DROSOPHILA (FRUIT FLY)






(SM) [6]

Classification / Diagnostic Characteristics

Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Family: Drosophilidae
Genus: Drosophila (SF)(20)

Drosophila melanogaster, otherwise known as the common fruit fly, is an insect arthropod that is often used as a model organism in scientific studies because it is easy to grow and study in a laboratory setting, has relatively simple and well documented genetics, and exhibits characteristics that represent a larger group of organisms, multicellular eukaryotes. Like all insects, drosophila is a hexapod, meaning it has six appendages and distinct body segments: a head, three thoracic segments, and eight abdominal segments. Insects are a unique clade within the hexapod category distinguished by an ancestral change to the structure of the homeobox gene Ubx. As an arthropod, drosophila also has an exoskeleton to which smooth internal muscles are attached and jointed appendages for sensing, transportation, copulating, and capturing and manipulating food. Drosophila also belongs to the pterygote insect group because it has two pairs of wings, and was actually the first species to achieve flight.


Relationship to Humans

Drosophila’s relationship with humans is mainly that of a pest. Flies are a commonly known annoyance throughout the world to both humans and livestock alike. Besides being a nuisance, drosophila may also carry disease and waste that can be transmitted through food with which it comes into contact with.

Drosophila has been a very important tool in laboratory work to study genetics and development because of its short life and reproductive cycles as well as its low maintenance. The study of Hox genes that control growth and development was first discovered and researched in the segments of fruit flies, and this information has helped scientists to better understand the underlying biological mechanisms that all organisms have in common ever since. With a knowledge of these vital genes that, when turned on or off, can determine the body structure of every organism, scientists have been able to use this information to perform further studies of human development. (3) In addition, fruit fly models provide a pathway for scientists to study components of heart and muscle disease in an effort to understand how these diseases are caused in humans and to find treatments for humans because many of the symptoms in humans can be replicated in fruit flies. (4) (CM)

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Explanation of the Hox genes. Each gene controls the development and location of a specific part of the body of the fruit fly, and these genes have been discovered in many other organisms as well. (5) (CM)



Habitat and Niche

Like all insects, drosophila’s metabolism is very conservative of water and key ions, allowing it to live in varied climates, including very dry ones, and its fast life cycle and profuse breeding allow it to live all over the world.

Drosophila can be found on every continent besides Antarctica. Its original habitat was the tropical regions of Europe, Africa, and Asia, but it can also be found in nearly all the temperate regions of the world. Drosophila needs a moist environment, and its scientific name actually means "lover of dew." The habitat of Drosophila is also limited by its need for warm temperatures. Its offspring need warm temperatures to develop properly, and the adults die in the cold. Drosophila living in temperate climates try to find shelter in the winter, often in man-made structures with an abundant food supply, such as grain elevators. It eats rotting fruit, and the females even lay their eggs on rotting fruit. As the larva grow, they eat the fungus and yeast that grow on the rotting fruit. (JM 13, 14)

Drosophila often seems to come out of nowhere. This was so commonly observed, that a few hundred years ago, people thought they were created from spontaneous generation. Although that theory was disproved long ago, how they appear in ones kitchen is still a mystery to many. The truth is, overripe and rotting fruits ferment, producing alcohol, which attract Drosophila. Drosophila can enter houses by flying through the smallest openings around windows and doors, even through insect screens. They eat and lay hundreds of eggs on the fruit. Since the temperature inside a house provides a stable temperature for the eggs to develop, and the Drosophila to live, once they get inside the house, they never seem to go away (17). (SP)

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http://ncsmallfruitsipm.blogspot.com/2009_09_01_archive.html

The drosphila eating, or possibly laying eggs on the fruit pictured. Since this fruit is in an outside environment,and the temperature is therefore, not stable, the drosphila is probably not staying there for a while, nor planning on keeping eggs there. (AG)

Predator Avoidance

Flight is so key to drosophila that “fly” is in its very name. Drosophila’s three thoracic segments bear, in order, no wings, two large forewings, and two vestigial hind wings called halteres. Flight is not only important for transportation, but also predator avoidance, allowing the fly to outmaneuver and escape its predators. Furthermore, drosophila has 800 small lenses in each of its compound eyes called ommatidia, which focus light onto photoreceptor cells rich in rhodopsin and containing microvilli to trap light, at which point axons transduce the stimuli and send the information to the nervous system. This compound-lens eye, a hallmark trait of arthropods, allows the fly to see a huge swath of angles, including the area behind itself, allowing it to sense predators long before they can strike. Unlike k-oriented species, fruit flies do not care for or raise their young, so predator avoidance is a programmed genetic reflex, not a learned behavior.

