Drosophila melanogaster is a dipteran insect that is commonly known as the fruit fly and is one of the most commonly used model organisms in studies in genetics, physiology and evolution. The Greek term Drosophila means “dew lover” and melanogaster means “dark gut”.
Wild type fruit flies have brick red eyes, yellowish-brown colour and transverse black rings across the abdomen. Females are about 2.5 millimetres long and males slightly smaller and darker. Males are distinguished from females by the sex combs (dark bristles on the fore tarsus) and claspers on the tip of abdomen.
BIOLOGY
The developmental period for Drosophila varies from 7 days at 28 °C to11 days at higher temperatures. Females lay about 400 eggs into rotting fruit or other decaying vegetable material. The eggs hatch after 12–15 and larvae complete development in about 4 days by moulting twice. Larvae feed on micro organisms that decompose fruit and on the fruit itself. Then the larvae pupate and adults take 4 days to emerge from pupae. The total lifespan is about 30 days at 29 °C.
SPECIAL FEATURES
Drosophila melanogaster is one of the most studied organisms in biological research, particularly in genetics and developmental biology because of the following reasons:
- It is small in size, morphologically easy to identify and easy to culture in small containers in laboratory.
- One can anesthetize them easily and manipulate individuals with unsophisticated equipment.
- Drosophila are sexually dimorphic, making differentiation of sexes easy.
- It is easy to obtain virgin males and females, as virgins are physically distinctive from mature adults.
- The care and culture requires little equipment of low cost and uses little space even for large cultures.
- It has a short generation time of about two weeks and high fecundity, so that large numbers could be reared in short time.
- The mature larvae possess giant chromosomes in the salivary glands called polytene chromosomes which are used in cytological studies.
- It has only 4 pairs of chromosomes; three autosomes, and one sex chromosome.
- Males do not show meiotic recombination, facilitating genetic studies.
- Genetic transformation techniques are available since 1987.
- Its genome has been completely sequenced and was first published in 2000.
- Its embryo grows outside the body and can easily be studied at every stage of development.
- The blastoderm of embryo is a syncytium, in which nuclei are not separated by cell membranes, so that DNA injected into the embryo can have easy access to all the nuclei.
- The genome is relatively small for an animal, less than a tenth that of humans and mice).
- Mutations can be targeted to specific genes.
THE DROSOPHILA GENOME
Drosophila has 4 pairs of chromosomes, the X/Y sex chromosomes and the autosomes 2, 3, and 4. The size of the genome is about 165 million bases that contain about 14,000 genes (human genome has 3,400 million bases and about 22,500 genes).
CONTRIBUTIONS TO SCIENCE
Charles W. Woodworth is credited with being the first biologist to breed Drosophila and suggesting its use in genetic research. Thomas Hunt Morgan began his studies in 1900 on heredity using Drosophila, and described sex-linked inheritance, which helped to confirm that genes were found on chromosomes and linked traits led to the construction of genetic maps that showed the locations of genetic loci on chromosomes. He reported his first finding of a white (eyed) mutant in 1910 to the academic community. His work on Drosophila earned him the 1933 Nobel Prize in Medicine for identifying chromosomes as vectors of inheritance for genes. The first genetic maps of Drosophila chromosomes were completed by Alfred Sturtevant.
About 75% of known human disease genes have a recognizable match in the genetic code of fruit flies (Reiter et al. (2001) and 50% of fly protein sequences have mammalian analogues.
Drosophila is being used as a genetic model for several human diseases including the neurodegenerative disorders such as Parkinson’s, Huntington’s, spinocerebellar ataxia and Alzheimer’s diseases.
The fly is also being used to study mechanisms underlying aging and oxidative stress, immunity, diabetes, and cancer, as well as drug abuse.
The use of Drosophila is a powerful tool in teaching the Life Sciences. In fact it allows the observation of sexual dimorphism, of mutants and of the life cycle. Moreover it permits the realization of crosses aimed to demonstrate the sex linked characters and crosses aimed to establish if a mutation is conferred by a dominant or a recessive gene.