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Drosophila melanogaster

2007 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and
Fish

           iDrosophila melanogaster
   Male Drosophila melanogaster
   Male Drosophila melanogaster
           Scientific classification

   Kingdom:          Animalia
   Phylum:           Arthropoda
   Class:            Insecta
   Order:            Diptera
   Family:           Drosophilidae
   Subfamily:        Drosophilinae
   Genus:            Drosophila
   Subgenus:         Sophophora
   Species group:    melanogaster group
   Species subgroup: melanogaster subgroup
   Species complex:  melanogaster complex
   Species:          D. melanogaster

                                Binomial name

   Drosophila melanogaster
   Meigen, 1830

   Drosophila melanogaster (from the Greek for black-bellied dew-lover) is
   a two-winged insect that belongs to the Diptera, the order of the
   flies. The species is commonly known as the fruit fly, and is one of
   the most commonly used model organisms in biology, including studies in
   genetics, physiology and life-history evolution. Flies belonging to the
   Tephritidae are also called fruit flies, which can lead to confusion.

Physical appearance

   Male (left) and female D. melanogaster
   Enlarge
   Male (left) and female D. melanogaster

   Wildtype flies have red eyes, are yellow-brown in colour, and have
   transverse black rings across their abdomen. They exhibit sexual
   dimorphism: females are about 2.5 millimetres long; males are slightly
   smaller and the back of their bodies is darker. Males are easily
   distinguished from females based on colour differences (males have a
   distinct black patch at the abdomen, less noticeable in recently
   emerged flies (see fig)) and the sexcombs (a row of dark bristles on
   the tarsus of the first leg). Furthermore, males have a cluster of
   spiky hairs (claspers) surrounding the anus and genitals used to attach
   to the female during mating. There are extensive images at Fly Base.

Life cycle

   Egg of D. melanogaster
   Egg of D. melanogaster

   The developmental period for Drosophila melanogaster varies with
   temperature, as with all cold-blooded species. The shortest development
   time (egg to adult), 7 days, is achieved at 28  °C. Development times
   increase at higher temperatures (30 °C, 11 days) due to heat stress.
   Under ideal conditions, the development time at 25 °C is 8.5 , at 18 °C
   it takes 19 days and at 12 °C it takes over 50 days. Under crowded
   conditions, development time increases , while the emerging flies are
   smaller. Females lay some 400 eggs (embryos), about five at a time,
   into rotting fruit or other suitable material such as decaying
   mushrooms and sap fluxes. The eggs, which are about 0.5 millimetres
   long, hatch after 12-15 h (at 25 °C). The resulting larvae grow for
   about 4 days (at 25 °C) while molting twice (into 2nd- and 3rd-instar
   larvae), at about 24 and 48 h after eclosion. During this time, they
   feed on the microorganisms that decompose the fruit, as well as on the
   sugar of the fruit themselves. Then the larvae encapsulate in the
   puparium and undergo a four-day-long metamorphosis (at 25 °C), after
   which the adults eclose (emerge).
   dorsal view
   Enlarge
   dorsal view

   Females become receptive to courting males at about 8-12 hours after
   emergence. Males perform a sequence of five behavioural patterns to
   court females. First, males orient themselves while playing a courtship
   song by horizontally extending and vibrating their wings. Soon after,
   the male positions itself at the rear of the female's adbdomen in a low
   posture to tap and lick the female genitalia. Finally, the male curls
   its abdomen, and attempts copulation. Females can reject males by
   moving away from males and extruding their ovipositor. The average
   duration of copulation, when successful, lasts 10 minutes, during which
   males transfer hundreds of very long sperm in seminal fluid to the
   female. Females store the sperm, which may need to compete with other
   males' stored sperm to fertilize eggs.

   The D. melanogaster lifespan is about 30 days at 29 °C.

Model organism in genetics

   Drosophila melanogaster is the most studied organism in biological
   research, particularly in genetics and developmental biology. There are
   several reasons:
     * It is small and easy to grow in the laboratory
     * It has a short generation time (about 2 weeks) and high
       productivity (females can lay 500 eggs in 10 days)
     * The mature larvae show giant chromosomes in the salivary glands
       called polytene chromosomes - "puffs" indicate regions of
       transcription and hence gene activity.
     * It has only 4 pairs of chromosomes: 3 autosomal, and 1 sex.
     * Males do not show meiotic recombination, facilitating genetic
       studies.
     * Genetic transformation techniques have been available since 1987.
     * Its compact genome was sequenced in 1998.

