Systematics of Tree Mosses (Climacium: Musci): Genetic and Morphological Evidence

Abstract

Three species of the moss genus Climacium are recorded from North America, but their systematic relationships have been unclear because of reports of morphological intergradation between them. Allopatric populations of C. americanum and C. dendroides appear to be morpholog- ically distinct, but the two species intergrade in a broad region of sympatry extending from Wis- consin to Massachusetts. We measured variation in two morphological traits and isozyme genotypes at 19 loci in 253 plants from six sites in New York state. The two species exhibited fixed allelic differences at three loci (Aco-i, Hex-i, and Me-i). Moreover, Nei's genetic identities between con- specific populations of the two species were substantially higher (mean = 0.980 and 0.988 for C. americanum and C. dendroides, respectively) than the mean genetic identity between the species (0.814). Levels of variation within populations were very low. The two species differed significantly in leaf cell length-width ratios and leaf auricle size, but there was substantial overlap between them. Our results indicate that C. americanum and C. dendroides are evolutionarily distinct, reproductively isolated species despite a lack of complete morphological discontinuity. We found no evidence that morphological intergradation is caused by interspecific hybridization.