HortScience 58, 2; 10.21273/HORTSCI16824-22

V. macrocarpon, is an eastern North American endemic that occurs as a woody trailing vine, frequently erect or ascending, 4 to 15 cm high. Its leaves are persistent and narrowly elliptical to elliptical, 3 to 4 mm wide and 7 to 10 mm long. Leaves are green above and glaucous below, with entire margins that are only slightly revolute. Flowers are borne singly in the axils of reduced leaves at the base of current shoots. Flowers have four corolla lobes, white to pink, that are strongly reflexed at anthesis. Flowers develop into red berries 9 to 14 mm in diameter. Chromosome number 2n = 24. V. macrocarpon’s main distribution lies between 40 and 50°N latitude, and 70 and 80°W longitude in open bogs, swamps, mires, wet shores, and headlands, and occasionally poorly drained upland meadows; occurrence is restricted to acidic soils and peat (Vander Kloet 1988).

Utility of V. meridionale for Breeding

Our original interest in V. meridionale was for its high number of flowers per bud, as well as their loose inflorescence structure that might make hybrids with highbush blueberry amenable to machine harvest (Luby et al. 1991). Plants of V. meridionale also have the potential to develop an upright, tree-like bush structure with a monopodial base.

In a previous study (Ehlenfeldt and Ballington 2017), 4x V. meridionale was found to produce high numbers of triploids with 2x V. corymbosum. We also reported that an exception to the observed crossing behavior was the production of a highly fertile 4x hybrid of 4x V. meridionale with a species of a different section, 2x V. vitis-idaea L. (section Vitis-idaea; lingonberry) (Ehlenfeldt and Ballington 2017). We conducted advanced studies with V. vitis-idaea (Ehlenfeldt et al. 2022), but we also recognized that V. vitis-idaea and V. macrocarpon could successfully hybridize, and we hypothesized that V. meridionale might similarly hybridize with V. macrocarpon. Such hybrids could broaden the cranberry gene pool and allow introgression of traits such as increased flower number and fruit set, and modified fruit quality parameters.

In the current study, we examined the crossability of 4x V. meridionale with 4x V. macrocarpon and the ability to recover fertile introgressed hybrids.

Materials and Methods

Plant material.

As reported in Ehlenfeldt and Luteyn (2021), the V. meridionale we used this study was derived from seed collected in Colombia by Dr. James L. Luteyn (Curator, New York Botanical Garden) and Dr. James R. Ballington (North Carolina State University) in 1990. Two V. meridionale clones are currently held at the USDA-ARS National Clonal Germplasm Repository: NC 3735 and NC 3737. Our particular clone of interest, NC 3735, is maintained as CVAC 1146. Among these two clones, NC 3735/CVAC 1146 has a slightly waxy fruit surface and is more male fertile, as judged by pollen shed. NC 3737/CVAC 1148 has darker fruit and, in our experience, has almost no pollen shed. Luteyn and Ballington also originally had a third genotype NC 3736. Our other genotype, US 2381 is a V. meridionale hybrid that was derived from some of the originally collected material, and is a self of an unnumbered clone with the pedigree NC 3737 × NC 3736.

In the cranberry material, CNJ 09-30-24 is a 4x clone that is among germplasm held by Rutgers University and was provided by N. Vorsa. US 2504-# clones are tetraploids of a population derived from 4x CNJ 09-30-24 × 4x US 94-70. US 94-70 is an older colchicine-doubled USDA breeding selection from Wisconsin, and is also held in the Rutgers collection.

Among other material used in this study, US 1930 is a 4x self of US 1184 (V. meridionale NC 3737 × V. vitis-idaea ‘European Red’) (see Ehlenfeldt et al. 2022).

For all plant material, pollen was extracted from open flowers by manual manipulation, and was collected on glassine weighing paper. If pollen was needed for longer term work, pollen was stored for up to a month under refrigerated, desiccated conditions.

To perform pollinations, a graphite pencil tip was dipped into the collected pollen and then was used to apply the pollen to the stigmas of unemasculated flowers in an insect-free greenhouse. Pollinations were made on what were judged to be mature stigmas.

