Evolutionary Patterns and Processes in Salix Sect. Longifoliae: Evidence from Chloroplast DNA

Chloroplast DNA (cpDNA) restriction site analysis was used to explore evolutionary relationships within a taxonomically difficult group of diploid willows comprising seven taxa (Salix sect. Longifoliae). Forty-five population samples from throughout North America were analyzed with up to 23 restriction endonucleases. Clades revealed by cpDNA data are generally congruent with groups delineated by enzyme electrophoresis. Both sets of molecular data disagree with traditional taxonomic concepts. Morphological characters such as stigma length and pubescence, traditionally viewed as taxonomically important, are not correlated with major genetic divisions in the section. High levels of intraspecific cpDNA variability were detected, mostly representing geographic pat- terns of divergence. Salix interior, for example, contains six distinct chloroplast genomes, and has a mean sequence divergence of 0.09%. Conversely, S. sessilifolia, S. fluviatilis, and populations of S. exigua from the Pacific Northwest all possess identical chloroplast genomes. Hybridization appears to be an important process in the section. Five cases of hybridization and subsequent cytoplasmic capture were documented. Chloroplast DNA data suggest a southern origin for the section and a northward radiation into geographically isolated montane and arid lowland habitats. The high incidence of cytoplasmic capture and intraspecific cpDNA variability detected in this study under- scores the importance of analyzing multiple cpDNA samples of a taxon in combination with nuclear markers in molecular systematic studies.