SYSTEMATICS OF TREE MOSSES (CLIMACIUM, MUSCI) - GENETIC AND MORPHOLOGICAL EVIDENCE

Three species of the moss genus Climacium are recorded from North Amer ica, but their systematic relationships have been unclear because of r eports of morphological intergradation between them. Allopatric popula tions of C. americanum and C. dendroides appear to be morphologically distinct, but the two species intergrade in a broad region of sympatry extending from Wisconsin to Massachusetts. We measured variation in t wo morphological traits and isozyme genotypes at 19 loci in 253 plants from six sites in New York state. The two species exhibited fixed all elic differences at three loci (Aco-1, Hex-1, and Me-1). Moreover, Nei 's genetic identities between conspecific populations of the two speci es were substantially higher (mean = 0.980 and 0.988 for C. americanum and C. dendroides, respectively) than the mean genetic identity betwe en the species (0.814). Levels of variation within populations were ve ry low. The two species differed significantly in leaf cell length-wid th ratios and leaf auricle size, but there was substantial overlap bet ween them. Our results indicate that C. americanum and C. dendroides a re evolutionarily distinct, reproductively isolated species despite a lack of complete morphological discontinuity. We found no evidence tha t morphological intergradation is caused by interspecific hybridizatio n.