Because all pollinations were performed in an insect-free greenhouse, and because it was expected that hybrids would be recognizable morphologically, the female cultivar parents were not emasculated.

Ploidy determinations.

Because previous crosses with V. meridionale had produced anomalous triploids, ploidy of F1 hybrids was verified using flow cytometry. Ploidy determinations were performed as described by Ehlenfeldt and Luteyn (2021) using the following procedures. Sampled leaf material (1 cm2/20–50 mg) together with leaf material of an internal standard with known DNA content (Zea mays L.) was chopped with a sharp razor blade in 500 mL of extraction buffer (CyStain PI absolute P buffer, catalog no. 05–5502; Partec, Münster, Germany) containing RNA-se, 0.1% dithiothreitol (DTT), and 1% polyvinylpyrrolidone (ice cold) in a plastic petri dish. After 30 to 60 s of incubation, 2.0 mL staining buffer (CyStain PI absolute P buffer) containing propidium iodide (PI) as fluorescent dye, RNA-se, 0.1% DTT, and 1% polyvinylpyrrolidone was added. The sample, containing cell constituents and large tissue remnants of the sample and the internal standard, was then filtered through a 50-mm mesh nylon filter. After an incubation of at least 30 min at room temperature, the filtered solution with stained nuclei was measured with a flow cytometer [CyFlow ML (Partec) with a green diode laser: 50 mW, 532 nm (for use with PI); software: Flomax version 2.4 d (Partec)]. The DNA amount of the unknown samples was calculated by multiplying the DNA amount of the internal standard with the DNA ratio of the relative DNA amount of the unknown sample and the internal standard. DNA amounts were measured and were compared with a set of standards covering a diploid to hexaploid range (2x V. darrowii ‘Fla 4B’, 4x V. corymbosum cv. Duke, and 6x V. virgatum cv. Powderblue) to determine basic ploidy levels.

Female fertility.

The number of flowers pollinated to evaluate female fertility varied depending upon flower availability. Pollinations and fruit set were recorded. Fruit were collected when ripe and measured for fruit size (in millimeters) at the time of seed extraction. Extraction was performed manually using a dissecting microscope, and the seeds were evaluated for number and quality. For our purposes, seeds were classified as good, good/fair, fair, fair/poor, poor, or aborted. “Good” and “fair” described seed that ranged subjectively from those considered fully normal to those somewhat reduced in size and/or development, but nonetheless were judged likely to be capable of germination. “Poor” described seed that displayed reduced size and/or development, often flattened or brown, and was judged less likely to be capable of germination. “Aborted seed” included those that were flat and brown, and generally translucent. Notes were made of the size of aborted seed. For purposes of this article, we report only seed totals that combine categories of good and good/fair.

Seeds were germinated on a greenhouse mist bench in a soil mix composed of a 50:50, peat: sand mixture. At about a three true-leaf stage, seedlings were transplanted to 36-cell flats. All primary hybrids were transferred to 3-L pots in their second season.

Male fertility.

Pollen samples were stained with acetocarmine jelly (75% acetic acid with iron acetate) prepared according to the recipe of Jensen (1962). Pollen samples were assayed for quantity, stainability, and general condition. For evaluating general pollen condition, our ratings were as follows: very good = almost all tetrads, good = tetrads and triads, fair = almost exclusively triads, and poor = mostly aborted grains.

Results

Primary crossing

4x V. meridionale × 4x V. macrocarpon.

In 2018, we made pollinations of V. meridionale US 2381 × 4x V. macrocarpon CNJ 09-30-24 on a very modest scale. From 30 pollinations, we produced 599 seeds, and we now have 500+ young and vigorous hybrids (Table 1). This cross was, by our standards, highly successful, for an intersectional cross, producing 20.7 seeds/fruit. This value is comparable to the best values found in our previously reported V. corymbosum × V. meridionale crosses (19.8 seeds/fruit; Ehlenfeldt and Luteyn 2021). The V. meridionale US 2381 × 4x V. macrocarpon cross was repeated in 2021 with a different, but related, 4x cranberry as the male parent: US 2504-1. In 2021, seed set was 13.7 seeds/fruit, ultimately producing more than 375 hybrid plants. While extracting seed from fruit of this second set of crosses, it was noted that seed size varied considerably within individual fruit. Subsequently, the seed extracted in 2021 was segregated subjectively based on size (normal vs. small), and plants derived from these sets will later evaluated for qualitative differences.

