EVOLUTION OF MULTILOCUS GENETIC-STRUCTURE IN AVENA-HIRTULA AND AVENA-BARBATA

Avena barbata, an autotetraploid grass, is much more widely adapted th an Avena hirtula, its diploid ancestor. We have determined the 14-locu s genotype of 754 diploid and 4751 tetraploid plants from 10 and 50 Sp anish sites, respectively. Allelic diversity is much greater in the te traploid (52 alleles) than in the diploid (38 alleles): the extra alle les of the tetraploid were present in nonsegregating heteroallelic qua driplexes. Seven loci were monomorphic for the same allele (genotypica lly 11) in all populations of the diploid: five of these loci were als o monomorphic for the same allele (genotypically 1111) in all populati ons of the tetraploid whereas two loci each formed a heteroallelic qua driplex (1122) that was monomorphic or predominant in the tetraploid. Seven of the 14 loci formed one or more highly successful homoallelic and/or heteroallelic quadriplexes in the tetraploid. We attribute much of the greater heterosis and wider adaptedness of the tetraploid to f avorable within-locus interactions and interlocus (epistatic) interact ions among alleles of the loci that form heteroallelic quadriplexes. I t is difficult to account for the observed patterns in which genotypes are distributed ecogeographically except in terms of natural selectio n favoring particular alleles and genotypes in the specific genetic st ructure of different local populations as well as the adaptive landsca pe of both the diploid and tetraploid.