Drosophila
by Frank Zhou

fly.jpg[1]

Table of Contents

1. Classification/Diagnostic Characteristics

2. Relationship to Humans

3. Habitat and Niche

4. Predator Avoidance

5. Nutrient Acquisition

6. Reproduction and Life Cycle

7. Growth and Development

8. Integument

9. Movement

10. Sensing the Environment

11. Gas Exchange

12. Waste Removal

13. Environmental Physiology

14. Internal Circulation

15. Chemical Control

16. Review Questions

17. References


Classification/Diagnostic Characteristics

Kingdom: Animalia
Phylum: Anthropoda
Class: Insecta
Order: Diptera
Family: Drosophilidae
Genus: Drosophila



drosophila 2.jpg[2] (JLev)

Otherwise known as a fruit fly, it is only about 3 mm long. The fruit fly known to accumulate around spoiled fruit, hence the name. It is a very valuable organism is genetics and is commonly used as a model organism for research. (LC) [3]

All members of the Drosophila genus have a segmented body (head, thorax, abdomen), six legs attached to the thorax, and two pairs of stiff, membranous wings also attached to the thorax. They also all have two antennae (see Touch in Sensing the Environment for details) and two compound eyes (see Vision in Sensing the Environment for details) on their heads.[4]

Males are smaller and have darker, more rounded abdomens. Males also have black tarsal sex combs on their first pair of legs which are very distinctive but can only be seen under high magnification.[5] (KG)

Relationship to Humans

Due to their fast reproductive cycle and their low upkeep requirements, fruit flies are frequently used for scientific research in genetic and developmental studies. Famous examples include studies on body segmentation, Hox genes, speciation, evolution under hyperoxic conditions, and genetic linkages.[6]


sadava_19_19_FULL.jpg

Hox genes in Drosophila[7] (NC)

Also, they have short life cycles (usually lasting around a duration of two weeks), so they can be studied in great depth over many generations at a time. Their mutations can also be identified, which can tell researchers much about genetic variation and accumulation of mutations. They can be artificially mutated very easily too, by exposing them to radiation or adding dangerous chemicals into their food. Students can benefit from studying these organisms because principles of genetics can be quickly seen and simply conveyed.[8] (E.S.S.)

Outside of the scientific world, fruit flies are often regarded as pests because they consume human food and are capable of spreading bacteria and other pathogens.

Along with their fast reproductive cycles, fruit flies are important to science because it is easy to differentiate between virgins and mature adults. Also, it is easy to differentiate between sexes and they can be easily manipulated with relatively simple laboratory equipment.[9] (SJ)

Habitat and Niche

Because the vast amounts of human waste often provide fruit flies with necessary nutrition, fruit flies are found around the world, often in high numbers near human civilization. In the natural world, fruit flies are responsible for the decomposition of fruits, vegetables, and other organic matter.[10]

Native habitats of Drosophila include those in the tropical regions of the Old Worlds, yet with the help of humans, the common fruit fly has been introduced to almost all temperate regions of the world. Only temperature and availability of water are factors that limit the habitats in which Drosophila can live because offspring development is extremely dependent on temperature, adults often cannot withstand colder temperatures of higher eleveations and latitudes, and food supplies are limited in these areas. Overall, Drosophila requires moist environments, as understood by the meaning of the name Drosophila: "lover of dew."[11] (JF)

The natural habitats of Drosophila itself are forest containing fruit bearing trees in the African tropics. (HSC)

Predator Avoidance

Fruit flies use their compound eyes (see Vision in Sensing the Environment for details) to see the world and detect motion. When movement is observed, the fruit fly uses its wings to quickly fly away.[12]

The drosophila is known to show a preference to seek for shelter when threatened by a predator, especially in recesses and small caves that can provide protection from the predators. Studies have shown that the fly may use redundant senses that are no longer used for other major functions to detect and avoid predators.[13] (BBV)

