Evolution and taxonomic revision of the “residual highbush” blueberries of Vaccinium sect. Cyanococcus (Ericaceae)

Abstract

The taxonomy of naturally occurring Vaccinium corymbosum has varied widely. The original monograph of V. sect. Cyanococcus treated the species narrowly as a tetraploid of the northeastern U.S. and adjacent Canada, whereas the most recent treatment of the section synonymized many species under a single polymorphic species ranging across eastern North America. Data from morphology, phylogenomics, and flow cytometry to assess ploidy have recently been used to resurrect several of the original species from the synonymy of V. corymbosum. This has left remaining a “residual highbush” blueberry group of uncertain species number and delimitation. We assessed the taxonomy of this group in the context of its origin and evolution with the same types of data and analyses used to segregate the prior species. Phylogenomic analysis yielded a monophyletic residual highbush group corroborated by several morphological characters. Neither the diploids nor the tetraploids in the clade formed monophyletic groups, but principal component and genetic structure analyses provided evidence of mostly distinct groups aligned with ploidy. The two ploidal levels have largely nonoverlapping geographic ranges, with the diploids in the southeastern U.S. and tetraploids in the northeastern U.S. and adjacent Canada, overlapping along the Atlantic Coastal Plain and East Gulf Coastal Plain. The nonmonophyly of the tetraploids, together with the prior finding of high frequencies of unreduced gametes in the residual highbush clade, suggest multiple autopolyploid origins. The lack of tetraploids throughout the entire western range of the diploids suggests that polyploid formation is localized or rare and that the two ploidy levels have largely followed separate evolutionary trajectories. We found higher genetic diversity in the tetraploid versus the diploid, suggesting that if gene flow is occurring between ploidal levels, it is not recent. Based on a synthesis of the data, we recognize two species, the diploid V. fuscatum and the tetraploid V. corymbosum, differentiated at least in part morphologically by characters that may be associated with the polyploid gigas effect. We provide a taxonomic revision, including a key, full synonymy, a discussion of character variation, nomenclatural and ecological notes, geographic distribution maps, and lists of representative specimens examined. We place four names variably recognized in recent floristic and horticultural literature in synonymy, i.e., the diploids V. atrococcum and V. caesariense under V. fuscatum, and the tetraploids V. formosum and V. marianum under V. corymbosum. We provide lectotypes for Cyanococcus holophyllus, V. atlanticum, V. caesariense, V. corymbosum, V. corymbosum var. atrocarpum, V. corymbosum var. glabrum, V. formosum, V. fuscatum, V. marianum, and V. vicinum. Vaccinium cuthbertii is placed under the synonym of V. fuscatum but warrants further investigation as to its possible taxonomic distinctness.