Population Dynamics of Growth of Drosophila

Marianne Anderson
1991 Woodrow Wilson Biology Institute

Objectives:

to determine the potential for increase of Drosophila.
to investigate various factors that affect the population dynamics of growth of Drosophila

Background:

The number of individuals in a population does not remain static. Rates of birth, death, maturation, etc fluctuate with seasons and resources ( the quality and quantity of food, space, air, predators, etc). Since it is too time consuming to study the parameters that affect the dynamics of growth of the human population, Drosophila have been chosen as the experimental organism. The basic features of growth and development of Drosophila have been studied in the laboratory. The fruit fly is a small organism, easy to rear and to count, with a short life cycle. One fact to consider is that the female lays for only about 5 days and then tapers off. Many variables affect the population dynamics of growth. This investigation will deal with variation in density of females, composition of food sources, and in environmenal factors such a temperature and light. Other factors can be considered and pursued.

Materials:

culture bottle of Drosophila containing adult flies 1-5 days old
petri dishes containing plain agar (recipe below)
fly nap or ether ( read safety directions on bottle) to anesthesize the flies
adhesive tape
stereomicroscopes
small artist's paint brushes
Optional:
- thermometers
- aluminum foil
- petri dishes containing plain agar with mashed banana chunks
- petri dishes containing plain agar with mashed banana chunks sprinkled lightly with yeast after agar cools

Recipe:

weigh 1.5g of agar and mix with 100 ml of water. Heat gently to boiling to dissolve the agar. Pour into petri dishes so that eaach dish contains about 20-25ml of agar. Let the plates cool and solidify. Store agar side up.

Experimental design:

To determine the potential for increase remove adult fruit flies from the culture bottle by allowing them to fly into an empty bottle. Anesthesize them and separate males from females. Put 50 females on plain agar in 1 dish, cover; 25 in another, then10, and 5, 2, and 1 in the last 4 plates. Wait 1 day. Liberate the flies outside and count the number of eggs laid per female. Graph eggs/female versus number of females.

Optional:

Repeat the above experiment varying the egg-laying medium, or the temperature, or the light. You might also try the effect of including males on the dishes.

Analysis:

Is there an effect of density on egg laying? How can you explain this effect?
Is there an effect of medium, or temperature, or light on egg laying? How can you explain this effect?
Consider the graphs. What shape are they? The experimental design was based on the assumption that female fruit flies have a constant egg laying rate. In reality she lays for about 5 days and then tapers off.
Extrapolate the number of eggs/female if 100 or 500 females were placed in a plain agar dish or in any of the other experimental setups. A group of students may want to test this prediction experimentally.
How many eggs does a female lay in 1 day? Usually, of course, half of the flies would be males. Eggs develop into adult flies in about 2 weeks and then those flies can lay eggs for 5 days. If you start with 2, 5, 10, 25, 50, and 100 flies and if 1/2 of each sample lay eggs how many flies would you have in 1 day, 1 week, and 1 year? Why isn't the earth covered solidly with flies?
Research the number of human offspring/female in a heavily populated city like NYC or LA compared to the number in your hometown or any lesser populated city. How do the results compare to the density relationships in flies?
Determine the population in 100 years if the rate of increase for humans is 2/female and the generation time is 20 years; 3/female; 4/female? What if the generation time is 30 years? Do this by starting with one pair of humans. What if you start with 10 pairs? 100 pairs?

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