The evolutionary pathway of the staphylococcal cassette chromosome element

The staphylococcal cassette chromosome (SCC) element can carry resistance genes to antibiotics, disinfectants, and heavy metals, contributing to the survival of strains in the environment and causing difficulties in the treatment of staphylococcal infections. Methicillin resistance in staphylococci, which is of particular clinical significance, is encoded by staphylococcal cassette chromosome

A study of ten families of transposable elements on X chromosomes from a population of Drosophila melanogaster

SummaryData were collected on the distribution of ten families of transposable elements among fourteen X chromosomes isolated from a natural population of Drosophila melanogaster, by means of in situ hybridization to polytene chromosomes. It was found that, with the exception of roo, the copy number per chromosome followed a Poisson distribution. There was no evidence for linkage disequilibrium, either within or between families. Some pairs of families of elements were correlated with respect to the identity of the sites that were occupied in the sample, although there was no evidence for a correlation with respect to the sites at which elements attained relatively high frequencies. Elements appeared to be distributed randomly along the distal part of the X chromosome. There was, however, a strong tendency for elements to accumulate at the base of the chromosome. Element frequencies per chromosome band were generally low, except at the base of the chromosome where bands in subdivisions 19E and 20A sometimes had high frequencies of occupation. These results are discussed in the light of models of the population dynamics of transposable elements. It is concluded that they provide strong evidence for the operation of a force or forces opposing transpositional increase in copy number. The accumulation of elements at the base of the chromosome is consistent with the idea that unequal exchange between elements at non-homologous sites is such a force, although other possibilities cannot be excluded at present. The data suggest that the rate of transposition per element per generation is of the order of 10−4, for the elements included in this study.

CHROMOSOMAL POLYMORPHISMS OF CONSTITUTIVE HETEROCHROMATIN AND INVERSIONS IN DROSOPHILA

ABSTRACT A simple technique for preparing mitotic metaphases from a larval ganglion of Drosophila is described. Parallel examination of polytene and metaphase chromosome groups shows that inversion polymorphism in chromosome 3 of D. recticilia from East Maui (Hawaii) manifests a one-to-one correlation with a metaphase karyotype polymorphism due to the presence of an extra heterochromatic portion. These observations are consistent with the previous findings on other species of Hawaiian Drosophila. They strongly support the hypothesis that when one breakpoint of a long inverted segment of a chromosome element occurs in the vicinity of the constitutive heterochromatin, it may exert an effect in eliciting the production of heterochromatic material in the same chromosome.