   Charles W. Woodworth is credited with being the first to breed
   Drosophila in quantity and for suggesting to W. E. Castle that they
   might be used for genetic research during his time at Harvard
   University. Beginning in 1910, fruit flies helped Thomas Hunt Morgan
   accomplish his studies on heredity. "Thomas Hunt Morgan and colleagues
   extended Mendel's work by describing X-linked inheritance and by
   showing that genes located on the same chromosome do not show
   independent assortment. Studies of X-linked traits helped confirm that
   genes are found on chromosomes, while studies of linked traits led to
   the first maps showing the locations of genetic loci on chromosomes"
   (Freman 214). The first maps of Drosophila chromosomes were completed
   by Alfred Sturtevant.
   lateral view
   Enlarge
   lateral view

The Drosophila genome

   The genome of Drosophila contains 4 pairs of chromosomes: an X/Y pair,
   and three autosomes labeled 2, 3, and 4. The fourth chromosome is so
   tiny that it is often ignored, aside from its important eyeless gene.
   The genome contains about 132 million bases and approximately 13,767
   genes. The genome has been sequenced and has been annotated.
   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.
   anterior view
   Enlarge
   anterior view

   Similarity to humans — Genetically, humans are about 44% similar to
   flies. About 61% 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. Drosophila is being used as a genetic model
   for several human diseases including the neurodegenerative disorders
   Parkinson's, Huntington's, and Alzheimer's disease. The fly is also
   being used to study mechanisms underlying immunity, Diabetes, and
   cancer, as well as drug abuse.

Development and embryogenesis

   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
   Enlarge
   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 nucelar 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 fruitfly 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 egg 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 the larval development
   (molting), the imaginal disks form, which are in essence the anlagen
   for the entire adult body. Cells of the imaginal disks are set aside
   early and mature over time into adult body structures, especially
   during pupation, whereas most other cells in the larva undergo
   apoptosis.

Behavioural genetics and neuroscience

   In 1971 Ron Konopka and Seymour Benzer published a paper titled "Clock
   mutants of Drosophila melanogaster" in which they described the first
   mutations that affected an animal's behaviour. Wild-type flies show an
   activity rhythm of 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, his students, and many others have used behavioural
   screens to isolate genes involved in vision, olfaction, audition,
   learning/memory, courtship, pain and other processes such as longevity.

   The first learning and memory mutants (dunce, rutabaga etc) were
   isolated by William "Chip" Quinn while in Benzer's lab, and were
   eventually shown to encode components of an intracellular signalling
   pathway involving cylic AMP, protein kinase A and a transcription
   factor known as CREB. These molecules were shown to be also involved in
   synaptic plasticity in Aplysia and mammals.

   Male flies sing to the females during courtship using their wing to
   generate sound.

   Furthermore, Drosophila has been used in neuropharmacological research,
   including studies of cocaine and alcohol in the labs of Jay Hirsh and
   Ulrike Heberlein.

Vision in Drosophila

   Stereo pair of images as viewed by fly eye
   Stereo pair of images as viewed by fly eye

   The compound eye of the fruit fly contains 800 unit eyes or ommatidia,
   and are one of the most advanced among insects. Each ommatidium
   contains 8 photoreceptor cells (R1-8), support cells, pigment cells,
   and a cornea. Wild-type flies have reddish pigment cells, which serve
   to absorb excess blue light so the fly isn't blinded by ambient light.

   Each photoreceptor cell consists of two main sections, the cell body
   and the rhabdomere. The cell body contains the nucleus while the
   rhabdomere is made up of toothbrush-like stacks of membrane called
   microvilli. Each microvillus is 1 mm to 1.5 mm in length and 50 nm in
   diameter. The membrane of the rhabdomere is packed with about 100
   million rhodopsin molecules, the visual protein that absorbs light. The
   rest of the visual proteins are also tightly packed into the
   microvillar space, leaving little room for cytoplasm.

