An Introduction to Drosophila Melanogaster
Drosophila melanogaster is a small, common fly found near unripe and rotted fruit. It has been in use for over a century to study genetics and lends itself well to behavioral studies. Thomas Hunt Morgan was the preeminent biologist studying Drosophila early in the 1900's. Morgan was the first to discover sex-linkage and genetic recombination, which placed the small fly in the forefront of genetic research. Due to it's small size, ease of culture and short generation time, geneticists have been using Drosophila ever since. It is one of the few organisms whose entire genome is known and many genes have been identified.
Fruit flies are easily obtained from the wild and most biological science companies carry a variety of different mutations. In addition these companies sell any equipment needed to culture the flies. Costs are relatively low and most equipment can be used year after year. There are a variety of laboratory exercises one could purchase, although the necessity to do so is questionable.
Why use Drosophila?
Teachers should use fruit flies for high school genetic
studies for several reasons.
1. They are small and easily handled
2. You can anesthetize them easily and manipulated individuals with very unsophisticated equipment.
3. Drosophila are sexually dimorphic (males and females are different), making it is quite easy to differentiate the sexes.
4. It is easy to obtain virgin males and females, as virgins are physically distinctive from mature adults.
5. Flies have a short generation time (10-12 days) and do well at room temperature.
6. The care and culture requires little equipment, is low in cost and uses little space even for large cultures.
By using Drosophila, students will:
1. Understand Mendelian genetics and inheritance of traits
2. Draw conclusions of heredity patterns from data obtained
3. Construct traps to catch wild populations of D. melanogaster
4. Gain an understanding of the life cycle of D. melanogaster, an insect which exhibits complete metamorphosis
5. Construct crosses of caught and known wild- type and mutated flies
6. Learn techniques to manipulate flies, sex them, and keep concise journal notes
7. Learn culturing techniques to keep the flies healthy
8. Realize many science experiments cannot be conducted and concluded within one or two lab session
National standards covered in these lessons:
Content:
1. Organisms require a set of instructions for specifying traits (heredity)
2. Hereditary information is located in genes.
3. Combinations of traits can describe the characteristics of an organism.
Inquiry:
Students will
1. Identify questions and concepts that guide scientific investigations
2. Design and conduct scientific investigations
3. Formulate and revise scientific explanations and models using logic and evidence
4. Communicate and defend a scientific argument
The genetics of Drosophila are well known and several web sites feature the complete genome. In additions, many gene loci are known and these, too, are in the public- domain web sites. Therefore, those teachers or students wishing to see where their mutations occur have a ready reference available.
Since Drosophila has been so widely used in genetics, there are many different types of mutations available for purchase. In addition, the attentive student may find mutations within their own cultures since, due to a short generation time, mutations are relatively common compared to other animal species.
Classification:
Domain: Eukarya
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Diptera
Family: Drosophilidae
Genus: Drosophila ("dew lover")
Species: melanogaster ("dark gut")
Life cycle of D. melanogaster
D. melanogaster exhibits complete metamorphism, meaning the life cycle includes an egg, larval (worm-like) form, pupa and finally emergence (eclosure) as a flying adult. This is the same as the well-known metamorphosis of butterflies and many other insects. The larval stage has three instars, or molts.
Life cycle by day
Day 0: Female lays eggs
Day 1: Eggs hatch
Day 2: First instar (one day in length)
Day 3: Second instar (one day in length)
Day 5: Third and final instar (two days in length)
Day 7: Larvae begin roaming stage. Pupariation (pupal formation) occurs 120
hours after egg laying
Day 11-12: Eclosion (adults emerge from the pupa case). Females become sexually
mature 8-10 hours after eclosion
·The time from egg to adult is temperature- dependent.
The above cycle is for a temperature range of 21-23 degrees C. The higher the
temperature, the faster the generation time, whereas a lower (to 18 degrees
C) temperature causes a longer generation time.
·Females can lay up to 100 eggs/day.
·Virgin females are able to lay eggs; however they will be sterile and few in
number.
After the eggs hatch, small larvae should be visible in the growing medium. If your media is white, look for the black area at the head of the larvae. Some dried premixed media is blue to help identify larvae however this is not a necessity and with a little patience and practice, larvae are easily seen. In addition, as the larvae feed they disrupt the smooth surface of the media and so by looking only at the surface one can tell if larvae are present. However, it is always a good idea to double check using a stereomicroscope. After the third instar, larvae will begin to migrate up the culture vial in order to pupate.