Pd. Cluster et Rw. Allard, EVOLUTION OF RIBOSOMAL DNA (RDNA) GENETIC-STRUCTURE IN COLONIAL CALIFORNIAN POPULATIONS OF AVENA-BARBATA, Genetics, 139(2), 1995, pp. 941-954
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.