PATTERNS OF SEQUENCE VARIATION IN THE MITOCHONDRIAL D-LOOP REGION OF SHREWS

Direct sequencing of the mitochondrial displacement loop (D-loop) of s hrews (genus Sorer) for the region between the tRNA(Pro) and the conse rved sequence block-F revealed variable numbers of 79-bp tandem repeat s. These repeats were found in all 19 individuals sequenced, represent ing three subspecies and one closely related species of the masked shr ew group (Sorer cinereus cinereus, S. c. miscix, S. c. acadicus, and S . haydeni) and an outgroup, the pygmy shrew (S. hoyi). Each specimen a lso possessed an adjacent 76-bp imperfect copy of the tandem repeats. One individual was heteroplasmic for length variants consisting of fiv e and seven copies of the 79-bp tandem repeat. The sequence of the rep eats is conducive to the formation of secondary structure. A terminati on-associated sequence is present in each of the repeats and in a uniq ue sequence region 5' to the tandem array as well. Mean genetic distan ce between the masked shrew taxa and the pygmy shrew was calculated se parately for the unique sequence region, one of the tandem repeats, th e imperfect repeat, and these three regions combined. The unique seque nce region evolved more rapidly than the tandem repeats or the imperfe ct repeat. The small genetic distance between pairs of tandem repeats within an individual is consistent with a model of concerted evolution . Repeats are apparently duplicated and lost at a high rate, which ten ds to homogenize the tandem array. The rate of D-loop sequence diverge nce between the masked and pygmy shrews is estimated to be 15%-20%/Myr , the highest rate observed in D-loops of mammals. Rapid sequence evol ution in shrews may be due either to their high metabolic rate and sho rt generation time or to the presence of variable numbers of tandem re peats.