Morphological and molecular evidence for hybridization and introgression in a willow (Salix) hybrid zone

Hybrid zones provide biologists with the opportunity to examine genetic and ecological interactions between differentiated populations. Accurate ident ification of hybrid genealogies is considered a necessary prerequisite to u nderstanding observed patterns of hybridization-related phenomena. We analy sed molecular and morphological data from individuals in a hybrid zone betw een two species of willows (Salix sericea Marshall and S. eriocephala Micha ux) and report the use of randomly amplified polymorphic DNA (RAPD), chloro plast DNA (cpDNA), and ribosomal DNA (rDNA) markers, as well as vegetative morphology and foliar chemistry data to identify individuals in terms of hy brid genealogy and to infer the direction and extent of backcrossing and in trogression within the hybrid zone. A novel version of a maximum likelihood estimate approach (developed for this study) was used to calculate hybrid index scores from RAPD marker data; this method produced results similar to those obtained using traditional arithmetic methods. Distribution of rDNA, cpDNA, and chemistry data were examined within the graphical context of RA PD-based hybrid index score histograms and principal component analyses (PC A) on RAPD and morphology data. Seven of the 21 plants classified as S, eri ocephala in the field were possible introgressants. Another plant presented an unequivocal example of backcrossed S. sericea chemistry and RAPD marker s. Inter- and intraspecific chloroplast diversity found within the hybrid z one suggests both historic introgression (perhaps in a glacial refugium), a nd contemporary hybridization. Patterns of inheritance and expression withi n the hybrid zone suggest that morphological characters are often not expre ssed in a simple additive fashion, and problems associated with both morpho logical and molecular data are considered.