Table 1.

Crosses of V. meridionale × V. macrocarpon germplasm.

Table 1.

Morphology of 4x V. meridionale × 4x V. macrocarpon hybrids

Among the large number of hybrids available, 20 were selected randomly for flow cytometry and all were verified as 4x. Based on this sample, we assumed all F1 hybrids were tetraploid, considering they were produced from crosses between confirmed 4x parents.

The F1 hybrids of V. meridionale × V. macrocarpon, like other V. meridionale hybrids, were distinctly recognizable in their morphology, having leaves that are relatively small (3.0 × 1.2 cm) and lance shaped (Fig. 2). Leaves under ideal conditions were medium green, smooth, and slightly glossy. Like most other V. meridionale interspecific hybrids, their leaves were very consistent in size along the length of any given shoot. In the greenhouse, plants produced extended vine-like growth, usually comprising only one or two shoots; however, plants grown outdoors were more compact, and showed multiple sturdy branching occurring near ground level. Early-season leaves were very symmetric, but later season foliage often gave the appearance reminiscent of a chimeric nature, with irregular and jagged margins. It is unknown whether this was a temperature effect or whether it was a reflection of generalized unsynchronized cellular growth; however, this effect was also seen in the V. meridionale NC 3735 parental material. Unlike cranberry, foliage of V. meridionale × V. macrocarpon hybrids did not turn significantly red as they entered dormancy. Leaves were retained, and often became chlorotic and mottled, but were not lost until plants began breaking dormancy, and sometimes not until new foliage growth began. This agreed with our subjective observation that hybrids had no chilling requirement (i.e., ectodormancy). Growth appeared to end when days got shorter and cooler, but no specific dormancy was apparent. Our subjective observations suggest the same is probably true for V. meridionale as a species.

Fig. 2.
Fig. 2.

Leaves of V. meridionale, an F1 hybrid, and 4x V. macrocarpon.

Citation: HortScience 58, 2; 10.21273/HORTSCI16824-22

Flowers of hybrids were generally campanulate and white (Fig. 1) and reminiscent of lily-of-the-valley (Convallaria majalis). In most cases, the styles were only very slightly exerted beyond the stamen tips. Some flowers had a slight pinkish tinge when unopened, and unopened buds had an angular shape reminiscent of cranberry buds. Flower clusters varied considerably among these young plants, but were more similar to V. meridionale than cranberry. Hybrids often had multiple buds containing multiple flowers, whereas cranberry typically has a single terminal floral bud per upright containing approximately six flowers and a terminal vegetative shoot (Fig. 3). V. meridionale × V. macrocarpon (cranberry) hybrids may have multiple inflorescence buds on a previous season’s shoot, and may or may not exhibit vegetative apical shoots ending the inflorescences.

Fig. 3.
Fig. 3.

(A) Growth from a typical cranberry inflorescence bud with approximately six flowers subtending a vegetative apical shoot. (B) V. meridionale × V. macrocarpon (cranberry) hybrids may have multiple inflorescence buds on a previous season’s shoot, and may or may not exhibit vegetative apical shoots ending the inflorescences.

Citation: HortScience 58, 2; 10.21273/HORTSCI16824-22

Fruit of the F1 hybrids was difficult to quantify completely because seed set was limited, and set affects fruit size and fruit shape, as well as development. Nonetheless, a few valid observations can be made. First, fruit was variable in shape among genotypes, ranging from almost perfectly round to broadly heart shaped (rounded at the base with a pointed tip). Second, across genotypes, ripe fruit skin color ranged from a mottled red blush over an apple-green base color (approximately RHS 150C, brilliant yellowish green) (The Royal Horticultural Society 2015), to a cranberry red (approximately RHS 45C, vivid red), to a deep red-purple on very ripe fruit (approximately RHS 187B, dark red). Third, the interior fruit structure was variable, but locularization similar to cranberry has been observed, but much reduced in magnitude. The flesh was typically pale green or whitish green, and retained some of the firmness and acidity of the V. macrocarpon parent. Within some very ripe fruit, flesh pigmentation was expressed.

