Chromosomes of a cell line of Dipodomys panamintinus (kangaroo rat). A banding and autoradiographic study

DNA topology in heterochromatin (a hypothesis)

A hypothesis explaining the known heterochromatin features--a compact DNA packaging, transcriptional inactivity, propensity to aggregate (stickiness) and position effect variegation-is described. The hypothesis is based on the assumption that DNA molecules in heterochromatin are topologically open and contain single-strand breaks in the regions with identical or similar primary sequences.

Cytogenetics of the parthenogenetic grasshopper Warramaba (formerly Moraba) virgo and its bisexual relatives. VI. DNA replication patterns of the chromosomes

The distribution of late-replicating segments along the chromosomes of five clones of W. virgo is described. Some, but not all of these segments correspond to C-bands. In general, the "autoradiographic profiles" (histograms of linear grain density along the length of chromosomes labeled with tritiated thymidine in late S-phase) show strong resemblances throughout the five clones. However, some significant differences exist, and these are particularly marked in the case of the Boulder clone, which is anomalous in many other respects. --A similar study has also been carried out on the two bisexual species of Warramaba ("P169" and "P196") that gave rise, by hybridization more than half a million years ago, to the parthenogenetic W. virgo. In the case of P169, the autoradiographic profiles of the three large chromosomes (X + A, B + 5, CD) which it has contributed to the W. virgo karyotype are extremely similar to those of the corresponding chromosomes in the virgo clones we have studied. In the case of P196 there is likewise, in most instances, a close resemblance of the autoradiographic profiles of the AB, X1 and CD chromosomes to those of the same chromosomes in the virgo clones, but that of the X1 shows no particular resemblance to the anomalous profile of the X1 in the Boulder clone, in which the X1 has undergone a structural reorganisation. The autoradiographic profile of the P196 CD chromosome does, however, show a much closer resemblance to that of the corresponding chromosome in the Boulder clone than to those of the CD196 in the other four virgo clones studied. These investigations confirm the considerable evolutionary stability of DNA replication patterns.

Similarity of satilite DNA properties in the order Rodentia

We have characterized satellite DNAs from 9 species of kangaroo rat (Dipodomys) and have shown that the HS-alpha and HS-beta satellites, where present, are nearly identical in all species as to melting transition midpoint (Tm), and density in neutral CsCl, alkaline CsCl, and Cs(2)SO(4)-Ag(+) gradients. However, the MS satellites exist in two internally similar classes. The satellite DNAs from three other rodents were characterized (densities listed are in neutral CsCl). The pocket gopher, Thomomysbottae, contains Th-alpha (1.713 g/ml) and Th-beta (1.703 g/ml). The guinea pig (Caviaporcellus) contains Ca-alpha, Ca-beta and Ca-gamma at densities of 1.706 g/ml, 1.704 g/ml and 1.704 g/ml, respectively. The antelope ground squirrel (Ammospermophilusharrisi) contains Am-alpha, 1.708 g/ml, Am-beta, 1.717 g/ml, and Am-gamma, 1.707 g/ml. The physical and chemical properties of the alpha-satellites from the above four rodents representing four different families in two suborders of Rodentia were compared. They show nearly identical Tm, nucleoside composition of single strands, and single strand densities in alkaline CsCl. Similar comparisons on the second or third satellite DNAs from these rodents also indicate a close relationship to each other. Thus the high degree of similarity of satellite sequences found in such a diverse group of rodents suggests a cellular function that is subject to natural selection, and implies that these sequences have been conserved over a considerable span of evolutionary time since the divergence of these rodents about 50 million years ago.

Satellite DNA and cytogenetic evolution. DNA quantity, satellite DNA and karyotypic variations in kangaroo rats (genus Dipodomys)

The genus Dipodomys (kangaroo rats) exhibits major interspecies variations in the proportions of highly reiterated satellite DNA sequences in the genome as well as in the chromosome number and the proportions of uni-armed and bi-armed chromosomes. For nearly all of the approximately 22 species of the genus and several subspecies, liver DNA was distributed in neutral CsCl buoyant density gradients into four fractions; principal DNA (1.698 g/ml), intermediate-density DNA (1.702 G/ML), MS satellite (1.707 g/ml) and HS (heavy satellites (1.713 g/ml). The total nuclear DNA content of diploid liver cells measured in eleven species by quantitative cytophotometry, ranged from 6.9 to 10.9 pg. These data were correlated with known features of the karotypes of individual species. The salient findings were: (1) that interspecies variations in diploid chromosome number cluster at 52-54, 60-64 and 70-72 (2) that high total nuclear DNA was associated with high chromosome number, and with relatively large amounts of satellite DNA (3) that a high ratio of HS satellites to intermediate-density DNA was generally correlated with a predominance of metacentric and submetacentric chromosomes (high fundamental number). The relationships of satellite DNA to karyotype structure reveal a new level of hierarchy in the genome that appears capable of exerting global control over environmental adaptation and the evolution of new species. This mechanism is consistent with recent hypotheses that changes in the macro-structure of the genome are more important than point mutations in facilitating the rapid phases of animal evolution.