Fruit Flies for the Classroom

Steve Binkley
Technical Support Manager, Living Materials Group

Much of what we know about inheritance was first discovered using Drosophila melanogaster, and Drosophila remains an important model organism today. Its short generation time, ease of culture, and the availability of a range of mutants for crosses make it one of the most widely used organisms for teaching genetics.

Drosophila has 4 chromosome pairs, with chromosome pair 1 being the sex chromosomes (X and Y), making it possible to study both autosomal and sex-linked inheritance patterns. Mutations at multiple gene loci are available for chromosome pairs 1, 2, and 3 and can be used for studies of gene linkage and crossing over.

Prior to 1965, many were deterred from working with Drosophila due to the messiness and time required to mix and cook media. Carolina solved this issue by introducing Formula 4-24® Instant Drosophila Medium. To prepare a vial of media, simply add a measured amount of dry Formula 4-24® to a vial, add an equal volume of water, and then add a few grains of yeast. The vial is now ready to receive flies.

Another deterrent to the wider use of Drosophila in science education was the use of ether to anesthetize the flies, which was necessary in order to select males and females for crosses and to score the phenotypes. This issue was addressed in 1980 when Carolina introduced FlyNap® as a safer, less flammable alternative to ether. The availability of these 2 products, Formula 4-24® and FlyNap®, allowed the use of Drosophila to expand from the college/university level to high school and middle school science labs.

Scheduling

A typical Drosophila lab involves crossing parent flies and observing the offspring for 2 generations (F₁ and F₂). This takes over 5 weeks to complete and makes planning essential. Use the following as a general guide for scheduling. Times given are approximations and can vary according to several factors, especially temperature.

Eight to 12 hours before the first lab, remove all adult flies from the parent vials. This is necessary because once mated, female flies can store sperm. New flies will emerge in the vials prior to the lab. This allows students to collect unmated female flies for controlled crosses.

Day 1: To set up a cross, select about 6 male of one parent type and 6 female flies of the other parent type and place them in a fresh vial with culture medium.
Days 7 to 10: Remove parent flies from the vials. This is necessary to prevent the parent flies from mating with their offspring (F₁), which would confuse the counts for the F₂.
Days 12 to 14: F₁ flies begin to emerge in the vials. Once sufficient F₁ flies are available, students can set up these flies in new culture vials to mate and produce an F₂. The F₁ females do not have to be virgins since you want them to mate with F₁ males anyway.
Days 21 to 24: Remove F₁ flies from the vials.
Days 26 to 28: F₂ flies begin to emerge in the vials. Begin to score phenotypes on the day after the first F₂ flies emerge. Once flies are counted, do not return them to the vials. Do the counts every other day for 10 days. This will make the last lab about 36 to 38 days after the first lab. Males tend to emerge later than females—as do some mutations—so if counts are ended too early, your data may be skewed.

Simplify, simplefly

Eliminate the need to clear flies from the parent vials before setting up crosses by using Easy Fly® cultures, which drastically reduce the pre-lab preparation time. Male Easy Fly® specimens have a heat-activated lethal gene on their Y chromosome. Cultures receive a series of heat treatments that activate the lethal gene, killing all the male larvae. Only females emerge, allowing us to ship vials containing only virgin female flies. These can be crossed with normal wild-type or mutant male flies.

Another approach is to begin with our F₁ cultures. We set up the parental crosses here and ship vials of F₁ cultures to you. F₁ flies will be emerging about the time you receive the vials or within a few days. This not only eliminates the need for you to clear vials for the emergence of virgin females, but it also cuts 10 days or more off the schedule given above.

If you have questions

We are often asked to recommend fly cultures for crosses. For sex-linkage, white crossed with wild-type is the classic. Good monohybrid crosses are vestigial x wild-type or sepia x wild-type. Good dihybrid crosses are vestigial x sepia and vestigial x ebony. These contrasting phenotypes are easily distinguished by beginners. Most mutant alleles are recessive to their wild-type allele, but Wrinkled is a dominant mutation. Bar is also dominant and sex-linked. It expresses differently in the homozygous and hemizygous conditions, making it a good mutation for study by more advanced students. A cross of brown with scarlet makes a good introduction to how genes interact in producing a phenotype.

Many customers want to know how many flies will be in the vial when they receive it. There is no easy answer to this question. Female flies lay their eggs over several days, which means that they do not all hatch at the same time. There may be no flies or only a few flies in the vial when you receive it, but there should be lots of larvae and pupae. Over a 10-day period, a vial of parent flies may produce between 50 to 100 flies. Vials of F₁ flies typically produce a bit fewer, perhaps 30 to 75 over the same time period. Notice that Easy Fly® cultures are exceptions to this. Easy Fly® vials contain only unmated female flies, so the number of flies in the vial when you receive the culture is the number that you will have. No other flies will emerge in these vials.

Sometimes we are asked about the information provided on the vial label. Fig. 1 shows the vial label for an F₁ cross. The top of the label tells us that this is a cross of female apterous flies with male wild type flies. The label gives the allele symbol (ap) for apterous in lower case letters, indicating that ap is recessive to the wild-type (+) allele. The (2) indicates that apterous is on chromosome 2. The lower part of the label is stamped with the date the parent flies were placed in the vial and the catalog number.

Figure 1   Vial label for an F₁ Drosophila cross.