Female fertility

F2 sibs and selfs.

In light of the large number of hybrids available, ∼30% of the population was transferred to Wisconsin during the first-year dormant season and ∼70% retained in New Jersey. The F1 hybrids flowered to varying degrees in their first year. In New Jersey, 63 of 358 plants (17.6%) flowered; in Wisconsin, 55 of 144 (38.2%) flowered. Female fertility assessments had to balance the availability of pollen sampling and crossing, and specific crossing depended on the pollen sources available in each location. In New Jersey, we focused on backcrosses to 4x V. macrocarpon, and a few sib matings. In Wisconsin, crossing focused primarily on selfing.

The selfing of F1 hybrids (across 15 clones; New Jersey and Wisconsin combined) yielded 2.0 seeds/fruit (Wisconsin, 13 clones) and 2.3 seeds/fruit (New Jersey, two clones). The seed generated on these V. meridionale US 2381 × 4x V. macrocarpon CNJ 09-30-24 hybrids was often noted as narrow or elongated compared with typical blueberry seed. Whether this is indicative of abnormal or atypical seed development was unclear.

BC1 crosses to V. macrocarpon.

Considering the remarkable ease and success of generating intersectional F1 hybrids between these two species, backcrosses to 4x cranberry had a relatively low success rate (2.1 seeds/fruit). This was considerably lower than the comparable BC1 of (V. corymbosum × V. meridionale) × V. corymbosum (5.9–15.0 seeds/fruit; Ehlenfeldt and Luteyn 2021), but also represents a much more morphologically diverse cross. Backcrosses of F1 (V. meridionale × V. macrocarpon) with V. meridionale have not yet been attempted.

Male fertility

Pollen shed was generally very good. Among the flowering clones, 26 were assayed microscopically for pollen quality. Among them, 18 plants (69.2%) were rated as very good or good (very good, 4 plants, 15.4%; good, 14 plants, 53.8%). Very good indicated that pollen was almost all well-formed tetrads. Good indicated both tetrads and triads. Pollen of one of the best V. meridionale × 4x V. macrocarpon clones, US 2505-22, displayed nearly 100% tetrads, F1 (Fig. 4).

Fig. 4.
Fig. 4.

Pollen from two F1 V. meridionale × 4x V. macrocarpon clones showing (A) high frequencies of viable tetrads and (B) elevated frequencies of dyads.

Citation: HortScience 58, 2; 10.21273/HORTSCI16824-22

Among the remaining hybrids, another eight plants (30.8%) were rated as fair or poor (fair, four plants, 15.4%; poor, four plants, 15.4%). Among the clones with irregular meiosis, US 2505-64 exhibited elevated frequencies of well-formed dyads (Fig. 4). Such clones may be useful if trying to cross to species with higher ploidy levels such as V. virgatum.

Because early indications (based on fruit set) suggested the success of F1 × F1 and F1 × V. macrocarpon backcrosses might be limited, we opted—as a test of male fertility—to test F1 pollen against US 1930, a particularly fertile 4x S1 V. meridionale × V. vitis-idaea hybrid (Ehlenfeldt and Luteyn 2021). Following the terminology of half-sib crosses, we referred to crosses with a common V. meridionale parent as half-species crosses. In 72 pollinations, these half-species crosses yielded 274 seeds at a frequency of 5.4 seeds/fruit (Table 1). Considering the high female fertility of US 1930, this value may be a good representation of potential male fertility; however, it should be noted that the hybridity of these progeny are currently unverified.

