Genetic diversity and adaptedness in tetraploid Avena barbata and its diploid ancestors Avena hirtula and Avena wiestii

A vena barbata, a tetraploid grass, is much more widely adapted and successful in forming dense stands than its diploid ancestors. The success of such polyploids has often been attributed to heterosis associated with ability to breed true for a highly heterozygous state in which allelic differences between the parents are fixed in the polyploid by chromosome doubling. We have examined the relationship between genetic diversity and adaptedness for 14 allozyme loci in A. barbata and its diploid ancestors in samples collected from diverse habitats in Israel and Spain. The relationship varied from locus to locus: superior adaptedness was associated with genetic uniformity for five loci, in part with genetic uniformity and in part with genetic diversity (monomorphism for a single heteroallelic quadriplex) for one locus, and with allelic diversity in the form of heteroallelic quadriplexes combined with genotypic diversity in the form of complex polymorphisms among different homoallelic and/or heteroallelic quadriplexes for the eight remaining loci. These results indicate that allelic diversity fixed in nonsegregating form through chromosome doubling was an important factor in the evolution of adaptedness in A. barbota. However, it is unlikely that heterosis associated with heterozygosity contributed significantly to superior adaptedness in either the diploids or the tetraploid because virtually all loci (≈99%) were homozygous in the Avena diploids and tetraploid.