Drosophila melanogaster, the fruit fly, is found around rotten and unripe fruits (Itskov, 2013). It is one of the model organism for research in a variety disciplines among fundamental genetics and the development of structures as tissues and organs. Almost 60 percent of Drosophila genome is homologous to humans. 75% ortholog human genes involving in diseases, are found in Drosophila. It also has a short generation time, small size, low maintenance costs, easy manipulation and sexually dimorphic features.
Life Cycle of Drosophila Melanogaster
- Day 0: Mating and eggs
- Day 1: Hatched eggs
- Day 2: First instar (one day length)
- Day 3: Second instar (one day length)
- Day 5: Third instar (two days in length)
- Day 7: Roaming of pupa/Pupariation
- Day 11-12: Eclosion from pupa case
The life cycle of Drosophila melanogaster includes metamorphosis with four distinguishable stages:eggs, larva, pupa and mature.
- Mating and Eggs: Mated females, which store sperm to make sure desired cross are occurring before laying their fertilized eggs. Oviposition of female flies are initiated after emerging from their pupal cases. And oviposition is increased through the week till 50-75 eggs are laid per day (≈ 400-500 eggs in 10 days). Ovoid eggs are protected by chorion, which is a strong and thin cover, but at the anterior end of it, there is a micropyle to pass down the spermatozoa. Other spermatozoa, are placed in the developing embryonic tissues.
- Larva: A segmented, white worm shaped, with black mount parts in the head part is called as larva. It develops spiracles for breathing, located in the anterior and posterior ends. Until larva become adult, the cuticle is periodically shed by along with the mouth. The periodic times are called as first instar, among hatching and first molt, and determined by the number of teeth on jaw hooks and size the larva has reached. Third instar is fed till pupation is occurred and crawls out from the culture to a dry place.
- Pupa: Prepupa will be formed after anterior spiracles everting, body of larva shortens and cuticle is getting hardened and pigmented. The stage is followed by formation of head, pads, wings, and legs, formation of preadult tissues known as imaginal discs. While using breakdown substances of larval tissues for pupal metabolism.
- Mature: Adult stage, which includes compound eyes, head, thorax and abdomen, wings and six jointed legs covered with bristles and hairs to have a typical insect anatomy. Virgin female flies cannot mate during several hours (around 8 hours at 25oC)* so they could be easily collected separately from male flies for genetic crosses to vials.
*The generation time of the Drosophila melanogaster depends on the temperature.
The above cycle description is based on 22oC (77oF) for 12-15 days.
Flies which are raised at low temperatures as 20oC, need more time to develop as 19 days. Besides, at high temperatures like 29oC, the generation time period takes around 7 days. The optimum condition for it is 25oC with taking about 9-10 days long.
Feeding in Drosophila melanogaster
Drosophila, intermittent feeder, usually eats up sugary fruits and plants. Also, the smell of yeast inside the plant materials attracts it. Although main carbohydrate sources for Drosophila is sucrose, glucose or fructose, different caloric and/or taste content of sugar also attract the Drosophila as arabinose, mannose. Since ethanol is produced by rotting fruit, low ethanol concentrations, 5-10%, attracts Drosophila. Optimize lifespan is provided by 1:2 protein-carbohydrate ratio . The enhancement in protein level increases the egg production.
Different behavioral states of feeding in Drosophila melanogaster. The food source is searched by odor attraction (a). Locomotion is inhibited when a food source is found (b). The proboscis extension response occurs by gustatory receptors (GR) in gustatory receptor neurons (GRN) with the palatability of food and nutrient content food(c). Food ingestion started with consumption (d). Internal state feedbacks to terminate the consumption (e). Food disengagement and reactivation of locomotion (F).
The feeding behavior in Drosophila melanogaster includes food source detection, physical contact with food, sense of nutrient content information, ingestion initiation, and discrete meal consumption based on internal state and environmental factors. When the information of food source is identified as caloric, taste, liquid or solid, the feeding is started with proboscis extension ressponse. Although liquid food directly is taken, the solid food needs to be predigested with extracted saliva, including digestive enzymes. The enhanced starvation state increases feeding time, but not the feeding speed (Qi et al., 2015). Around 260 taste sensilla is distributed primarily in the tarsi, wings and labellum of Drosophila.
Each of sensilla includes multiple gustatory receptor (GR) neurons (GRNs). These neurons provides detection of taste, of salts, water, bitter compounds, caffeine, fatty acids, amino acids, carbohydratesin taste pegs of the inner labellum and the internal taste organs lining inside pharynx, lateral sensory organ (LSO), ventral cibarial organ (VCSO), and dorsal cibarial sensory organ (DCSO). Gr64f is one of the gustatory receptors to respond variety of sugars as glucose, fructose, maltose, trehalose to promote consumption.