Nutrient Acquisition

An adult Drosophila receives nutrients from rotting plants and fruit (9). It intakes nutrients through its mouth, which has small appendages next to it that manipulate food, where it is digested internally. The organs that make up its digestive system are the foregut, midgut, hindgut, and malpighian tubes. The foregut contains the pharynx, esophagus, crop or food-storing organ, salivary glands, and the sucking pump, or cibarium, used in nutrient uptake. From the crop the food travels into the midgut, where the proventriculus organ grinds and pulverize food particles (10). The hindgut contains the rectal ampulla, a variety of rectal sacs that store material until excretion. The malpighian tubes are then used in waste removal. (11)

Reproduction and Life Cycle

The entire life cycle of drosophila, from fertilized egg to adult, occurs within just two weeks. Larval segment number, boundaries, and polarity are controlled by segmentation genes, which are expressed within three hours of fertilization, and within twenty-four hours a larva with distinct segment emerges. Drosophila then transitions from a larva to a pupa, and subsequently from a pupa to an adult.

Adults emerging from rotting fruits are ready to mate within eight hours. A male drosophila chases a female and taps her bodies with his forelegs. If the female does not escape, the male extends one wing and vibrates it to generate a courtship song, and proceeds to copulate. Researchers have observed that a mutation on the per gene alters the frequency of wing vibrations and interrupts courtship, and that a mutation on the fru gene interferes with the ability of males to discriminate between sexes, also inhibiting reproduction.


The embryonic development of a fruitfly (15) (LK)


Growth and Development

Drosophila develops from a fertilized egg to a larva to a pupa to an adult within its life cycle. When a drosophila egg is fertilized, the Bicoid and nanos mRNA diffuses the from mother’s cells into the anterior end of the egg, and help determine the anterior-posterior axis. Sex is determined by the Sxl gene, which has four exons. In females, splicing results in two active forms of the Sxl gene, but the male protein contains all four exons and is inactive. Cytokinesis does not occur within the first thirteen mitotic cycles of the fertilized egg, resulting in the formation of a multinuclear embryo and allowing for the easy diffusion of morphogens, which end up affecting transcription factors in the nuclei.

When the egg enters multinucleate stage, the Bicoid protein acts to stimulate the transcription of the hunchback gene in the embryo, leading to the establishment of the head / anterior region. Nanos, which is deposited in the posterior end of the embryo, inhibits the hunchback protein and cofunctions with Bicoid to distinguish the anterior and posterior regions.

During development, segmentation genes, Hox genes, and myriad other less famously studied genes all direct the growth of different body parts, organs, and systems in drosophila. Gap genes organize broad areas along the anterior-posterior axis, pair rule genes divide the embryo into units of two segments each, and segment polarity genes determine the boundaries and anterior-posterior organization of individual segments. The rest of the drosophila’s growth and development are described in the previous section.


Integument

Drosophila’s covering is an exoskeleton, a commonality to all arthropods. This covering protects soft interior tissue, but does not provide the structure of an internal skeleton.

The exoskeleton helps stabilize cells and tissues while helping maintain body morphology. In Drosophila, the exoskeleton is known as the cuticle, which is produced by epidermal cells. The cuticle is composed of lipids and chitin, which is a fibrous material that consists of polysaccharides. As Drosophila grow, the cuticle forms into a multilayered structure. The correct cuticle structure is essential for the efficient activity of enzymes beneath the cuticle. Drosophila with mutated chitin synthase genes have mutated sclerotization, which is the process of hardening the cuticle, and pigmentation, which is the process of coloration. (12) (RS)


Movement

As discussed in the predator avoidance section, drosophila’s primary mode of transportation is flight. Drosophila’s flight muscles must reach 35-40 degrees celsius before it can fly, and this temperature must be maintained throughout flight. Muscular contractions analogous to shivering in mammals are performed by drosophila to maintain this temperature. Drosophila’s flight is achieved through the very rapid beating of its wings, for which the Vg (wild-type wing) allele is dominant to the vg (vestigial, or very small wing) allele. In addition to wings, drosophila also has jointed legs that it can use to walk around a surface on which it has landed.


