GENETICS OF CRONARTIUM-RIBICOLA .3. VARIATION IN THE RIBOSOMAL GENE-CLUSTER

The ribosomal gene repeat in Cronartium ribicola J.C. Fisch is highly variable among spore samples from British Columbia, Canada. Both restr iction site variation and length variation occur. Length heterogeneity results from differences in the number of subrepeats in the intergeni c spacer (IGS). The number of IGS size classes in haploid cultures is limited but is very large and highly variable in aeciospores from sing le cankers. The proportions of different size classes vary among canke rs on different trees, and among subsamples taken around the periphery of large old cankers. The results are consistent with the fungus havi ng a haploid infective mycelium that produces functional pycnia that r esult in localized dikaryotic areas following fusion between flexuous hyphae and pycnia. Restriction site variation appears lower than has b een reported in range-wide samples of endemic fungal species, consiste nt with the hypothesis that introduction of C. ribicola to western Nor th America was limited and does not represent the full genetic range o f the species. No particular restriction site variants or IGS size cla sses characterize samples from particular geographic areas. No evidenc e for geographic races of the fungus was obtained.