INTERSPECIFIC COMMON REPETITIVE DNA SEQUENCES IN SALAMANDERS OF GENUS PLETHODON

Intermediate repetitive sequences of P. cinereus which comprised .apprx. 30% of the genomic DNA were isolated and iodinated with 125I. About 5% of the 125I-repetitive fraction hybridized with a large excess of DNA from P. dunni at Cot 20. About half of the 125I-DNA in the hybrids was resistant to extensive digestion with S-1 nuclease. The average molecular size of the S-1 nuclease-resistant fraction was .apprx. 100 nucleotide pairs. The melting temperature of the S-1 nuclease-resistant fraction was about 2.degree. lower than that of the corresponding fraction made with P. cinereus DNA. The genomes of P. cinereus and P. dunni seem to possess evolutionarily stable common repetitive sequences. The average frequency of repetition of the common repetitive sequences is .apprx. 6000 .times. in both species. The common repetitive fraction is also present in the genomes of other species of Plethodon, although the general populations of intermediate repetitive sequences are markedly different from one species to another. The P. cinereus-P. dunni common repetitive sequences could not be detected in plethodontids belonging to different tribes, nor in more distantly related amphibians. The profiles of binding of the common repetitive sequences to CsCl or Cs2SO4-Ag+ density gradient fractions of P. dunni DNA suggested that these sequences consisted of heterogeneous components with respect to base compositions, and that they did not include large amounts of the genes for ribosomal RNA, 5S RNA, 4S RNA, or histone messenger RNA. In situ hybridization of the 3H-labeled intermediate repetitive sequences of P. cinereus to male meiotic chromosomes of the same species gave autoradiographs after an exposure of 7 days showing all 14 chromosomes labeled. The labeling pattern appeared not to be random, but was impossible to analyze on account of the irregular shapes and different degrees of stretching of diplotene and prometaphase chromosomes. In situ hybridization of the same sequences to meiotic chromosomes from P. dunni gave autoradiographs after 60 days exposure in which all chromosomes were labeled. These heterologous in situ hybrids can only have involved the common repetitive sequences.