Discussion and Conclusion

V. meridionale represents a remarkable bridging species that is just beginning to demonstrate its potential to allow germplasm movement among V. species. The primary cross in this study, V. meridionale × V. macrocarpon is an intersectional cross between sections Pyxothamnus and Oxycoccos. This cross is notable in that it is a cross between species recognizable as blueberry and cranberry. This study parallels and complements our previous reports of crosses between V. meridionale and V. corymbosum (Ehlenfeldt and Luteyn 2021), and crosses of V. meridionale and 2x V. vitis-idaea (Ehlenfeldt et al. 2022).

When considering these hybrids, several important points can be made concerning them. These crosses were intersectional crosses. Nonetheless, it was possible to generate hybrids with 4x V. meridionale with remarkable ease and success. More than 500 hybrids were generated in our initial crossing cycle, and a subsequent cycle with a different V. macrocarpon genotype has also produced large numbers of potential hybrids.

By our standards, female fertility was limited, but as a proof-of-concept, fertility has been demonstrated by the reasonable success observed in self crosses, BC1-type backcrosses (to V. macrocarpon), and S1 sib crosses. In such hybrids, the expression of fertility may be greatly influenced by the significant morphological differences between V. meridionale and V. macrocarpon that must be reconciled in floral development. Pollen–stylar interactions may also play a role. The generation of additional hybrids and subsequent cycles of selfing/inbreeding might be expected to stabilize and improve self-fertility.

Male fertility among our hybrids, particularly with respect to pollen shed, was good. Further visual assays confirmed the existence of individual clones producing good levels of well-formed tetrads, and several of these clones were used very successfully as males with US 1930, a 4x V. meridionaleV. vitis-idaea clone. The high female fertility of this S1 V. meridionaleV. vitis-idaea clone allows perhaps the best estimation of potential male fertility. Fertility of F1 allotetraploids might be expected because every chromosome is expected to have a homologous pairing partner (Clausen and Goodspeed 1925). Furthermore, high levels of synteny among V. genomes suggest little differentiation in general among V. genomes (Qi et al. 2021; Wu et al. 2021). These high levels suggest that ongoing recombination and fertility may be possible.

Crosses and hybrids of somewhat similar natures have been documented previously. Christ (1977) made crosses of V. vitis-idaea and V. macrocarpon, and Zeldin and McCown (1997) created similar hybrids between V. vitis-idaea and V. macrocarpon and conducted further crosses. Vorsa et al. (2009) generated a hybrid of V. darrowii × (V. macrocarpon × V. oxycoccos). All of these crosses occurred between diploids, and typically resulted in hybrids with limited or no fertility. A critical difference, however, between these previous hybrids and the current ones are the resultant ploidy levels. Our hybrids are all at the tetraploid level.

As we noted in a previous article (Ehlenfeldt et al. 2022, p. 530):

Much remains to be understood about intersectional hybrids in Vaccinium; however, given our results, and the results of several other studies, intersectional crossing barriers appear to be modest at best (Morozov 2007; Lyrene 2011; Ehlenfeldt and Polashock 2014; Lyrene 2016). The hybridization of V. padifolium with V. corymbosum (Ehlenfeldt and Polashock 2014) would have seemed to be an extraordinarily difficult one based upon modern molecular-based taxonomic determinations that listed sect. Hemimyrtillus species amongst the furthest outliers from section Cyanococcus (Powell and Kron 2002), yet these hybrids were made with only moderate difficulty, and the resultant tetraploid hybrids were highly fertile. There is no doubt that diploid intersectional hybrids may present fertility issues, but if hybrids can be made or brought to the tetraploid level, allotetraploid fertility and apparent tetraploid buffering almost surely guarantee that some level of success will exist in the backcrossing of intersectional hybrids to tetraploid cultivated germplasm.

