MICROSATELLITES AND ASSOCIATED REPETITIVE ELEMENTS IN THE SHEEP GENOME

To determine the frequency and clustering of a variety of simple di- a nd trinucleotide repeats, an Artiodactyl short interspersed element (S INE), an ovine satellite repeat, and a human Alu 1 repeat were used to screen a random selection of cosmids containing inserts of ovine geno mic DNA. In total, 197 individual cosmids were digested with EcoRI and the fragments separated on 0.7% agarose gels. Southern blots of these gels were then sequentially probed with (AC)7, (CT)9, and (CAC)6 olig onucleotides, and the repeats described above. The frequency at which (AC)n, (CT)n, and (CAC)n repeats were found in the cosmids indicated t hat they occurred at average intervals of 65 kb, 367 kb, and 213 kb re spectively within the ovine genome. The Artiodactyl SINE was the most common, occurring at an average interval of 20 kb. No human Alu 1 sequ ences were detected. There was a significant positive association betw een the (AC)n and the Artiodactyl SINE. This association is quite stro ng as there was significant clustering of the two repeats both within cosmids and also within the EcoRI fragments of the digested genomic fr agments. With the exception of the sheep satellite sequence, which occ urs in tandem arrays, none of the other repeats showed significant clu stering within the 41-kb (average size) cosmid inserts. The first 25 o vine microsatellites we characterized had an average polymorphic infor mation content (PIC) of 0.65. The different microsatellite types, cont aining either perfect, imperfect, or compound repeats, had similar ave rage PICs of 0.64, 0.65, and 0.66 respectively. There was a weak regre ssion relationship (R2(adj)% = 21.9) between the length of the longest uninterrupted dinucleotide repeat in the largest allele and the PIC o f the microsatellite.