   The photoreceptors in Drosophila express a variety of rhodopsin
   isoforms. The R1-R6 photoreceptor cells express Rhodopsin1 (Rh1) which
   absorbs blue light (480 nm). The R7 and R8 cells express a combination
   of either Rh3 or Rh4 which absorb UV light (345 nm and 375 nm), and Rh5
   or Rh6 which absorb blue (437 nm) and green (508 nm) light
   respectively. Each rhodopsin molecule consists of an opsin protein
   covalently linked to a carotenoid chromophore, 11-cis-3-hydroxyretinal.
   Expression of Rhodopsin1 (Rh1) in photoreceptors R1-R6
   Enlarge
   Expression of Rhodopsin1 (Rh1) in photoreceptors R1-R6

   As in vertebrate vision, visual transduction in invertebrates occurs
   via a G protein-coupled pathway. However, in vertebrates the G protein
   is transducin, while the G protein in invertebrates is Gq (dgq in
   Drosophila). When rhodopsin (Rh) absorbs a photon of light its
   chromophore, 11-cis-3-hydroxyretinal, is isomerized to
   all-trans-3-hydroxyretinal. Rh undergoes a conformational change into
   its active form, metarhodopsin. Metarhodopsin activates Gq, which in
   turn activates a phospholipase Cβ (PLCβ) known as NorpA.

   PLCβ hydrolyzes phosphatidylinositol (4,5)-bisphosphate (PIP[2]), a
   phospholipid found in the cell membrane, into soluble inositol
   triphosphate (IP[3]) and diacylgycerol (DAG), which stays in the cell
   membrane. DAG or a derivative of DAG causes a calcium selective ion
   channel known as TRP (transient receptor potential) to open and calcium
   and sodium flows into the cell. IP[3] is thought to bind to IP[3]
   receptors in the subrhabdomeric cisternae, an extension of the
   endoplasmic reticulum, and cause release of calcium, but this process
   doesn't seem to be essential for normal vision.

   Calcium binds to proteins such as calmodulin (CaM) and an eye-specific
   protein kinase C (PKC) known as InaC. These proteins interact with
   other proteins and have been shown to be necessary for shut off of the
   light response. In addition, proteins called arrestins bind
   metarhodopsin and prevent it from activating more Gq.

   A potassium-dependent sodium/calcium exchanger known as NCKX30C pumps
   the calcium out of the cell. It uses the inward sodium gradient and the
   outward potassium gradient to extrude calcium at a stoichiometry of 4
   Na^+/ 1 Ca^++, 1 K^+.

   TRP, InaC, and PLC form a signaling complex by binding a scaffolding
   protein called InaD. InaD contains five binding domains called PDZ
   domains which specifically bind the C termini of target proteins.
   Disruption of the complex by mutations in either the PDZ domains or the
   target proteins reduces the efficiency of signaling. For example,
   disruption of the interaction between InaC, the protein kinase C, and
   InaD results in a delay in inactivation of the light response.

   Unlike vertebrate metarhodopsin, invertebrate metarhodopsin can be
   converted back into rhodopsin by absorbing a photon of orange light
   (580 nm).

   Approximately two-thirds of the Drosophila brain (about 200,000 neurons
   total) is dedicated to visual processing. Although the spatial
   resolution of their vision is significantly worse than that of humans,
   their temporal resolution is approximately ten times better.

Drosophila flight

   The wings of a fly are capable of beating at up to 220 times per
   second. Flies fly via straight sequences of movement interspersed by
   rapid turns called saccades. During these turns, a fly is able to
   rotate 90 degrees in fewer than 50 milliseconds.

   Drosophila, and probably many other flies, have optic nerves which lead
   directly to the wing muscles (while in other insects they always lead
   to the brain first), making it possible for them to react even more
   quickly.

   It was long thought that the characteristics of Drosophila flight were
   dominated by the viscosity of the air, rather than the inertia of the
   fly body. However, recent research by Michael Dickinson and Rosalyn
   Sayaman has indicated that flies perform banked turns, where the fly
   accelerates, slows down while turning, and accelerates again at the end
   of the turn. This indicates that inertia is the dominant force, as is
   the case with larger flying animals.

Courtship and mating rituals

   When two drosophila melanogaster of the opposite sex encounter one
   another, they often first exhibit "cleaning behavior". After this
   behaviour is engaged for some time, the male will proceed towards the
   rear of the female from either the left or right side. He will then
   begin a "dance" in which he describes a semi-circle around the female.
   During this dance the male may or may not vibrate his wings. If the
   female is not receptive, she will move away and courtship ends. If she
   is receptive however, the male will approach her rear and make contact
   with her using his proboscis. The female may then scissor her wings and
   allow the male to mount. When intercourse has finished, the female will
   dislodge the male with a violent kicking of her hind legs.
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