Drosophila Melanogaster
gene expression control. Determination of sex in Drosophila occurs by the ratio of X chromosomes to autosomes, not because of the presence of a Y chromosome as in human sex determination. Although the Y chromosome is entirely heterochromatic, it contains at least 16 genes, many of which are thought to have male-related functions.
Similarity to humans
About 75% of known human disease genes have a recognizable match in the genetic code of fruit flies, and 50% of fly protein sequences have mammalian analogues. An online database called Homophila is available to search for human disease gene homologues in flies and vice versa. Drosophila is being used as a genetic model for several human diseases including the neurodegenerative disorders Parkinson’s, Huntington’s, spinocerebellar ataxia and Alzheimer’s disease. The fly is also being used to study mechanisms underlying aging and oxidative stress, immunity, diabetes, and cancer, as well as drug abuse.
Development
Main article: Drosophila embryogenesis
Embryogenesis in Drosophila has been extensively studied, as its small size, short generation time, and large brood size makes it ideal for genetic studies. It is also unique among model organisms in that cleavage occurs in a syncytium.
Drosophila melanogaster oogenesis
During oogenesis, cytoplasmic bridges called “ring canals” connect the forming oocyte to nurse cells. Nutrients and developmental control molecules move from the nurse cells into the oocyte. In the figure to the left, the forming oocyte can be seen to be covered by follicular support cells.
After fertilization of the oocyte the early embryo (or syncytial embryo) undergoes rapid DNA replication and 13 nuclear divisions until approximately 5000 to 6000 nuclei accumulate in the unseparated cytoplasm of the embryo. By the end of the 8th division most nuclei have migrated to the surface, surrounding the yolk sac (leaving behind only a few nuclei, which will become the yolk nuclei). After the 10th division the pole cells form at the posterior end of the embryo, segregating the germ line from the syncytium. Finally, after the 13th division cell membranes slowly invaginate, dividing the syncytium into individual somatic cells. Once this process is completed gastrulation starts.
Nuclear division in the early Drosophila embryo happens so quickly there are no proper checkpoints so mistakes may be made in division of the DNA. To get around this problem the nuclei which have made a mistake detach from their centrosomes and fall into the centre of the embryo (yolk sac) which will not form part of the fly.
The gene network (transcriptional and protein interactions) governing the early development of the fruit fly embryo is one of the best understood gene networks to date, especially the patterning along the antero-posterior (AP) and dorso-ventral (DV) axes (See under morphogenesis).
The embryo undergoes well-characterized morphogenetic movements during gastrulation and early development, including germ-band extension, formation of several furrows, ventral invagination of the mesoderm, posterior and anterior invagination of endoderm (gut), as well as extensive body segmentation until finally hatching from the surrounding cuticle into a 1st-instar larva.
During larval development, tissues known as imaginal discs grow inside the larva. Imaginal discs develop to form most structures of the adult body, such as the head, legs, wings, thorax and genitalia. Cells of the imaginal disks are set aside during embryogenesis and continue to grow and divide during the larval stages – unlike most other cells of the larva which have differentiated to perform specialized functions and grow without further cell division. At metamorphosis, the larva forms a pupa, inside which the larval tissues are reabsorbed and the imaginal tissues undergo extensive morphogenetic movements to form adult structures.
Behavioral genetics and neuroscience
In 1971, Ron Konopka and Seymour Benzer published “Clock mutants of Drosophila melanogaster”, a paper describing the first mutations that affected an animal’s behavior. Wild-type flies show an activity rhythm with a frequency of about a day (24 hours). They found mutants with faster and slower rhythms as well as broken rhythms – flies that move and rest in random spurts. Work over the following 30 years has shown that these mutations (and others like them) affect a group of genes and their products that comprise a biochemical or biological clock. This clock is found in a wide range of fly cells, but the clock-bearing cells that control activity are several dozen neurons in the fly’s central brain.
Since then, Benzer and others have used behavioral screens to isolate genes involved in vision, olfaction, audition, learning/memory, courtship, pain and other processes, such as