EVOLUTION OF RIBOSOMAL DNA (RDNA) GENETIC-STRUCTURE IN COLONIAL CALIFORNIAN POPULATIONS OF AVENA-BARBATA

DNA samples from 980 plants of Avena barbata from 48 ecologically dive rse sites in California and Oregon were assayed to determine their gen otype for two duplicated loci governing rDNA variants. More than 40 di fferent rDNA genotypes were observed among which 5 made up 96% of our sample in environmentally homogeneous sites; predominant genotypes wer e less frequent and recombinant genotypes were more frequent in enviro nmentally heterogeneous sites. The spatial distribution of each predom inant rDNA genotype was nearly an exact overlay on both macro- and mic rogeographical scales of a distinctive habitat and also of the distrib ution of an eight-locus morphological-allozyme variant genotype. In al l, seven different habitat-genotype combinations (ecotypes) were disti nguishable on the basis of their morphological-allozyme-rDNA genotypes . None of these seven genotypes has been found in ancestral Spanish po pulations; thus the above predominant multilocus genotypes (ecotypes) of the colonial populations evidently evolved subsequent to the recent introduction (within 150-200 generations) of A. barbata to California . The precise associations of specific alleles and genotypes of the mo rphological allozyme and rDNA loci with different specifiable habitats leads us to the conclusion that natural selection favoring particular multilocus combinations of alleles in different habitats was the main guiding force in shaping the internal genetic structure of local popu lations as well as the overall adaptive landscape of A. barbata over C alifornia and Oregon.