Satellite DNA properties of the germ line limited DNA and the organization of the somatic genomes in the nematodesAscaris suum andParascaris equorum

Abstract

During the early cleavage divisions in some Ascarids, parts of the chromosomes are eliminated from the somatic blastomeres ("chromatin diminution", Boveri, 1887) while the chromosomes in the germ line cells maintain their integrity. To characterize the germ line and soma genome, DNA was isolated from gametes and embryonic somatic cells of two Ascarid species, Parascaris equorum var. univalens and Ascaris suum. It was shown that the germ line limited DNAs of these species have the same density and almost identical reassociation kinetics: in CsCl the predominant component of the germ line limited DNA of P. equorum and A. suum has the buoyant density of 1.697 g/cm3, while soma DNA of both species bands at 1.700 g/cm3. In P. equorum there is a small additional germ line limited satellite DNA component with the density of 1.690 g/cm3, identical to that of mitochondrial DNA of both organisms. Comparison of the reassociation kinetics of germ line and soma DNA demonstrates for both species that the eliminated DNA sequences are highly repetitive. In contrast to these similarities between the germ line limited DNAs of P. equorum and A. suum the analysis of their base composition revealed differences (40% guanine plus cytosine in P. equorum and 36% in A. suum). The only very fast reassociating DNA sequences which we could isolate from soma DNA was demonstrated to be foldback DNA. The reassociation kinetics of total A. suum soma DNA was investigated by hydroxylapatite chromatography. Least squares analysis of the data revealed about 10% of intermediate repetitive DNA sequences. Their interspersion between single copy DNA sequences was analyzed by comparing the reassociation kinetics of DNA fragments 0.35 and 7.2 kilobases long. Thus the DNA sequence arrangement of Ascaris does not follow the short period interspersion pattern observed in most organism.

Organization and dynamics of satellite and telomere DNAs in Ascaris: implications for formation and programmed breakdown of compound chromosomes

“…The outer eggshell layers were removed, as described by Roth (1979). The resulting peeled eggs, which are still surrounded by the perivitelline membrane and the ascaroside layer, were stored in distilled water at 3°C.…”

“…Here we identify a second repeat, a decanucleotide, and demonstrate that the penta-and decanucleotides constitute the terminal heterochromatic blocks, which are characteristic of the P. univalens plurivalent chromosomes. Nevertheless, the germline-specific DNA contains noncoding single-copy DNA (Teschke et al 1991), middle repeated sequences (Roth 1979;Tobler et al 1992), and even a few genes (Etter et al 1991;Spicher et al 1994;reviewed in Müller et al 1996) in both species. The latter finding has given rise to an 'ancient partial genome duplication' hypothesis according to which DNA segments containing genes of selective advantage would be tolerated in the eliminated genome portion, because they do not affect the genetic balance in somatic cells .…”

“…In both species, satellite DNAs were found to be present in the germline at amounts that correspond to the differences between the sizes of the germline and soma genomes. Reassociation studies pointed to low sequence complexity of the satellites (Moritz and Roth 1976;Roth 1979). In A. suum the germline-limited satellite (gll sat) DNA consists of one single 123 bp repeat (Roth and Moritz 1981;Streeck et al 1982).…”

Cytogenetic and molecular characterization of a highly repeated DNA sequence in the peach potato aphid Myzus persicae

“…Studies carried out on satellite DNA sequences in organisms possessing holocentric chromosomes are scanty (Lagowsky et al 1973;Roth 1979;Collet and Westerman 1987;Naclerio et al 1992;Grenier et al 1996Grenier et al , 1997Castagnone Sereno et al 1998a, b). Only two highly repeated DNA sequences have been described in aphids to date: a 600 bp repeat unit that has been localized in the intercalary heterochromatin of Megoura viciae X chromosomes ) and a 169 bp tandem repeat that occurs at a subtelomeric location on all chromosomes of three species of the Myzus persicae complex (Spence et al 1998).…”

Nucleotide sequence of satellite DNA contained in the eliminated genome ofAscaris lumbricoides

“…Earlier work has already shown that the eliminated DNA is enriched for repetitious DNA sequences (5)(6)(7)(8)(9). Recently it has been demonstrated by our group (10) and independently by Moritz Nucleic Acids Research (11) that the eliminated material contains highly repetitive satellite sequences.…”