Drosophila

Drosophila
Male Drosophila melanogaster
Scientific classification
Kingdom: Animalia Phylum: Arthropoda Class: Insecta Order: Diptera Family: Drosophilidae Genus: Drosophila
Species
Drosophila melanogaster Drosophila subobscura

Drosophila is a genus of fruit fly; however, members of Drosophila are more appropriately termed vinegar flies, wine flies, pomace flies, grape flies, and picked fruit-flies. One species in particular, Drosophila melanogaster, has been heavily used in research in genetics and is a common model organism in developmental biology.

Drosophila is part of the phylum Arthropoda, a phylum of segmented animals with paired, jointed appendages and a hard exoskeleton made of chitin. Open circulatory system with dorsal heart. Hemocoel occupies most of body cavity, and coelom is reduced.

Table of contents

1 Physique 2 Lifecycle and ecology 2.1 Habitat 2.2 Respiration 2.3 Reproduction 2.4 Predators 3 The Drosophila research project 3.5 The Drosophila genome 3.5.1 Genetic nomenclature 4 Vision in Drosophila 5 References 5.6 Further Readings 5.7 External links

Physique

Drosophila, also the vinegar flies, patterned in yellow and dark gray with red eyes, are probably familiar to everyone. They appear on overripe fruit in kitchens, they swarm in thousands about the residue produced by the pressing of grapes or apples for wine. They nibble on marmalade and other preserves, and wherever vinegar is standing open, they are there; thus the name.

Typically Drosophila are an orange-brown color and range from about 1/8 to 1/6 inches in length. Most species have red eyes. The adult are yellowish, with dark crossbands on abdomen; the feathered arista is characteristic of the family.

Lifecycle and ecology

Habitat

A genus of fruit flies, sometimes called wine flies or vinegar flies, found primarily in tropical regions. The fruit fly, is a cosmopolitan holometabolous insect, that is found in all warm countries, while in cooler regions, it is established by migrants during the summer or can over winter in warm places. These flies are found in all warm countries in abundance of overripe soft fruits like grapes, bananas and plums. Adult flies as well as larvae feed on the fruit juices and the yeast growing on rotting fruit. Some feed on other decaying organic matter or on plant exudations; a few are leaf miners, parasites, or predators.

Respiration

The insect respired by means of air-filled internal tubes, the tracheae. This ectoderm-derived organ forms a highly branched tubular network which provides the organs with oxygen.

Reproduction

A female lays up to 2000 pearly white eggs, each with a pair of “wings” or respiratory “horns” near the anterior end; the eggs of all known Drosophila have one or more of these horns, the tips of which extend above the surface of the moist media in which the eggs develop.

Predators

Some predators are the orchid mantis who feed on free living insects, primarily consisted of fruit flies.

The Drosophila research project

Drosophila is one of the most studied organisms in biological research, particularly genetics. Their short generation time (about 2 weeks), high reproductively rate (females can lay 500 eggs in 10 days), small size (1/8" to 1/15" long), and multitude of genes for study make them an ideal specimen for studying genetic mutations. Species of Drosophila have migrated with man over history and can now be found all over the world.

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. The genome contains about 165 million bases and approximately 14,000 genes. The genome has been sequenced and is currently being annotated.¹

Genetic nomenclature

Genes named after recessive alleles begin with a lowercase letter, while dominant alleles begin with a uppercase letter. Genes named after a protein product begin with an uppercase letter. Genes are typically written in italics. The convention for writing out genotypes is X/Y; 2nd/2nd; 3rd/3rd.²

Vision in Drosophila

The compound eye of the fruit fly contains 800 unit eyes or ommatidia. 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 1-1.5 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 has 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.³

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.

A diagram of a single microvillus in a Drosophila photoreceptor cell.

PLCβ hydrolyzes phosphoinositol-4,5-bisphosphate (PIP₂), a phospholipid found in the cell membrane, into soluble inositol triphosphate (IP₃) and diacylgycerol (DAG), which stays in the cell membrane. DAG or a derivitive of DAG causes a calcium selective ion channel known as TRP (transient receptor potential) to open and calcium and sodium flows into the cell. IP₃ is thought to bind to IP₃ 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.⁴

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⁺.⁵

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).

References

Further Readings

1. K. Haug-Collet, et al. (1999). Cloning and Characterization of a Potassium-dependent Sodium/Calcium Exchanger in Drosophila. J. Cell Biol. 147(3):659-669.
2. P. Raghu, et al. (2000). Normal Phototransduction in Drosophila Photoreceptors Lacking an InsP₃ Receptor Gene. Molec. & Cell. Neurosci. 15:4289-445.
3. R. Ranganathan, et al.(1995). Signal Transduction in Drosophila Photoreceptors. Annu. Rev. Neurosi. 18:283–317.

External links

1. A quick and simple introduction to Drosophila melanogaster
2. Flybase
3. FlyMove