Sensing the environment

Compound eyes provide drosophila with superb vision from all angles, but they are not its only sense organs. Antennae, which are located on drosophila’s head, are also central in environmental sensation, allowing it to sense both vibrations, temperature, and chemical signals. Drosophila also has a motion-sensitive receptor on its antennae called the Johnston’s organ, which is typical of all insects.


Gas exchange

Atmospheric gases are uptaken by drosophila through openings on its thorax known as spiracles. Air then travels through a system of internal tubes called tracheae, allowing oxygen to diffuse to all the cells within it. Rhythmic contractions of the fly’s body also facilitate ventilation in the circulatory system.

Fruit flies are so small because they rely on gas exchange to occur through their skin, and therefore must maintain a high surface area to volume ratio. In studies on the effects of hyperoxia (increased oxygen levels) on evolution, results showed larger body sizes being selected for in drosophila within just a few generations. This data also illustrated that stabilizing selection for body size in current atmospheric conditions can be changed to directional selection when O2 levels are altered.


Waste Removal

Drosophila’s excretory system consists of Malpighian tubules, which actively transport uric acid, potassium ions, and sodium ions from the extracellular fluid into themselves.This high concentration of solutes causes an osmotic flow of water, flushing the tubule contents toward the gut. The final waste product created is a semisolid mixture of uric acid and other wastes, and is very conservative of water.


Environmental physiology

Drosophila sense the environment through their antennae and their skin and their central nervous system and brain respond accordingly to regulate temperature, and internal sensors measure nutrient levels in bodily fluids and react accordingly.

Heat for Flight

Because fruit flies are exotherms, they do not regulate their core body temperature. However, the flight muscles of fruit flies, like many other insects, must reach 35°C to 40°C before flight. To overcome this obstacle, fruit flies contract both the muscles used to move the wings up and the muscles used to move the wings down, generating the heat necessary for flight. (SM) [16]

Drosophila must take in a lot of water in order to survive, they have a high surface area to volume ratio and are susceptible to dehydration. They release as little water as possible in order to maintain a stable internal environment.
(SL 21)

Drosophila have a segregated population of olfactory sensory neurons that are highly selective for acidity. These olfactory sensory neurons contain IR64a, a member of the ionotropic receptor (IR) family of putative olfactory receptors. IR64a+ neurons project to the antennal lobe that are specifically activated by acids, thus projecting the organism's ability to sense acids in the environment. (18) (NU)


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Head of Drosophila w/ Antennal Lobe containing IR64a (19) (NU)

Internal Circulation

All arthropods, drosophila included, contain a hemocoel, or blood chamber, in which fluids from the open circulatory system wash over internal organs before returning to blood vessels. Insects do not have a single, central heart like mammals, but rather have contractile blood vessels that push blood along.

Drosophila have an open circulatory system with a hemocoel and dorsal blood vessels. The heat and aorta are dorsal. The drosophila's heart may have a few pacemakers, but each segment can maintain a pacemaker role. The drosophila heart isn't driven by a single pacemaker, so it isn't like the central hear of humans. Unlike in humans, drosophila have no red blood cells, but they do have hemoglobin. Oxygen is delivered throughout the Drosophilas body by the tracheal system. The dorsal blood vessel is open in the heart area by small openings or ostia. When in the relaxed stage, the ostia open and hemolymph enter. When the heart contracts, the valves of the ostia are closed by pressure and the hemolymph is moved forward into the head area of the Drosophila. Alary muscles are a special set of muscles that aid the heart in contraction. Blood circulation is aided by the presence of the dorsal and ventral diaphragms. The drosophila had to evolve a dorsal and ventral diaphragm to aid with hemolymph movement. The dorsal and ventral diaphragm consists of sets of muscles that divide the hemocoel into various sinuses. The contraction of the diaphragms aids in movement within these various sinuses. The hemolymphs are lubricants for tissues and are the transport medium for many molecules and this includes wastes. Hemolymphs help with the storage of amino acids and glycerol, protection, and no-cellular immune responses (1). (SM)

