Teacher Material: Lesson 3

Teacher Lesson Three
Collecting and crossing flies from the wild

This is an interesting extension, utilizing information from lesson two. Are wild populations of flies different than ones obtained from cultures? Students will collect their own flies and construct crosses, then compare them to the data from lesson two.

Materials:
1. Stereomicroscope
2. Anesthetizing system
3. Paint brushes
4. Culture vials with media
5. Various mutant strains of flies (as per lesson two)
6. Wild-type and caught
7. Pictures of D. melanogaster and D. simulans
8. Fly Trap

Time
This depends on ascertaining the specie of fly. The crossing of caught and wild will result in F1 within 10 days of mating, F2 after another 12 days. To determine the species of fly takes minutes on visual inspection. If mating is used to determine species, 5 days minimum is required.

Part 1: Collecting flies from the wild.
Construction of a fly trap is very easy. Other insects may be present in the trap, which is not a problem. Also, even though there are no adult flies present there may be eggs or larvae on the banana.If so, transfer the banana to a fresh tube with media and ask the student to try again. Collection of flies is best done when the temperatures are moderate.

Pictures of a fly trap

Part 2: A problem with wild flies, and a solution
D. melanogaster and D. simulans are morphologically different in the arrangement of the claspers in the male genitalia, used for copulation. This can be difficult to perceive unless one has a trained eye for such things. However, once one sees the difference it is quite easy to determine the species. Therefore, there are two ways to determine the species.
1. Visual inspection of males in the population. It is possible to have mixed populations but visual inspection of females is very difficult.
2. Crossing caught species with a known wild-type D. melanogaster. Remember to use virgin females for the cross. If larvae fail to develop in the media then the caught species is not D. melanogaster. Eggs may be seen, however they will be sterile. It is interesting to note that it is possible to have some eggs hatch and a hybrid melanogaster/simulans F1 products. However, the frequency of this occurrence is very low. If, however, larvae do develop, they will be sterile and unable to produce viable offspring with either D. melanogaster or D. simulans.

Pictures comparing male D. melanogaster with D. simulans

Now could be a time to introduce the biological definition of a species. Since a melanogaster/simulans cross may produce hybrids, students may think they are the same species. However, the offspring are sterile (not viable), therefore, not the same species. It is the same with a cross between a horse and donkey. Mules are produced, however they are sterile. Therefore, a horse and donkey are not of the same species.

Part 3: Crossing collected flies with known phenotype flies.
Once the species has been determined to be D. melanogaster, you can proceed with mating between caught flies and mutant and caught flies and bought wild types. Since most caught flies will exhibit wild-type phenotypes, there may be no noticeable difference in a wild caught and wild cultured cross, however it might be interesting to see the outcome. In any case, comparing the outcomes between wild caught x mutant and cultured x mutants could open up avenues for discussion.

Overview

Lesson 1
Lesson 2
Lesson 3

Student Material

References