A population of Vaccinium from San Bruno Mountain in San Mateo County, California has been confused with V. cespitosum Michx. (V. sect. Myrtillus), a species documented from the mountain, since its initial collection in 1961. These plants resemble V. cespitosum and other species in V. sect. Myrtillus in several characters, but differ most notably in their evergreen habit and well developed calyx lobes. The latter characters are shared by V. ovatum Pursh (V. sect. Pyxothamnus), the only other species of Vaccinium known from San Bruno Mountain, suggesting that the population is a hybrid between V. cespitosum and V. ovatum. We used data from gross morphology, leaf anatomy, and DNA sequences from the nuclear ribosomal internal transcribed spacer (ITS) and plastid matK and ndhF regions to test the hybrid status of this population. Hybrid status is supported by: 1) a plastid sequence profile identical to that of V. cespitosum from San Bruno Mountain and different from the profiles of all other Vaccinium samples in the study, 2) an ITS profile that differs from V. cespitosum from the mountain by only two polymorphic sites, and 3) the possession of gross morphological and anatomical characters that are either shared with one putative parent or the other, or are intermediate between them. Although ITS variation was non-additive and closely matched one of the putative parents, uniparental inheritance has been observed in other hybrids. Two novel gross morphological characters were also observed in the hybrid. The study documents a case of likely intersectional hybridization within Vaccinium, only rarely observed under natural conditions. Vaccinium cespitosum may have been outcompeted by the hybrid to the point of its extirpation from San Bruno Mountain. The population is here newly described as the nothospecies Vaccinium ×brunoense P.W.Fritsch, occurring in an area of only ca. 200 m2 with the number of individuals remaining uncertain because of clonal growth.
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Vol. 67 • No. 4