Differential Cytoplasmic versus Nuclear Introgression in Helianthus

Diagnostic chloroplast DNA and nuclear ribosomal DNA markers were used to assess the occurrence and the extent of introgression from the common sunflower, Helian-thus annuus, into the cucumber-leaf sunflower, H. debilis ssp. cucumerifolius. Anal-ysis of 11 populations (270 individuals) of H. debilis ssp. cucumerifolius revealed that 193 individuals from nine populations had the chloroplast DNA genotype of H. annuus, whereas only 20 individuals from nine populations had detectable nuclear ribosomal DNA markers of H. annuus. Not only does the introgression of chloroplast DNA genotypes into H. debilis ssp. cucumerifolius appear to occur more readily than that of nuclear genes, but the flow of chloroplast DNA genotypes is greater than that previously documented in the opposing direction (into H. annuus). It is speculated that the differential, asymmetric gene flow observed in H. debilis ssp. cucumerifolius results from the greater abundance of H. annuus in the area of sympatry coupled with strong selection against "annuus " nuclear genes.