Several receptors could sense the same sugars just as trehalose sensing receptor Gr5a. Many clade of ionotropic receptors (IR), IR20a, was found in the labellum, legs, and pharynx. Although IR functions are still unknown, two of the IR20a receptors are responsible in male mating behavior. There is one sucrose sensing receptor, IR60b to limit sucrose consumption. IR25a and IR76b also provide amino acid sensation in larvae. Drosophila melanogaster percepts post-ingestive cues like gut stretch, nutrient value so these cues are feedback to stop ingestion and it disengage from food source via reactivated locomotion.
Fly taste sensing. Gustatory receptor neurons (GRN) of sensilla are stimulated with taste in labella, wings and legs. Several gustatory receptors are responsible for attractive and aversive taste sensing as in mammals.
Insulin Like Signaling in Drosophila melanogaster
Insulin which is peptide hormone, triggers the metabolic process such as glycogen and fatty acid synthesis and cellular glucose uptake to maintain blood/hemolymph glucose level. It also acts as mitogen about fibroblasts. Regulatory system to maintain energy homeostasis from circulating sugars is highly conserved between mammals and Drosophila. In mammals, there is one insulin secretion interacting with insulin receptor (InR) with high affinity while expressing two insulin-like growth factors (IGF); relaxin and insulin-like peptides (INSL 3-7) to interact with InR with low affinity.
The insulin gene codes for preproinsulin, insulin precursor composed of A-chain, C-peptides and B-chain. Cleavage of preproinsulin to polypeptide is occurred to transport into the endoplasmic reticulum (ER) to form proinsulin. The proper folding is applied in ER for transportation into trans-Golgi network where immature granules made upon by proinsulin. The maturation will occur when disulfide bond formation occurs after separation of C peptide of proinsulin from A and B-chains by cleavage. And, the insulin receptor has two spliced variants; InR-A and InR-B with receptor tyrosine kinase activity (RTKs).
In Drosophila, sugars in food are in taken and transport to fat body to convert into theralose. Theralose is stored as glycogen in fat body to release into hemolymph when energy homeostasis is imbalanced. The sugar level in Drosophila melanogaster is controlled antagonistically by Adipokinetic hormone (AKH), Drosophila insulin-like peptides (DILPs), and glucagon-like peptides. Together with DILPs, they are released into the hemolymph circulation. The InR proreceptor is proteolytically processed to generate binding site, alpha subunit (InR 120 kDa) and beta subunit (InR 170 kDa or 90 kDa) with protein kinase domain for DILP.
The InR 170 is composed of carboxyterminal side that produce alpha 120, beta 170, beta 90, and free 60 kDa C terminus after proteolytic cleavage to mature InR (alpha2(Beta170)2) and InR (alpha2(Beta90)2 (Fernandez et al., 1995). There are eight DILPs (1-8) in Drosophila. They are structurally similar to insulin, IGFs and relaxin in mammals due to their conserved Cys position and disulfide bonds. Except the DILP8, all of them bind to InR. DILP8 stimulates the relaxin-type membrane receptor, Lgr3. Each of DILP has its unique expression pattern tied to developmental stage.
Although DILP 1, 4 are restricted developmentally, DILP 6 is found in fat body while showing homology with IGF. DILP 2, 3, and 5 is found in adult to regulate energy homeostasis and they are procured by insulin producing cells (IPCs) in pars intracerebralis in the brain. DILP 2, 3, 5 projects into aorta, corpora cardiaca and crop for hemolymph circulation. DILP 6, unpaired 2 (Upd 2) , neuropeptide F (NPF), serotonin, GABA, octopamine, alllatostatin A, corazonin, limostatin and AKH modulate DILP 2, 3 and 5 expression. 20 cells in ventral nerve cord (VNC) generate DILP7 which projects into the hindgut and in female reproductive organ in adult fly. Larval imaginal discs and adult ovaries express DILP 8. DILP7-8 shows homology with human relaxin.
DILP and InR triggers receptor tyrosine kinase activity to activate phosphoinositide 3-Kinase (PI3K) to create phosphatidylinositol (3, 4, 5) triphosphate (PIP3) with ATP by phosphatidylinositol 4,5-biphosphate (PIP2). PDK is recruited to membrane by PIP3 to activate AKT. AKT repress FOXO which controls the life cycle of an organism as development and growth, aging and life-span, stress response, neural activity and behavior modulation, fecundity and ovarian development, carbohydrate and lipid metabolism.
Insulin signaling in Drosophila melanogaster. Fat body releases Upd2 and DILP6 in the head and abdomen. DILP 2, 3, and 5 is produced by 14 IPC cells in pars-intercerebralis. 20 cells in the VNC stimulate DILP7 production. Principal cells inside renal tubes generates DILP5 (A). DILPs activated by glucose/fat stimulates InR to activate PI3K with Chico sites of InR. PDK1 is activated by conversion of PIP2 to PIP3 to trigger AKT. Activated AKT results in repressed FOXO to prevent its intercellular transport to nucleus to control growth.