Tandem repeat polymorphism and heteroplasmy in the mitochondrial DNA control region of threespine stickleback (gasterosteus aculeatus)

Length polymorphism and heteroplasmy (multiple forms within a single individual) in the mitochondrial DNA (mtDNA) control region have been reported for several species of fish. In this report we demonstrate its existence in the threespine stickleback (Gasterosteus actileatus). The repeat region, responsible for the length variation and heteroplasmy is located near the 5' end of the control region and consists of slightly imperfect palindromic sequences. The repeat sequences vary in copy number from 1.5 to 16.5 and are capable of forming secondary structures. All individuals examined were heteroplasmic for 4 or more different copy number of repeats. Our results indicate that the repeats are a result of frequent competitive misalignment in the repeat region prior to replication and support the "illegitimate elongation model" as an explanation of the origin and maintenance of length heteroplasmy in the 5' end of the control region.