GENOME ORGANIZATION OF REPETITIVE ELEMENTS IN THE RODENT, PEROMYSCUS-LEUCOPUS

To document the frequency and distribution of repetitive elements in P eromyscus leucopus, the white-footed mouse, a cosmid genomic library w as examined. Two thousand thirteen randomly chosen recombinants, with an average insert size of 35 kb and representing 2.35% of the haploid genome of P. leucopus, were screened with probes representing microsat ellites, tandem repeats, and transposable elements. Of the four dinucl eotides, (GT)n was present in 87% of the clones, (CT)n was present in 59% of the clones, and (AT)n and (GC)n each was represented in our sam ple by a single clone (0.05%). (TCC)n was present in 8% of the clones. Of the tandem repeats, the 28S ribosomal probe and the (TTAGGG)n telo mere probe were not represented in the library, whereas a heterochroma tic fragment was present in 9% of the clones. A transposable element, mys, was estimated to occur in 4700 copies, whereas a long intersperse d element (LINE) was estimated to occur in about 41,000 copies per hap loid genome. LINE and mys occurred together in the same clones more fr equently than expected on the basis of chance. Hybridizing the library to genomic DNA from P. leucopus, Reithrodontomys fulvescens, Mus musc ulus, and human produced general agreement between phylogenetic relate dness and intensity of hybridization. However, dinucleotide repeats ap peared to account for a disproportionately high number of positive clo nes in the more distantly related taxa.