Book Viewer

Short Contents | Full Contents | Other books @ NCBI

Modern Genetic Analysis Chapter 13. Transposable Genetic Elements
Transposable Elements in Drosophila

It is now estimated that many spontaneous mutations and chromosomal rearrangements in Drosophila are caused by transposable elements. As much as 10 percent of the Drosophila genome may be composed of families of dispersed, repetitive DNA sequences that move as discrete elements. Three types of transposable elements have been characterized: the copia-like elements, the fold-back (FB) elements, and the P elements. Their structures are summarized in Figure 13-16

Copia-like Elements

The copia-like elements constitute at least seven families, ranging in size from 5 to 8.5 kb. Members of each family appear at 10100 positions in the Drosophila genome. Each member carries a long, direct terminal repeat and a short, imperfect inverted repeat (Figure 13-16) and is structurally similar to a yeast Ty element (Figure 13-15). Copia-like elements also cause a duplication of a characteristic number of base pairs of Drosophila DNA on insertion. Certain classic Drosophila mutations result from the insertion of copia-like and other elements. For example, the white-apricot (wa) mutation for eye color is caused by the insertion of an element from the copia family into the white locus. Some copia-like families have surprising properties. For instance, all the insertion mutations detected so far that result from the gypsy family of copia-like elements are suppressible by a specific allele at one particular outside locus. That is, the phenotypes resulting from the gypsy insertions are affected by unlinked genes. The mechanism of the effect is unknown.

FB Elements

The FB elements range in size from a few hundred to a few thousand base pairs. These elements have sequence homologies, but different elements have sequence differences also. Each carries long inverted repeats at its termini (see Figure 13-16). Sometimes the entire element consists of inverted repeats, but a central sequence separates the inverted repeats in other elements. In either case, FB elements can literally fold back on themselves owing to the inverted repeats (hence, their name). Several unstable mutations in Drosophila have been shown to be caused by the insertion of FB elements. Mutations can result either from the interruption of the coding sequence of a gene by FB-element insertion or from the effects on gene expression due to FB-element insertion in or near a control region. The properties of some of the FBinsertion mutations suggest that FB elements can excise themselves from the genome and promote chromosomal rearrangements at high frequencies.

P Elements

Of all the transposable elements in Drosophila, the most intriguing and useful to the geneticist are the P elements. These elements were discovered as a result of studying hybrid dysgenesisa phenomenon that occurs when females from laboratory strains of Drosophila melanogaster are mated with males derived from natural populations. In such crosses, the laboratory stocks are said to possess an M cytotype (cell type), and the natural stocks are said to possess a P cytotype. In a cross of M × P, the progeny show a range of surprising phenotypes that are manifested in the germ line, including sterility, a high mutation rate, and a high frequency of chromosomal aberration and nondisjunction. These hybrid progeny are dysgenic, or biologically deficient (hence, the expression hybrid dysgenesis). Interestingly, the reciprocal cross P × M produces no dysgenic offspring. An important observation is that a large percentage of the dysgenically induced mutations are unstablethat is, they revert to wild-type or to other mutant alleles at very high frequencies. This instability is generally restricted to the germ line of an individual possessing an M cytotype.

These findings led to the hypothesis that the mutations are caused by the insertion of foreign DNA into specific genes, thereby rendering them inactive. According to this hypothesis, reversion would usually result from the spontaneous excision of these inserted sequences. This hypothesis has been critically tested by isolating dysgenically derived unstable mutants at the eye-color locus white. A plasmid constructed to carry the white locus was used as a probe to recover the dysgenesis-mutated white genes. (This type of experiment is explained in Chapter 11). Most of the mutations were found to be caused by the insertion of a genetic element into the middle of the white+ gene. The element, called the P element, was found to be present in 30 to 50 copies per genome in P strains but to be completely absent in M strains. The P elements vary in size, ranging from 0.5 to 2.9 kb in length (this size difference is due to partly deleted elements derived from a single complete P element), but there is always a 31-bp perfect inverted repeat at their ends. The full-sized P element carries a gene containing four exons, coding for a transposase.

The current explanation of hybrid dysgenesis is based on the proposal that P elements encode both the transposase product and P-repressor products. According to this model, which is depicted in Figure 13-17, the transposase is responsible for mobilization of the P elements, whereas the repressor prevents transposase production, thereby blocking transposition of the element. (We consider repressor proteins in general in Chapter 14.) In the P cytotype, the high copy number of P elements leads to the abundant production of repressor, so the P elements are immobilized. For some reason, most laboratory strains have no P elements; consequently, there is no repressor in the cytoplasm. In hybrids from the cross M × P, the P elements are in a repressor-free environment and can transpose throughout the genome, causing a variety of damage expressed as the various manifestations of hybrid dysgenesis. On the other hand, as noted earlier, P × M crosses do not result in dysgenesis. This is because P-cytotype females already have high levels of P repressor.

Quite apart from their interest as a genetic phenomenon, the P elements have become major tools of the modern Drosophila geneticist, being used to tag genes for cloning and to insert genes transgenically.

© 1999 by W. H. Freeman and Company. All rights reserved.