Organization of the large mitochondrial genome in the isopod Armadillidiumvulgare

The mitochondrial DNA (mtDNA) in animals is generally a circular molecule o f similar to 15 kb, but there are many exceptions such as linear molecules and larger ones. RFLP studies indicated that the mtDNA in the terrestrial i sopod Armadillidium vulgare varied from 20 to 42 kb. This variation depende d on the restriction enzyme used, and on the restriction profile generated by a given enzyme. The DNA fragments had characteristic electrophoretic beh aviors. Digestions with two endonucleases always generated fewer fragments than expected; denaturation of restriction profiles reduced the size of two bands by half; densitometry indicated that a number of small fragments wer e present in stoichiometry, which has approximately twice the expected conc entration. Finally, hybridization to a 550-bp 16S rDNA probe often revealed two copies of this gene. These results cannot be due to die generic rearra ngements generally invoked to explain large mtDNA. We propose that the larg e A. vulgare mtDNA is produced by the tripling of a 14-kb monomer with a si ngular rearrangement: one monomer is linear and the other two form a circul ar dimer. Densitometry suggested that these two molecular structures were p resent in different proportions within a single individual. The absence of mutations within the dimers also suggests that replication occurs during th e monomer phase.