Cloning of DNA sequences from the white locus of D. melanogaster by a novel and general method

The numbers of members of three families of copia-like elements were counted on twenty X, 2nd and 3rd chromosomes collected from a natural population of Drosophila melanogaster. Theoretical predictions were computed for two models of copy number stabilization: (1) element frequencies are regulated by a simple genetic process such as copy number dependent transposition or excision, independent of chromosomal location; (2) elements are eliminated by natural selection against mutational effects of their insertion into the chromosome. Since insertions into the X can be expected to suffer more selection than autosomal insertions, due to expression of mutant phenotypes in the hemizygous state, hypothesis 2, called the disproportional model, predicts that the proportion of elements on the X will be smaller than the proportion of the genome contributed by the X, while hypothesis 1, called the equiproportional model, predicts that this proportionality will be unaffected. Two of the elements, 297 and roo, showed no evidence for deficiency of X-linked elements, but the data for a third element, 412, were consistent with the prediction based on the selective model.These results indicate that simple selection against mutational effects of insertions of transposable elements is not generally adequate to account for their distribution within populations. We argue that a mechanism such as recombination between elements at different chromosomal sites, leading to rearrangements with highly deleterious, dominant effects could play a role in stabilizing copy number. This process would lead to a higher abundance of elements in genomic regions with restricted crossing over. We present some data indicating such an effect, and discuss possible interpretations.

A test for the role of natural selection in the stabilization of transposable element copy number in a population of Drosophila melanogaster.

Publisher Summary Transposable elements are the DNA sequences that move to new genomic locations at a much higher rate than that of the bulk of the cellular DNA. The ability to cause deletions or chromosomal rearrangements is characteristic of these elements, and many have been shown to affect the expression of chromosomal genes by inserting adjacent to or into these genes. Saccharomyces cerevisiae is very useful as a eukaryotic model system for molecular biologists. Features of a yeast system that make this organism advantageous include the ease of growth, small genome size, the availability of selectable markers, the ease of genetic manipulation, and the availability of techniques for transformation with exogenous DNA. Yeast transposable element (Ty) is a family of disperse, repetitive DNA sequences, each of which is homologous to the original Ty 1 sequence. Yeast has quite a small genome, as compared to other eukaryotes, and much less repeated DNA. Transposable elements in many systems appear to cause chromosomal rearrangements, such as deletions, inversions, and translocations. Several types of chromosomal rearrangements have been reported to be associated with yeast Ty elements or delta sequences.

Transposable elements in yeast.

The presence in cultured Drosophila melanogaster cells of retrovirus-like particles containing 5 kilobase RNA molecules homologous to the transposable element copia provides support for the suggested evolutionary relationship between retroviruses and eukaryotic movable genetic elements.

Retrovirus-like particles containing RNA homologous to the transposable element copia in Drosophila melanogaster.

The important role insect viruses have in the biological control of pests has inspired a considerable effort towards understanding their molecular biology. The nuclear polyhedrosis viruses (NPVs) are particularly interesting and accessible owing to their unique nuclear replication cycle and large (∼100 kilobase, kb) double-stranded, circular DNA genome. Serial passage of NPVs in cell culture results in a gradual reduction in the ability of the virus to carry out the biphasic growth cycle of producing extracellular virus particles (non-occluded virus, NOV), followed by the occlusion of virus particles in proteinaceous polyhedra within the nucleus. Instead, NOVs become the predominant replication product of serially passaged NPVs, with occlusion being a rarely observed, and probably aberrant, phenomenon1–5. Plaque purification of Autographa californica (the alfalfa looper, Lepidoptera; Noctuidae) NPV (AcNPV) after 25 passages in cell cultures of Trichoplusia ni (the cabbage looper, Lepidoptera; Noctuidae) resulted in the isolation of few-polyhedra (FP) mutants. One of these plaque isolates, FP-D, exhibited a restriction endonuclease fragment pattern indicative of an insertion, probably of non-viral DNA, at 86.4–86.6 on the physical map. Of additional interest was the observation of restriction fragments in sub-molar quantities6. We have now characterized this mutant; it contains an insertion of a copia-like transposable element, designated TED, derived from the host T. ni cells.

E. Young

Papers

copia-like Transposable Elements in the Drosophila Genome

Copia like transposable elements in the drosophila melanogaster genome