Drosophila Melanogaster As A Genetic Model

Over the past 100 years, geneticists have built a large "toolbox" of specialized methods that allow them to manipulate the genome of D. melanogaster with more deftness than is possible with any other organism. These methods have largely taken advantage of some of Drosophilas inherent characteristics, such as the lack of recombination in males. Some widely used techniques include polytene chromosome visualization and in situ hybridization, using balancers and other cytological aberrations for genetic crosses, and germline transformation using P elements and other transposons to examine gene expression and to tag genes for cloning.

The chromosomes found in the larval salivary glands are highly duplicated, allowing a characteristic banding pattern to be visualized with a compound light microscope (Fig. 3). Polytene chromosomes, which allow researchers to observe and study large-scale genetic rearrangements such as inversions, duplications, translocations, and deletions, have been used by geneticists to answer a variety of questions. Early work focused on understanding chromosome mechanics and using deletions to map the location of specific genes. Molecular geneticists have used the polytene chromosome in conjunction with in situ hybridization to more specifically localize the chromosomal site of specific cloned genes or gene fragments. For example, small fragments of DNA can be amplified by using the polymerase chain reaction (PCR), incorporating radioactive or bioluminescent probes as labels and with hybridization to the polytene chromosomes. In addition, evolutionary and population geneticists have used inversion patterns to reconstruct the history of species and populations.

8 species

D. erecta, D. melanogaster, D. mauritiana,

D. teissieri, D. yakuba

FIGURE 2 Placement of D. melanogaster within the family Drosophilidae. [Modified after Powell, J. R. (1997). "Progress and Prospects in Evolutionary Biology. The Drosophila Model." Oxford University Press, New York.]

FIGURE 3 Polytene chromosome of D. melanogaster: X, X chromosome; 2R, right arm of second chromosome; 2L, left arm of second chromosome; 3R, right arm of third chromosome; 3L, left arm of third chromosome; 4, fourth chromosome; CH, chromocenter. [From Krimbas C., and Powell, J. R. (eds.) (1992). "Drosophila Inversion Polymorphism," Fig. 2B, p. 344. CRC Press, Boca Raton, FL, with permission.]

FIGURE 3 Polytene chromosome of D. melanogaster: X, X chromosome; 2R, right arm of second chromosome; 2L, left arm of second chromosome; 3R, right arm of third chromosome; 3L, left arm of third chromosome; 4, fourth chromosome; CH, chromocenter. [From Krimbas C., and Powell, J. R. (eds.) (1992). "Drosophila Inversion Polymorphism," Fig. 2B, p. 344. CRC Press, Boca Raton, FL, with permission.]

Balancers are multiply inverted chromosomes that repress recombination and are useful for making controlled genetic crosses as well as keeping homozygous lethal mutant genes in culture. In addition to being marked with a visible phenotype, such as curly wings, balancer chromosomes are often homozygous lethal, making crosses and the establishment of multiple mutant stocks much simpler.

Transposable elements (TEs) are native components of the genomes of nearly all organisms. TEs typically encode a protein, called transposase (some also move via a method that is mediated by reverse transcriptase), which can catalyze the movement of the element throughout the genome. Transposons have been used to mutagenize and clone genes, as well as to study spatial and temporal patterns of gene expression. The P transposable element was isolated after several researchers noticed an aberrant syndrome of hybrid sterility when certain geographic strains were crossed. This sterility was caused by the introduction of P elements into a genetic background lacking these transposons. This transposon has become the most versatile and widely used tool in modern Drosophila genetics.

Since their discovery, P elements have been heavily modified and are now used extensively to manipulate Drosophila germline DNA. The transposase coding regions have been removed and replaced with a wild-type marker gene, resulting in an inactive transposon with a dominant marker. It is possible to introduce such a P element into the germline of mutant embryos by injecting the embryos with cloned P element DNA and a buffer containing the active transposase. The offspring carrying the P element construct will have a wild-type phenotype because the marker will rescue the mutant phenotype of the recipient strain. Such transformed lines can be used in mutagenesis screens by crossing the inactive P element construct line to a stock engineered to contain active transposase. Because P elements insert at random into the genome, they are very effective mutagenic agents and can insert into a gene, thereby disrupting its function. Once a phenotype has been observed, the transposase can be "crossed out," leaving a stable P element insertion into a gene of interest. The mutagenized gene can be easily cloned by means of a variety of techniques (e.g., inverse PCR) because the sequence of the P element is known and a "transposon tag" is present in the gene of interest. Other powerful techniques that exploit transposons are enhancer trapping and the flipase recombination system.

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