The genus Corylus L., comprises about 15 species distributed in the temperate zone of the northern hemisphere. This genus has a boreotropical pattern of distribution with most species occurring in Eurasia. Only two species, each with a few infraspecific taxa, are present in North America. A phylogenetic analysis was performed using DNA sequences from the non-transcribed spacer of the 5S-rRNA gene. The Asian genus Ostryopsis Decne., considered to be a close relative of Corylus based on morphological characters, was used as an outgroup. The 5S spacer has an average length of 339 bp in Corylus and of 384 bp in Ostryopsis. Among Corylus species, sequence divergence ranges from 0.6% to 11%, while the mean divergence between Corylus and Ostryopsis is 25%. Because of intra-individual variation in spacer sequences, cloning was performed on several individuals per species. Insertions/deletions and small repetitive sequences were observed throughout the genus. All species of Corylus examined share an "ACTT" repeated pattern, variable in length, which is not evident in Ostryopsis. Preliminary cladistic analysis of the molecular data tends to support the traditional sectional classification system of Spach. The Asian Corylus ferox Wall. (sect. Acanthochlamys) occurs as sister to the remainder of the genus. Section Tubo-avellana is strongly supported as monophyletic. Section Avellana also is supported as monophyletic, but includes three clades not recognized in traditional classification systems. Several species groups are supported by morphological characters (e.g., involucral bracts, staminate catkins, leaf vestiture characters). These data suggest that the North American C. americana Walt. (sect. Avellana) and C. cornuta Marsh. (sect. Tubo-avellana) have distinct origins, as do the European-West Asian C. maxima Mill. and C. colurna L. (sect. Avellana), all of which are derived independently from Asian relatives.

Key words: 5S-rRNA, Betulaceae, biogeography, Corylus, non-transcribed spacer, phylogeny