The relationship between DNA value and chromosome volume in the coleopteran genus Dermestes

DNA values and chromosome volumes were determined for six species of the coleopteran genus Dermestes which have very similar karyotypes. Large differences in DNA value occur which give no indication of a geometric series. DNA value is approximately proportional to chromosome volume, indicating that the DNA differences lie in the chromosomes. Evidence from the variation in volume of the sex chromosomes, along with the DNA values, cannot be interpreted in terms of a chromosome model with a variable number of strands. DNA values are often at variance with the taxonomic grouping of species within the genus.     

The influence of DNA content and nuclear volume on the frequency of radiation-induced chromosome aberrations in Bufo species

The frequency of chromosome aberrations was compared in X-irradiated blood lymphocytes of Bufo marinus, B. calamita and B. pardalis which have similar chromosome numbers (2n=22) and karyotypes but differ in chromosomal DNA content and nuclear volume. — For each species the yield, per cell of centric exchange aberrations (dicentrics+rings) and of deletions (interstitial and terminal) increased approximately as the 1.5th power of the dose. — The 55% higher chromosomal DNA content of B. pardalis compared with both B. marinus and B. calamita resulted in the same increase in the frequency of deletions as a 55% increase in radiation dose, approximately doubling the yield. Both factors probably lead to a similar increase in the frequency of primary lesions from which the deletions are derived. — In contrast, an increase in chromosomal DNA content did not result in a higher yield of dicentric and ring exchanges, probably because the greater nuclear volume of B. pardalis (twice that of the other species) offset the potential increase in exchanges by increasing the average distance between chromosomes and chromosome arms. The data support the hypothesis that, in order to be involved in exchange, chromosome regions must be close together at the time of irradiation. The frequency of deletions is unaffected by changes in nuclear volume.     

Fragile X chromosome and chromosome condensation

The effect of chromosome condensation on the frequency of expression of the fragile X chromosome was examined. Chromosome decondensation substances were tested for their ability to elicit expression or improve frequencies of expression of the fragile X chromosome in five patients. The substances tested included the AT specific DNA ligands ethidium bromide, Hoechst 33258, and netropsin, and the GC specific substances actinomycin D and olivomycin. Under culture conditions appropriate for eliciting fragile X expression none of the decondensation compounds studied significantly altered frequencies of expression, nor did any of the substances elicit fragile X expression under conditions that normally suppress fragile X expression. The fragile X was found to be more frequently evident in less condensed chromosome preparations from fibroblasts. The implications of these findings with respect to the nature of fragile sites are discussed.     

Mouse chromosome 19 and distal rat chromosome 1: a chromosome segment conserved in evolution

Through a combination of radiation hybrid mapping and studies by FISH and zoo-FISH we have made a comparative investigation of the distal portion of rat chromosome 1 (RNO1) and the entire mouse chromosome 19 (MMU19). It was found that homologous segments of RNO1 and MMU19 are similar in banding morphology and in length as determined by several different methods, and that the gene order of the 46 genes studied appears to be conserved across the homologous segments in the two species. High-resolution zoo-FISH techniques showed that MMU19 probes highlight only a continuous segment on RNO1 (1q43-qter), with no detectable signals on other rat chromosomes. We conclude that these data suggest the evolutionary conservation of a chromosomal segment from a common rodent ancestor. This segment now constitutes the entire MMU19 and a large segment distally on RNO1q in the mouse and rat, respectively.     

The three vibrio cholerae chromosome II-encoded ParE toxins degrade chromosome I following loss of chromosome II

Three homologues of the plasmid RK2 ParDE toxin-antitoxin system are present in the Vibrio cholerae genome within the superintegron on chromosome II. Here we found that these three loci-two of which have identical open reading frames and regulatory sequences-encode functional toxin-antitoxin systems. The ParE toxins inhibit bacterial division and reduce viability, presumably due to their capacity to damage DNA. The in vivo effects of ParE1/3 mimic those of ParE2, which we have previously demonstrated to be a DNA gyrase inhibitor in vitro, suggesting that ParE1/3 is likewise a gyrase inhibitor, despite its relatively low degree of sequence identity. ParE-mediated DNA damage activates the V. cholerae SOS response, which in turn likely accounts for ParE's inhibition of cell division. Each toxin's effects can be prevented by the expression of its cognate ParD antitoxin, which acts in a toxin-specific fashion both to block toxicity and to repress the expression of its parDE operon. Derepression of ParE activity in ΔparAB2 mutant V. cholerae cells that have lost chromosome II contributes to the prominent DNA degradation that accompanies the death of these cells. Overall, our findings suggest that the ParE toxins lead to the postsegregational killing of cells missing chromosome II in a manner that closely mimics postsegregational killing mediated by plasmid-encoded homologs. Thus, the parDE loci aid in the maintenance of the integrity of the V. cholerae superintegron and in ensuring the inheritance of chromosome II.     

Mouse chromosome 1

Committee Members: D. Epstein, T.A. Howard, D. Malo, F. Mancino, M. Mehrabian, K.J. Moore, R.J. Oakey, S. Reinehez-Videl, K. Steel and M.L. Watson.