Patterns of sequence variation in the mitochondrial D-loop region of shrews

Direct sequencing of the mitochondrial displacement loop (D-loop) of shrews(genus Sorex) for the region between the tRNA(Pro) and the conservedsequence block-F revealed variable numbers of 79-bp tandem repeats. Theserepeats were found in all 19 individuals sequenced, representing threesubspecies and one closely related species of the masked shrew group (Sorexcinereus cinereus, S. c. miscix, S. c. acadicus, and S. haydeni) and anoutgroup, the pygmy shrew (S. hoyi). Each specimen also possessed anadjacent 76-bp imperfect copy of the tandem repeats. One individual washeteroplasmic for length variants consisting of five and seven copies ofthe 79-bp tandem repeat. The sequence of the repeats is conducive to theformation of secondary structure. A termination-associated sequence ispresent in each of the repeats and in a unique sequence region 5' to thetandem array as well. Mean genetic distance between the masked shrew taxaand the pygmy shrew was calculated separately for the unique sequenceregion, one of the tandem repeats, the imperfect repeat, and these threeregions combined. The unique sequence region evolved more rapidly than thetandem repeats or the imperfect repeat. The small genetic distance betweenpairs of tandem repeats within an individual is consistent with a model ofconcerted evolution. Repeats are apparently duplicated and lost at a highrate, which tends to homogenize the tandem array. The rate of D- loopsequence divergence between the masked and pygmy shrews is estimated to be15%-20%/Myr, the highest rate observed in D-loops of mammals. Rapidsequence evolution in shrews may be due either to their high metabolic rateand short generation time or to the presence of variable numbers of tandemrepeats.