ASSOCIATION BETWEEN CHLOROPLAST AND MITOCHONDRIAL LINEAGES IN OAKS

Patterns of chloroplast DNA (cpDNA) and mitochondrial DNA (mtDNA) vari ation were studied in 378 populations of oak trees sampled throughout the southern half of France. Six cpDNA haplotypes detected in a previo us European survey and three new cpDNA haplotypes were found in this r egion. Two mitochondrial polymorphisms detected earlier by restriction analysis of PCR-amplified fragments alone, or in combination with sin gle-strand conformation polymorphism (SSCP), were compared with the cp DNA data. Sequencing revealed the nature of the two mitochondrial muta tions: a single-base substitution and a 4-bp inversion associated with a 22-bp hairpin secondary structure. The single-base substitution was then analyzed by allele-specific amplification. Results for the two c ytoplasmic genomes were combined, which allowed the identification of 12 cpDNA-mtDNA haplotypes. The 4-bp mtDNA inversion has appeared indep endently in different cpDNA lineages. Given the peculiar nature of thi s mtDNA mutation, we suggest that intramolecular recombination leading to repeated inversions of the 4-bp sequence (rather than paternal lea kage of one of the two genomes) is responsible for this pattern. Furth ermore, the geographic locations of the unusual cpDNA-mtDNA associatio ns (due to the inversion) usually do not match the zones of contact be tween divergent haplotypes. In addition, in southern France, the group ings of populations based on the mtDNA substitution were strictly cong ruent with those based on cpDNA. Because many populations that are pol ymorphic for both cpDNA and mtDNA have remained in contact since postg lacial recolonization in this area without producing any new combinati on of cytoplasms involving the mitochondrial substitution, we conclude that paternal leakage is not a significant factor at this timescale. Such results confirm and expand our earlier conclusions based on contr olled crosses.