We believe V. macrocarpon stands to benefit significantly from introgression of new germplasm. The accessible gene pool of cranberry is small, and with the current crosses, V. meridionale has expanded the pool of utilizable germplasm significantly. An important consideration for crosses with V. macrocarpon, is the highly defined morphology and ecology of 2x V. macrocarpon as a commercial crop. Our introgressed allotetraploids are more upright, stockier, and more woody than diploid cranberry, and it is likely that the de novo tetraploids, being a mix of two divergent species would not tolerate wintertime flooding, as is practiced in current cranberry management systems. However, with a return to higher percentages of cranberry germplasm, and selection for commercial type morphology, considerable interesting possibilities might exist. Alternatively, an allotetraploid “cranberry” hybrid with appropriate amounts of germplasm introgressed from V. meridionale might open new options for production of a cranberry-like fruit under more upland conditions.

Based on our studies, we feel that V. meridionale is destined to play a critical role in the future in producing wide hybrids among Vaccinium species, as it appears to be a very accepting parent. With our V. meridionale studies, we believe we will soon gain access to a wide array of tertiary Vaccinium germplasm that will benefit both conventional and molecular aspects of Vaccinium breeding.

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  • Powell, EA & Kron, KA. 2002 Hawaiian blueberries and their relatives: A phylogenetic analysis of Vaccinium sections Macropelma, Myrtillus, and Hemimyrtillus (Ericaceae) Syst Bot. 27 768 779 https://doi.org/10.1043/0363- 6445-27.4.768

    • Search Google Scholar
    • Export Citation
  • Qi, X, Ogden, EL, Bostan, H, Sargent, DJ, Ward, J, Gilbert, J, Iorizzo, M & Rowland, LJ. 2021 High-density linkage map construction and QTL identification in a diploid blueberry mapping population Front. Plant Sci. 12 692628 https://doi.org/10.3389/fpls.2021.692628

    • Search Google Scholar
    • Export Citation
  • Swartz, O. 1788 Nova genera & species plantarum; seu, Prodromus descriptionum vegetabilium, maximam partemin cognitorum 1783–87 vol 25 Bibliopoliis Acad. M. Swederi, Holmiae, Upsaliae and Aboae Sweden https://doi.org/10.5962/bhl.title.4400

    • Search Google Scholar
    • Export Citation
  • The Royal Horticultural Society 2015 The Royal Horticultural Society Colour Chart 6th ed London

  • Vander Kloet, SP. 1988 The genus Vaccinium in North America (no. 1828) Agriculture Canada Ottawa

  • Vorsa, N, Johnson-Cicalese, J & Polashock, J. 2009 A blueberry by cranberry hybrid derived from a Vaccinium darrowii × (V. macrocarpon × V. oxycoccos) intersectional cross Acta Hortic. 810 187 190 https://doi.org/10.17660/ActaHortic.2009.810.24

    • Search Google Scholar
    • Export Citation
  • Wu, C, Deng, C, Hilario, E, Albert, NW, Lafferty, D, Grierson, ERP, Plunkett, BJ, Elborough, C, Saei, A, Günther, CS, Ireland, H, Yocca, A, Edger, PP, Jaakola, L, Karppinen, K, Grande, A, Kylli, R, Lehtola, V-P, Allan, AC & Chagné, D. 2021 A chromosome-scale assembly of the bilberry genome identifies a complex locus controlling berry anthocyanin composition Mol Ecol Resour. 22 345 360 https://doi.org/10.1111/1755-0998.13467

    • Search Google Scholar
    • Export Citation
  • Zeldin, EL & McCown, BH. 1997 Intersectional hybrids of lingonberry (Vaccinium vitis-idaea, section Vitis-idaea) and cranberry (V. macrocarpon, section Oxycoccus) to Vaccinium reticulatum (section Macropelma) Acta Hortic. 446 235 238 https://doi.org/10.17660/ActaHortic.1997.446.34

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  • Fig. 1.

    Flowers of (A) V. meridionale, (B) V. meridionale × 4x V. macrocarpon (cranberry), and (C) V. macrocarpon. Fruit of (D) V. meridionale, (E) V. meridionale × 4x V. macrocarpon (cranberry), and (F) V. macrocarpon.

  • Fig. 2.

    Leaves of V. meridionale, an F1 hybrid, and 4x V. macrocarpon.

  • Fig. 3.