Drosophila do not have very many defenses against predation but sheer force of number could be looked at as a way to avoid predation. The organisms reproduce so rapidly that reproduction outweighs heavy predation.[14] (CC)

Nutrient Acquisition

Fruit flies acquire nutrients by first ingesting food through their mouths, where the food enters the foregut. The food then moves to the crop, a storage sac that holds the food. Next, the food passes to the gizzard, a muscular organ that physically grinds larger food particles into smaller ones. Finally, the food moves onto the midgut and the intestines, where food is digested and nutrients are absorbed.[15]

As their common name implies, fruit flies (drosophilia) most commonly feed on fruit and other sugary substances.
They are also attracted to the fermenting sugars present in spilled alcoholic beverages. Fruit flies may also feed on organic material present in unclean drains.[16] (KG)

Reproduction and Life Cycle

Like all other insects, a fruit fly's life consists of four stages: an egg stage, a larval stage, a pupal stage, and an adult stage (see Growth and Development for details).[17]

The adult fly reproduces by mating with a fruit fly of the opposite sex. When a male spots a female, he follows her and taps the female's body with his forelegs. If the female does not fly away, the male will vibrate a wing to produce a courtship song. If the female still hasn't flown away, the male will lick the female's genitals and then proceed to engage in sexual intercourse.

Male drosophila ready to reproduce, aggregate in areas of appropriate mating conditions and form leks. Leks are groups of males competing for females by displaying themselves and their attractive qualities. Once a male attracts a female they may begin copulation.[18] (BH)

Many species in the drosophilia genus reproduce by traumatic insemination. In this process the male creates a wound in the female's abdomen with his penis. Then the male injects his sperm through the wound into the female's abdominal cavity, where the sperm diffuses through many layer of cell, eventually reaching the ovaries. [19] (WSS)

The drosophila egg is about half a millimeter long. Fertilization takes about one day and it develops into a worm-like larva. The larva eats and grows continuously, molting one day, two days, and four days after hatching, the first, second, and third instars. After two days as a third instar larva, it molts one more time to form an immobile pupa. The body is then completely remodeled to give the adult wings. [20] (TM)


p2000a433g162001.jpeg[21] (ES)

Growth and Development

Following fertilization, a fruit fly egg undergoes a rapid series of incomplete mitoses that do not result in cytokinesis (the DNA is duplicated and repackaged into nuclei, but the cell doesn't split in two). As a result, until the 13th division cycle, the fruit fly embryo (earliest stage of development) is still a single cell, but it has up to thousands of nuclei.[22]

Maternal effect genes, segmentation genes, and Hox genes are responsible for the determination (deciding the fate of a cell or group of cells) of each body segment in the embryo.[23]

The maternal effect genes help determine the anterior-posterior (front-back) and dorsal-ventral (top-bottom) axes of the fruit fly embryo. While the egg is still inside the mother fly's body, the transcription, or copying of DNA to mRNA, of maternal effect genes in the mother's ovarian cells causes the production of mRNA. The mRNA is then is transferred to the anterior portion of the egg by cytoplasmic bridges (strand of cytoplasm linking the mother's ovarian cells to the embryo). The anterior-posterior axis of the embryo is determined by the mRNA from the bicoid and nanos maternal effect genes. Because bicoid mRNA is deposited by the mother's cells at the anterior end of the egg, it is translated (using mRNA code to synthesize proteins) by ribosomes to produce Bicoid protein at the anterior end of the egg. The Bicoid protein diffuses towards the posterior end of the egg, creating a concentration gradient (uneven distribution of a substance) that is most concentrated at the anterior region. While bicoid is being translated, the nanos mRNA is transported to the posterior end of the cell, where it is then translated to produce the Nanos protein. Since Nanos protein is produced at the posterior end of the egg, the resulting gradient of the Nanos protein is greatest at the posterior end of the egg. Furthermore, because the Bicoid protein promotes the transcription of the hunchback gene and the Nanos protein inhibits the transcription of the hunchback gene, the Bicoid and Nanos proteins work together to establish a concentration gradient of the Hunchback protein. This concentration gradient of Hunchback protein that is most concentrated on the anterior side of the embryo and least concentrated on the posterior side of the embryo is what determines the anterior-posterior axis.[24]

