The common fruit fly Drosophila melanogaster is a human commensal typically seen hovering around garbage cans or the bananas in kitchen fruit bowls. These small flies have been model organisms for genetic studies since about 1910. The reason these flies have been so useful to biologists is because they are small and thus take up little lab space, they are easily cultured, they have a short generation time, and they are extremely fertile.
In 1910 Thomas Hunt Morgan of Columbia University in New York City discovered a white-eyed mutant in Drosophila melanogaster which differed from the standard red-eyed fruit fly. Mutations such as this allow geneticists to narrow down the chromosomal location of the gene or genes responsible for a particular phenotype, such as eye color. Since then thousands of other mutations in Drosophila have been identified and mapped, including mutations that alter behavior and learning. At the genetic level, more is known about Drosophila than any other multicellular organism. Furthermore, much of our knowledge of Drosophila is relevant to humans. For example, genetic mutations causing tumors in flies have homologues in other animals. With the sequencing of the entire genome of Drosophila in 2000, Drosophila will continue to be an important tool in understanding how the genotype controls the phenotype of complex organisms.
One aspect of biology in which Drosophila proved to be extremely useful was in the study of development. The life cycle of Drosophila is made up of four stages: egg, larva, pupa, and adult. Eggs are typically laid in a food source such as a rotting fruit and develop into larvae after about one day. The larvae resemble small segmented worms, and rove about eating the food for several days until they pupate. At pupation the larvae encase themselves and remain stationary for four days as they metamorphose into adult flies. As are all arthropods, the adult flies are composed of three main sections— the head, thorax, and abdomen.
In 1995, three developmental biologists—Americans Edward B. Lewis and Eric F. Wieschaus, and German Christiane Nüsslein-Volhard—won the Nobel Prize for Physiology or Medicine for determining the genes that control the developmental processes in Drosophila. They showed that once maternally transcribed genes determine the anterior / posterior polarity of the egg, the zygote's own genes control the rest of development. First the zygote's gap genes divide the embryo into broad bands. This activates the pair-rule genes which further divide the embryo into seven bands, each representing two of the larval segments. The pair-rule genes then activate the segment polarity genes, which establish the anterior/posterior polarity of each segment. Finally, the segment polarity genes activate the homeotic selector genes, which establish segment identity in both the larval and adult stages.
Interestingly, complexes of the homeotic selector genes are arranged along the chromosome in the same general order that they are expressed along the length of the fly. For example, genes transcribed in the first segments of the fly are located in front of the genes transcribed in the last segments. Also, the first homeotic gene in the complex is active slightly earlier than the second gene and so on. The homeotic selector genes of Drosophila were later found to be arranged in the same order as the homologous homeotic selector genes in humans and other animals. This implies that the ordering of the homeotic selector gene complex has been conserved since the beginning of animal evolution.
Todd A. Schlenke
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Aperia, Anita, Hugo Lagercrantz, Ralf Pettersson, Nils Ringertz, and Björn Vennström. The Nobel Prize in Physiology or Medicine 1995. Sweden: The Nobel Committee for Physiology or Medicine at the Karolinska Institute, 1995. <http://www.nobel.se/medicine/laureates/1995/illpres/index.html,Stockholm>.
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dro·soph·i·la / drəˈsäfələ/ • n. a small fruit fly (genus Drosophila, family Drosophilidae), used extensively in genetic research because of its large chromosomes, numerous varieties, and rapid rate of reproduction.