    (A) Growth from a typical cranberry inflorescence bud with approximately six flowers subtending a vegetative apical shoot. (B) V. meridionale × V. macrocarpon (cranberry) hybrids may have multiple inflorescence buds on a previous season’s shoot, and may or may not exhibit vegetative apical shoots ending the inflorescences.

  • Fig. 4.

    Pollen from two F1 V. meridionale × 4x V. macrocarpon clones showing (A) high frequencies of viable tetrads and (B) elevated frequencies of dyads.

  • Christ, E. 1977 Crossbreedings between cranberries (Vaccinium macrocarpon Ait.) and cowberries (Vaccinium vitis-idaea L.) Acta Hortic. 61 285 294 https://doi.org/10.17660/ActaHortic.1977.61.34

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  • Clausen, RE & Goodspeed, TH. 1925 Interspecific hybridization in Nicotiana II: A tetraploid glutinosa-tabacum hybrid: An experimental verification of Winge’s hypothesis Genetics. 10 278 284

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  • Dunal, MF. 1839 Vaccinieae 552 579 De Candolle, AP Prodromus systematis naturalis regni vegetabilis vol 7 Treuttel & Würtz Paris, France

  • 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

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  • 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

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  • Ehlenfeldt, MK & Polashock, JJ. 2014 Highly fertile intersectional blueberry hybrids of Vaccinium padifolium section Hemimyrtillus and V. corymbosum section Cyanococcus J Am Soc Hortic Sci. 139 30 38 https://doi.org/10.21273/JASHS.139.1.30

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  • 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. [accessed 2 Dec 2022]

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  • Ehlenfeldt, MK, Polashock, JJ, Rowland, LJ, Ogden, E & Luteyn, JL. 2022 Fertile intersectional hybrids of 4x Andean blueberry (Vaccinium meridionale) and 2x lingonberry (V. vitis-idaea) HortScience. 57 525 531 https://doi.org/10.21273/HORTSCI16285-21

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  • Gaviria, CA, Ochoa, CI, Sánchez, NY, Medina, CL, Lobo, M, Galeano, PL, Mosquera, AJ, Tamayo, A, Lopera, YE & Rojano, BA. 2009 Propiedades antioxidantes de los frutos de agraz o mortiño (Vaccinium meridionale Swartz) 93 112 Ligaretto, GA Perspectivas de cultivo de agraz o mortiño (Vaccinium meridionale Swartz) 1st ed Universidad Nacional de Colombia Bogota, Colombia

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  • Jensen, WA. 1962 Botanical histochemistry W.H. Freeman San Francisco, CA, USA

  • Luby, JJ, Ballington, JR, Draper, AD, Pliszka, K & Austin, ME. 1991 Blueberries and cranberries (Vaccinium) Acta Hortic. 290 391 456 https://doi.org/10.17660/ActaHortic.1991.290.9

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  • Lyrene, PM. 2011 First report of Vaccinium arboretum hybrids with cultivated highbush blueberry HortScience. 46 563 566 https://doi.org/10.21273/HORTSCI.46.4.563

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  • Lyrene, PM. 2016 Phenotype and fertility of intersectional hybrids between tetraploid highbush blueberry and colchicine-treated V. stamineum HortScience. 51 15 22 https://doi.org/10.21273/HORTSCI.51.1.15

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  • Morozov, OV 2007 The prospects for using Vaccinium uliginosum L. × Vaccinium vitis-idaea L. hybrids in breeding Int J Fruit Sci. 6 43 56 https://doi.org/10.1300/J492v06n04_05

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  • Powell, EA & Kron, KA. 2002 Hawaiian blueberries and their relatives: A phylogenetic analysis of Vaccinium sections Macropelma, Myrtillus, and Hemimyrtillus (Ericaceae) Syst Bot. 27 768 779 https://doi.org/10.1043/0363- 6445-27.4.768

    • Search Google Scholar
    • Export Citation
  • Qi, X, Ogden, EL, Bostan, H, Sargent, DJ, Ward, J, Gilbert, J, Iorizzo, M & Rowland, LJ. 2021 High-density linkage map construction and QTL identification in a diploid blueberry mapping population Front. Plant Sci. 12 692628 https://doi.org/10.3389/fpls.2021.692628

