Observations on centromeric heterochromatin and satellite DNA in salamanders of the genus Plethodon.

Abstract

DNA from Plethodon cinereus cinereus separates into two fractions on centrifugation to equilibrium in neutral CsCl. The smaller of these fractions has been described as a high-density satellite. It represents about 2% of nuclear DNA from this species, and it has a density of 1.728 g/cm3. It is cytologically localized near the centromeres of all 14 chromosomes of the haploid set. In P. c. cinereus the heavy satellite DNA constitutes about 1/4 of the DNA in centromeric heterochromatin. The nature of the rest of the DNA in centromeric heterochromatin is unknown. The number of heavy satellite sequences clustered around the centromeres in a chromosome from P. c. cinereus is roughly proportional to the size of the chromosome, as determined by in situ hybridization with satellite-complementary RNA, and autoradiography. Likewise the amount of contromeric heterochromatin, as identified by its differential stainability with Giemsa, shows a clear relationship to chromosome size. — The heavy satellite sequences identified in DNA from P. c. cinereus are also present in smaller amounts in other closely related forms of Plethodon. Plethodon cinereus polycentratus and P. richmondi have approximately half as many of these sequences per haploid genome as P. c. cinereus. P. hoffmani and P. nettingi shenandoah have about 1/3 as many of these sequences as P. c. cinereus. P. c. cinereus, P. c. polycentratus, and P. richmondii all have detectable heavy satellites with densities of 1.728 g/cm3. Among these forms, satellite size as determined by optical density measurements, and number of satellite sequences as determined from hybridization studies, vary co-ordinately. P. c. cinereus heavy satellite sequences are not detectable in P. nettingi, P. n. hubrichti, or P. dorsalis. The latter species has a heavy satellite with a density of 1.718 g/cm3, representing about 8% of the genomic DNA, and two light satellites whose properties have not been investigated. The heavy satellite of P. dorsalis is cytologically localized in the centromeric heterochromatin of this species. — These observations are discussed in relation to the function and evolution of highly repetitive DNA sequences in the centromeric heterochromatin of salamanders and other organisms.