Drosophila open circulatory system (2) (SM)
Drosophila open circulatory system (2) (SM)


Chemical control

The endocrine system of insects consist of four main hormone-producing cells. The first type make up endocrine glands, which are structures that secrete hormones, releasing into the circulatory system of the insect. The second type of hormone-producing cell compose neurohemal organs, which are similar to glands but the release of hormones from these organs are triggered by signals from the nervous system or another hormone. Neurosecretory cells are specialized neurons that also secrete chemical messengers when stimulated, but they serve more as a link between the nervous system and the endocrine system. Lastly, the fourth category of hormone-producing cells make up the internal organs of insects; these cells make up bodily structures such as reproductive organs, the fat body, and the digestive system and their ability to control chemicals affect the associated functions of these structures. (SM) [7]

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(SM) [8]

Review Questions

1. Name the hormones involved in the growth of fruit flies and explain their role in the maturation process. (SM)
2. Explain the structure and function of the cuticle. (S.S)
3. What do Malpighian tubules transport? (SM)

4. What growth and developmental genes were first discovered and researched in fruit flies? What do these genes primarily determine? (AC)

5. What two major organ diseases are studied with the fruit fly model? (AC)

6. Drosophila rely on which organ for gas exchange? (AC)

7. The fruit fly doesn't just use it's special eyes for sensing the environment. What else does it use and how? (RG)


Sources
1. "Blood Circulatory System." Circulatory System. N.p., n.d. Web. 24 Nov. 2013.
2. Fruit Fly open circulatory system
3. http://evolution.berkeley.edu/evolibrary/article/evodevo_05
4. Service, Elizabeth. "The Fruit Fly and Genetics." The Fruit Fly and Genetics. University of North Carolina Chapel Hill, n.d. Web. 18 Nov. 2013.
5. "Hox Genes in Drosophila." Nature.com. Nature Publishing Group, n.d. Web. 18 Nov. 2013.
6. http://www.news.wisc.edu/newsphotos/gompel.html
7. http://www.cals.ncsu.edu/course/ent425/tutorial/endocrine.html
8. http://www.docstoc.com/docs/25694502/The-endocrine-system-of-insects
9. http://animaldiversity.ummz.umich.edu/accounts/Drosophila_melanogaster/
10. http://www.cals.ncsu.edu/course/ent425/tutorial/stomo.html
11. http://rice.bio.indiana.edu:7082/allied-data/lk/interactive-fly/aimain/1adult.htm
12. Moussian, B., H. Schwarz, S. Bartoszewski, and C. Nusslein-Volhard. "Involvement of Chitin in Exoskeleton Morphogenesis in Drosophila Melanogaster." J Morphol (2005): 117. NCBI. U.S. National Library of Medicine, Apr. 2005. Web. 18 Nov. 2013.
13. Miller, Conrad. "Drosophila Melanogaster." Http://animaldiversity.ummz.umich.edu. University of Michigan Museum of Zoology, n.d. Web. 18 Nov. 2013.
14. "Fruit Flies." Entoweb.okstate.edu. Oklahoma State University, n.d. Web. 18 Nov. 2013.
15. The Embryonic Development of a Fruit Fly
16. Hillis, David M., David Sadava, H. C. Heller, and Mary V. Price. Principles of Life High School Edition. Sudnerland, MA: Sinauer Associates, 2012. Print.
17. "Where Do Fruit Flies Come From?" MNN. Mother Nature Network, n.d. Web. 23 Nov. 2013.
18. Ai, Minrong, Soohong Min, Yael Grosjean, Charlotte LeBlanc, Rati Bell, Richard Benton, and Greg S.B Suh. "Acid Sensing by the Drosophila Olfactory System." //Nature: International Weekly Journal of Science//. Nature Publishing Group, 17 Nov. 2010. Web. 24 Nov. 2013.
19. http://www.nature.com/nrn/journal/v8/n5/images/nrn2098-f1.jpg
20."Drosophila Melanogaster — Classifications." //Encyclopedia of Life//. N.p., n.d. Web. 25 Nov. 2013.
21.
http://jeb.biologists.org/content/207/13/2313.full.pdf