Relationships among the Spruces (Picea, Pinaceae) of Southwestern North America

Numerous populations from six spruce taxa, including four relict endemics, Picea chihuahuana (Chihuahua spruce), P. martinezii (Martínez spruce), P. mexicana (Mexican spruce), and P. breweriana (Brewer spruce), and two widespread species, P. engelmannii (Engelmann spruce) and P. pungens (blue spruce), were compared at homologous isozyme loci to test various hypotheses about their affinities and origins. Each of the species was clearly separated, and Neighbor-Joining and Unweighted Pair Group analyses of Nei's genetic distance grouped all populations within a taxon into their own clusters. Spruces from Flys Peak, Chiricahua Mountains, Arizona, joined a P. engelmannii cluster and were not a bridge to P. mexicana as previously believed. Spruces from Cerro Mohinora, Chihuahua, were clearly P. mexicana , not phantom hybrids of P. chihuahuana and P. pungens. Nuclear random amplified polymorphic DNA and chloroplast simple sequence repeat and cleaved amplified polymorphic genetic markers were compared in a smaller sample of populations, using distance and parsimony approaches. DNA markers, like isozymes, clearly identified spruces from Cerro Mohinora as P. mexicana. In contradiction to the most recent taxonomic treatment, P. chihuahuana and P. martinezii were separated as distinct species by both isozyme and DNA markers, and formed a sister-species group. Picea engelmannii and P. mexicana formed a separate cluster, and the genetic distance between them was similar to values associated with closely related species but greater than distances typical of subspecies or varieties in conifers. Picea pungens, which is so similar to P. engelmannii that the two are frequently misidentified, was clearly distinguished from it, sometimes joining a P. chihuahuana-martinezii group andsometimes a P. engelmannii-mexicana group, depending on analysis. Picea breweriana was well isolated from all other taxa. Both DNA and isozyme phylogenies agreed with results from crossability studies and contradicted intrageneric relationships constructed largely on cone morphology.