After the maternal effect genes have created the gradient of Hunchback protein, there are about 6000 nuclei in the embryo and segmentation genes (gap, pair rule, and segmentation genes) begin to act on the embryo. First, gap genes organize the continuous gradient of the Hunchback protein into several broad regions. Next, pair rule genes within each gap-gene region divide the broad regions established by the gap genes into smaller units of two segments. Finally, segment polarity genes determine the anterior and posterior end of each individual pair-rule segment and also establish the boundaries between individual segments.[25]

Finally, Hox genes are responsible for the differentiation (process by which a less specialized cell develops into a specialized cell) of each segment determined by the maternal effect and gap segmentation genes. Because each segment expresses a different selection of Hox genes and Hox genes code for transcription factors (proteins that affect the rate at which DNA is copied to mRNA) that determine the fate of each cell, Hox gene expression causes specific cells in the fruit fly embryo to develop into different body parts.[26]
Growth of a Fruit Fly (MC)
Growth of a Fruit Fly (MC)

After maternal effect genes, segmentation genes, and Hox genes have affected the embryo, the fruit fly embryo differentiates and emerges from the embryo as a larva. As a larva, the fruit fly goes through a series of instars (growth stages) and molts, during when the larva sheds its exoskeleton. Following a set amount of instars, the fruit fly does not molt, but pupates to become a pupa. During this stage of life, the body of the fly larva is broken down and reorganized. When the fly emerges from the pupa, the fly larva has undergone metamorphosis (abrupt change in the physiology of an organism) to become an adult fly.[27]


Drosophilia embryo development[28] (SM)

Integument

Fruit flies are covered by a rigid exoskeleton (external skeleton that supports and protects the organism) made out of chitin (polymer derived from glucose) that protects them from the environment.[29]

Early in adult development fruit flies the terga (the dorsal portion of an arthropod segment other than the head) becomes covered with bristles. The bristles function from pure sensory receptors to pure secretors.[30] (BS)

As larvae the fruit flies go through 3 stages of molting their outer part of their body. The second instar larva has a hard cuticle covering that it molts to become a third instar larva.[31] (MDS)


Movement

Fruit flies can move because their rigid exoskeleton gives them the structural integrity necessary for movement. Their locomotive muscles are attached to the inside of their exoskeleton and control either their jointed legs or their wings. By extending or contracting their locomotive muscles, the fruit flies either moves their legs to walk or their wings to fly.[32]

Sensing the Environment

Vision
Like other arthropods (invertebrates with exoskeletons), fruit flies have compound eyes. Each eye is made up of hundreds of ommatidia, or optical units. Each ommatidium is responsible for perceiving a certain area of the fly's environment. In each ommatadium, light from the environment first passes through a corneal lens and a crystalline cone before reaching the photoreceptor (specialized neurons) cells that detect light. The inside border of each photoreceptor cell is lined with microvilli (small projections that increase surface area) containing rhopodsin, a pigment that traps light. The stimulated photoreceptor cells then send the signals along their axons (long tail of the neuron) to the brain. The brain compiles the sensory information of the ommatadia to create an image consisting of "pixels" that each ommatadium senses.[33]

Touch
The Johnston's organ, found in the second segment, or pedicel, is a conglomeration of sensory cells. It detects motion through the accompanying flagellum and is a characteristic that separates Class Insecta from group Entognatha. The flagellum rotates in the funiculus, where air will deform the cuticle where the Johnston's organ remains.[34] (AC)


