Linear mitochondrial molecules and intraspecific mitochondrial genome stability in a species of Pythium

Abstract

Restriction maps of the mitochondrial genomes of 11 isolates of Pythium oligandrum revealed sizes ranging from 69.8 to 72.7 kb. While most isolates had circular genomes, two isolates contained predominantly linear molecules with the position of the ends corresponding to the small unique region of the circular molecules. The termini of these molecules were resistant to Exo III and λ exonuclease digestion and had hairpin loop structures. Several isolates with circular genomes also appeared to contain what may be a small percentage of linear molecules. The inverted repeat and large unique region were 28.6 and 11.5 kb, respectively, and were equal in size among isolates. The small unique region was the most variable portion of the genome and ranged from 0.98 to 3.92 kb in size. Polymorphisms among isolates were due mainly to insertions–deletions in the small unique region, which not only contributed to size differences but also generated additional unique restriction sites. The only variation detected in the inverted repeat or the large unique region was the addition–loss of several restriction sites in four isolates. Less sequence divergence was observed in the inverted repeat than in the large unique region (maximum of 0.83 compared with 2.50%, respectively). Percent sequence divergence for the entire genome (excluding one arm of the inverted repeat) ranged from 0 to 1.00%. Excluding differences caused by insertions–deletions in the small unique region, 94% or greater of the restriction sites are conserved among all isolates.Key words: Pythium oligandrum, mtDNA, inverted repeat, intraspecific variation, linear genome.