ORIGIN AND EVOLUTION OF HOMOLOGOUS REPEATED SEQUENCES IN THE MITOCHONDRIAL-DNA CONTROL REGION OF SHREWS

The complete mitochondrial DNA (mtDNA) control region was amplified an d directly sequenced in two species of shrew, Crocidura russula and So rer araneus (Insectivora, Mammalia). The general organization is simil ar to that found in other mammals: a central conserved region surround ed by two more variable domains. However, we have found in shrews the simultaneous presence of arrays of tandem repeats in potential locatio ns where repeats tend to occur separately in other mammalian species. These locations correspond to regions which are associated with a poss ible interruption of the replication processes, either at the end of t he three-stranded D-loop structure or toward the end of the heavy-stra nd replication. In the left domain the repeated sequences (R1 repeats) are 78 bp long, whereas in the right domain the repeats are 12 bp lon g in C. russula and 14 bp long in S. araneus (R2 repeats). Variation i n the copy number of these repeated sequences results in mtDNA control region length differences. Southern blot analysis indicates that leve l of heteroplasmy (more than one mtDNA form within an individual) diff ers between species. A comparative study of the R2 repeats in 12 addit ional species representing three shrew subfamilies provides useful ind ications for the understanding of the origin and the evolution of thes e homologous tandemly repeated sequences. An asymmetry in the distribu tion of variants within the arrays, as well as the constant occurrence of shorter repeated sequences flanking only one side of the R2 arrays , could be related to asymmetry in the replication of each strand of t he mtDNA molecule. The pattern of sequence and length variation within and between species, together with the capability of the arrays to fo rm stable secondary structures, suggests that the dominant mechanism i nvolved in the evolution of these arrays is unidirectional replication slippage.