Gas Exchange

Fruit flies, like all insects, use tracheae, tubes made out of chitin, to exchange gas with the environment. Because fruit flies do not have a respiratory organ that pumps air through the body, gas exchange occurs solely through diffusion, or the natural intermingling of particles resulting from their kinetic energy. Air enters the body through small openings called spiracles on the thorax and abdomen and moves through the body in tracheae that split into smaller tubes called tracheoles. The abundant tracheoles are in close proximity with all cells in the body and end in air capillaries, where the gas exchange between the air and cells takes place.[35]

The spiracles are are located throughout the the body, with usually 2 pairs per body segment. The spiracles are controlled by small muscles that contract or relax to close or open the spiracle. The air then passes through the tracheal trunk, into smaller tubes called tracheal tubes, and then into the tracheoles. The tracheoles are cells that provide a moist surface that allows exchange of gasses between the fly and the environment. The gas diffuses out of the tracheoles into all other body cells. Waste gasses diffuse out at the same time.In order to maintain the flexibility of the tracheal tubes without causing creases and cutting off air flow, there are thin "wires" made of cuticle that wind throughout the walls of the tube.[36] (AA)

Waste Removal

Fruit flies have numerous Malpighian tubules, tubes that are closed on one end and open on the other, that help them filter waste from their blood and empty between the midgut and hindgut in the abdomen. Because fruit flies have an open circulatory system, cells must use active transport (uses energy) to move uric acid (nitrogen waste)), potassium ions (K+), and sodium ions (Na+) from the extracellular fluid into the Malpighian tubules, creating an concentration gradient of water that causes water to enter the tubule and wash the contents of the tubule toward the gut, where it joins with indigestible waste from the digestive tract. Once the waste has passed through the hindgut and rectum, cells then actively transport sodium and potassium ions back into the extracellular fluid. This creates another osmotic gradient that causes the water to diffuse back into the extracellular fluid, allowing the fruit fly to conserve water. The remaining waste of uric acid and other material is then excreted through the anus as a semisolid mixture.[37]


external image 1-s2.0-S1934590911000592-gr1.jpg[38] (JLau)

Environmental Physiology

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.[39]

Internal Circulation

Fruit flies have an open circulatory system, meaning there is no distinction between the blood and interstitial fluid of the fly.[40]

Unlike a vertebrate heart, which is often looped, the Drosophila uses a simple, linear-tubule structure to pump blood in the open circulatory system.[41] (SJ)

Chemical Control

Molting and Maturation
In the larval stage of a fruit fly's life, the larva eats until it is too large for its exoskeleton. At this point, endocrine cells (cells that produce and secrete hormones) in the fly's brain produces a peptide hormone (protein hormone) called prothoraciotropic hormone (PTTH), which is transported to two glands called corpus cardiaca at the base of the brain and released in to the body. When the PTTH reaches the prothoracic gland (endocrine gland located in the first part of the thorax), it stimulates the gland to release ecdysone, a hormone that causes the larva to molt.[42]

However, what dictates whether the larva molts into a larger larva or an adult fly is another hormone, called juvenile hormone (JH). Throughout the larva's life, the two glands called corpus allata (not the same as corpus cardiaca) that are attached to the brain constantly release JH. But, as the larva gets older, the amount of JH released decreases. If JH is present in high levels during a molt, the fly larva molts into a larger larva. However, after a certain number of molts, the level of JH gets too low, and the fly larva pupates.[43]

DrosophilaMaturation.jpg[44]

Review Questions
1. Name the hormones involved in the growth of fruit flies and explain their role in the maturation process. (AY)
2. Explain the life cycle of a fruit fly and the role of maternal effect genes in development. (Shwetha)
3. Since Drosophilia do not have great predator avoidance, what do they do to avoid heavy predation? (CC)
4. How are Drosophila's able to pump air throughout the body without a respiratory organ? Explain the role of tracheoles, air capillaries, and spiracles in Drosophila gas exchange. (JF)
5. Explain the importance/advantage of the exclusively male features such as the smaller, rounded abdomen and the black tarsel sex combs. (PS)
6. Explain how Drosophila remove unnecessary chemicals from their bodies. (DM)


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