    • Search Google Scholar
    • Export Citation
  • Swartz, O. 1788 Nova genera & species plantarum; seu, Prodromus descriptionum vegetabilium, maximam partemin cognitorum 1783–87 vol 25 Bibliopoliis Acad. M. Swederi, Holmiae, Upsaliae and Aboae Sweden https://doi.org/10.5962/bhl.title.4400

    • Search Google Scholar
    • Export Citation
  • The Royal Horticultural Society 2015 The Royal Horticultural Society Colour Chart 6th ed London

  • Vander Kloet, SP. 1988 The genus Vaccinium in North America (no. 1828) Agriculture Canada Ottawa

  • Vorsa, N, Johnson-Cicalese, J & Polashock, J. 2009 A blueberry by cranberry hybrid derived from a Vaccinium darrowii × (V. macrocarpon × V. oxycoccos) intersectional cross Acta Hortic. 810 187 190 https://doi.org/10.17660/ActaHortic.2009.810.24

    • Search Google Scholar
    • Export Citation
  • Wu, C, Deng, C, Hilario, E, Albert, NW, Lafferty, D, Grierson, ERP, Plunkett, BJ, Elborough, C, Saei, A, Günther, CS, Ireland, H, Yocca, A, Edger, PP, Jaakola, L, Karppinen, K, Grande, A, Kylli, R, Lehtola, V-P, Allan, AC & Chagné, D. 2021 A chromosome-scale assembly of the bilberry genome identifies a complex locus controlling berry anthocyanin composition Mol Ecol Resour. 22 345 360 https://doi.org/10.1111/1755-0998.13467

    • Search Google Scholar
    • Export Citation
  • Zeldin, EL & McCown, BH. 1997 Intersectional hybrids of lingonberry (Vaccinium vitis-idaea, section Vitis-idaea) and cranberry (V. macrocarpon, section Oxycoccus) to Vaccinium reticulatum (section Macropelma) Acta Hortic. 446 235 238 https://doi.org/10.17660/ActaHortic.1997.446.34

    • Search Google Scholar
    • Export Citation
Mark K. Ehlenfeldt USDA-ARS, Philip E. Marucci Center for Blueberry and Cranberry Research and Extension, Chatsworth, NJ 08019

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James J. Polashock USDA-ARS, Philip E. Marucci Center for Blueberry and Cranberry Research and Extension, Chatsworth, NJ 08019

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Nicholi Vorsa Rutgers University, Philip E. Marucci Center for Blueberry and Cranberry Research and Extension, Chatsworth, NJ 08019

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Juan Zalapa USDA-ARS, University of Wisconsin, Madison, WI 53705

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Fernando de la Torre University of Wisconsin, Madison, WI 53705

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James L. Luteyn New York Botanical Garden, The Bronx, NY 10458

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Contributor Notes

Current address for J.L.L.: 32075 East Side Drive, Beaver Island, MI 49782.

M.K.E. is the corresponding author. E-mail: mark.ehlenfeldt@usda.gov.

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  • Fig. 1.

    Flowers of (A) V. meridionale, (B) V. meridionale × 4x V. macrocarpon (cranberry), and (C) V. macrocarpon. Fruit of (D) V. meridionale, (E) V. meridionale × 4x V. macrocarpon (cranberry), and (F) V. macrocarpon.

  • Fig. 2.

    Leaves of V. meridionale, an F1 hybrid, and 4x V. macrocarpon.

  • Fig. 3.

    (A) Growth from a typical cranberry inflorescence bud with approximately six flowers subtending a vegetative apical shoot. (B) V. meridionale × V. macrocarpon (cranberry) hybrids may have multiple inflorescence buds on a previous season’s shoot, and may or may not exhibit vegetative apical shoots ending the inflorescences.

  • Fig. 4.

    Pollen from two F1 V. meridionale × 4x V. macrocarpon clones showing (A) high frequencies of viable tetrads and (B) elevated frequencies of dyads.

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