Cytonuclear disequilibrium and genetic drift in a natural population of ponderosa pine

We measured the cytonuclear disequilibrium between 11 nuclear allozyme loci and both mitochondrial and chloroplast DNA haplotypes in a natural populat ion of ponderosa pine (Pinus ponderosa, Laws). Three allozyme loci showed s ignificant associations with mtDNA variation, while two other loci showed s ignificant association with cpDNA. However, the absolute number of individu als involved in any of the associations was small, such that in none of the nuclear-organellar combinations was the difference between observed and ex pected numbers > 11 individuals. Patterns of association were not consisten t across loci or organellar genomes, suggesting that they are not the resul t of mating patterns, which would act uniformly on all loci. This pattern o f disequilibria is consistent with the action of genetic drift and with exi sting knowledge of the structure of this population and thus does not imply the action of other evolutionary processes. The overall magnitude (normali zed disequilibrium) of associations was greater for maternally inherited mt DNA than for paternally inherited cpDNA, though this difference was neither large nor significant. Such significant disequilibria involving the patern ally inherited organelle indicate that not only are there a limited number of seed parents, but the effective number of pollen parents is also limited .