Experiment 01Model Organism CytogeneticsDrosophila melanogaster
Culture, Handling, Etherization & Phenotypic Sex Differentiation of Drosophila melanogaster
Lab Duration: 4 Hours • Maintenance: Ongoing
🎯 Aim: To master the preparation of standard cornmeal-agar-molasses culture medium, aseptic stock transfer, safe anesthetization of adult flies using diethyl ether/CO2, and unambiguous microscopic differentiation of male and female Drosophila melanogaster using morphological markers (sex combs, abdominal pigmentation, body size, and genital apparatus).
🔬 Principle & Theoretical Background
Drosophila melanogaster (the common vinegar fly) was established as the premier multicellular eukaryotic model organism by Thomas Hunt Morgan at Columbia University in 1910. Its genetic utility stems from its rapid 10-12 day life cycle at $25^circtext{C}$, high fecundity (females lay 400-500 eggs), minimal chromosome complement ($2n = 8$: three autosomes and one sex chromosome pair), and the presence of giant polytene chromosomes in larval salivary glands.
Accurate genetic crosses depend strictly on isolating virgin females. Female flies store viable sperm in internal seminal receptacles (spermathecae and seminal receptacle) for over two weeks following a single mating. Because adult females do not become sexually receptive until 8 to 10 hours after eclosion (emergence from puparium) at $25^circtext{C}$, all adult flies must be cleared from the culture bottle early in the morning, and newly emerged virgin females collected within an 8-hour window.
Microscopic sex differentiation relies on four distinct morphological criteria:
- Sex Combs (♂ only): A comb-like longitudinal row of 10-12 thickened, black chitinous teeth located on the distal aspect of the metatarsus (first tarsal segment) of the forelegs in males. They are completely absent in females. Males utilize sex combs during courtship to grasp the female’s abdomen.
- Abdominal Pigmentation & Tergite Number: The male abdomen is shorter, cylindrical, and possesses only 5 visible dorsal abdominal tergites; the posterior 2 tergites (5th and 6th) are completely coalesced into a solid, heavily pigmented dark black cap. The female abdomen is larger, distended, oval, and displays 7 visible tergites, each marked by a distinct transverse dark band that remains separated without posterior coalescence.
- External Genitalia: The male genital apparatus includes heavily sclerotized, dark brown/black claspers and anal plates situated ventrally at the posterior abdominal tip. In contrast, the female posterior terminates in a light-colored, pointed ovipositor flanked by anal plates.
- Body Size: Females are substantially larger (~2.5-3.2 mm) than males (~2.0-2.5 mm), especially when gravid with mature eggs.
🧪 Reagents, Media & Laboratory Apparatus
Culture Medium Ingredients (per Liter)
- Yellow Cornmeal: $85,text{g}$
- Agar-agar powder (bacteriological grade): $10,text{g}$
- Dextrose / Sucrose: $50,text{g}$
- Blackstrap Molasses or Malt Extract: $60,text{mL}$
- Brewer’s Inactive Dried Yeast: $30,text{g}$
- Propionic acid / Tegosept (antifungal): $5,text{mL}$ of $10%$ methylparaben in $95%$ ethanol
- Distilled Water: q.s. to $1000,text{mL}$
Apparatus & Dissection Tools
- Stereomicroscope with incident (top) LED illumination
- Standard Drosophila culture bottles and vials ($25 times 95,text{mm}$)
- Non-absorbent cotton plugs or foam plugs (Flugs)
- Glass etherizer bottle and re-etherizer porcelain spot plate
- Diethyl ether (laboratory reagent grade) or FlyNap
- Camel-hair brush (#000 or #0) for gentle fly manipulation
- Fine dissection needle or bent glass probe
- Morgue jar containing mineral oil or $70%$ isopropanol
📋 Step-by-Step Experimental Protocol
- Preparation of Standard Cornmeal Medium: Suspend agar and cornmeal in $750,text{mL}$ of distilled water; bring to a rolling boil while continuously stirring with a magnetic stirrer. Separately dissolve yeast, dextrose, and molasses in $200,text{mL}$ warm water, then combine. Simmer for 15 minutes until smooth and viscous. Cool to $55^circtext{C}$, mix in $5,text{mL}$ of propionic acid (antifungal preservative), and dispense into culture vials (~10 mL per vial) or bottles (~40 mL). Plug with foam flugs and allow to solidify at room temperature for 24 hours. Add a pinch of dry active baker’s yeast to the surface before introducing flies.
- Anesthetization Technique: Add 3 to 4 drops of diethyl ether to the cotton wad of the etherizer sleeve. Tap the stock culture bottle firmly on a rubber mousepad to knock the flies to the bottom. Invert the etherizer over the culture bottle mouth, invert both, and tap again to transfer the flies into the etherizer chamber.
- Monitoring Anesthesia: Observe the flies continuously through the glass chamber. Flies will become motionless within 15 to 30 seconds. Critical checkpoint: When their wings remain folded flat along the abdomen and legs are relaxed, immediately invert and pour them onto a white porcelain dissection plate or index card. Warning: If wings project upward at a $45^circ$ angle, the flies have been over-etherized and killed!
- Stereomicroscopic Examination: Place the plate under a stereomicroscope at $10times$ to $40times$ magnification. Using a moistened camel-hair brush, gently orient flies dorsal-side up, then ventral-side up.
- Foreleg Sex Comb Inspection: Focus on the forelegs at $40times$ magnification. Examine the distal portion of the first tarsal joint. Identify the comb of dark, stout bristles (sex combs) in males. Confirm their complete absence in females.
- Abdominal & Genital Inspection: Count visible abdominal tergites. Observe the rounded, solid black posterior tip of the male versus the segmented, striped, pointed abdomen of the female. Turn the fly on its back and examine the dark genital claspers of the male.
- Sorting & Recovery: Separate males into one group and females into another using the brush. Transfer living sorted flies into an inverted fresh culture vial (laid horizontally on its side) until full recovery (5-10 minutes) so they do not sink into the moist food medium. Discard unwanted dead flies into the morgue jar.
📊 Morphological Diagnostic Matrix
| Diagnostic Trait | Male (♂) D. melanogaster | Female (♀) D. melanogaster | Diagnostic Reliability |
|---|---|---|---|
| Foreleg Sex Comb | Present (10-12 dark chitinous teeth on metatarsus) | Completely Absent | 100% Definitive (Absolute Marker) |
| Abdominal Tergites | 5 visible tergites; 5th & 6th coalesced | 7 visible tergites; distinct bands | High |
| Posterior Tip Pigmentation | Solid, heavily melanized dark black rounded cap | Light colored, alternating stripes; pointed | High (in mature flies >2h post-eclosion) |
| External Genitalia | Dark, sclerotized genital arch, claspers & penis apparatus | Light brown, pointed ovipositor with anal plates | Definitive |
| Body Size | Smaller (~2.0 – 2.5 mm length) | Larger (~2.5 – 3.2 mm length, distended abdomen) | Variable (dependent on larval nutrition) |
⚠️ Mandatory Experimental Precautions
- Never allow diethyl ether liquid to touch the flies directly; liquid ether causes rapid cuticular desiccation and immediate death.
- Never leave etherized flies upright in a culture vial; unanesthetized flies recover, but unconscious flies will stick to the agar surface and suffocate. Always keep vials on their side until flies walk.
- When collecting virgin females for experimental crosses, always clear the stock bottle within 8 hours of pupal emergence. In cultures kept at $18^circtext{C}$, the virgin window extends to 16 hours.
