Chapter 1. Drosophila Genetics I

Drosophila Genetics I

This two-week laboratory exercise is designed to pull together many of the skills and concepts covered (and yet to be covered) this semester, including hypothesis testing, statistics, lab writing, and genetics. Today’s lab focuses on introducing you to a very important model organism in biological research: the red-eyed fruit fly, Drosophila melanogaster.

Drosophila melanogaster, the “common” or “vinegar” fruit fly, is the species of fruit fly that you often see flying around the bananas at the supermarket. Unlike the Medfly, these fruit flies are not an agricultural pest because they only eat the yeast that grows on rotting fruit. Scientists have been using D. melanogaster (and other Drosophila species) as a so-called “model organism” in biology for over 100 years now. Originally, the fruit fly was picked as a test animal because of its large reproductive capacity, its medium size (between microscopic organisms and larger animals like mice), its short generation time (two weeks at room temperature), and its inexpensive room and board (glass bottles, bananas, and molasses).

Currently there are tens of thousands of Drosophila researchers around the world. We now know more about the biology of the fruit fly than any other living thing, including humans. Additionally, because its genome and metabolic pathways have been so well studied, we can use the information gathered from them to illuminate the same systems in other species, including our own. For example, 61% of human genetic diseases have been found in fruit flies as well! This allows medical scientists to initially develop possible treatments and test them on the flies, which have a two-week generation time and can be cheaply and reliably raised in the laboratory in huge numbers.

Quick Facts about Drosophila melanogaster

Quick Facts about Drosophila melanogaster

• Humans and flies share at least 5,000 genes. Drosophila has about 15,000 genes, Homo sapiens about 40,000. Of course, even in the shared genes, some differences have arisen in the 600+ million years since our last common ancestor.

• A female fly has roughly the same number of eggs as a female human, from 400 to 1,000.

• Male flies spend most of their time chasing after and singing to females to try and obtain a mating. They succeed about once a day.

• Fruit flies sleep. Caffeine keeps them awake longer.

• Flies live about two weeks in the wild, up to six months in the lab (if bred for longevity).

• None of the life stages can survive being frozen. (They originated in Africa.)

• Most larval tissue, save the germ line and neurons, is consumed in the pupal stage to make an adult.

• Typically, the human genetic diseases not shared by flies are the cancers; the only cancer flies are known to get is ovarian cancer. Aside from producing eggs, adult flies do not have any cell division.

• Another reason flies were chosen for study is the “giant” chromosomes of their salivary gland cells, which allows researchers to see the banding patterns on the chromosomes and track genes.

• The first scientists who worked on Drosophila, and later won Nobel Prizes, used to obtain the glass milk bottles for their flies from people’s porches in the middle of the night.

You will be performing two genetic crosses with Drosophila. We will be setting up these crosses today so that the offspring will be mature two weeks from now. Before observing these animals, you need to prepare two culture tubes (one for each cross). Our lab flies will eat yeast cells growing on commercially prepared high carbohydrate media.

Drosophila Life Cycle

Drosophila Life Cycle

Drosophila has a four-stage life cycle: egg, larva, pupa, adult (Figure 7-1). Temperature affects the rate of development such that time from egg to adult varies from 90 days at 15°C to about 10 days at 25°C. Your crosses will be kept at about 25°C so that you can get your results two weeks from today. Development times listed refer to this temperature.

Larvae usually hatch out from the eggs after about 22 hours. The larvae are transparent, and you can see the inside organs such as the coiled intestines, whitish fat bodies, and gonads. Larvae grow and feed for four days, proceeding through three larval stages (instars), each larger than the last. During the third instar, the larvae crawl up the sides of the culture container in preparation for pupation.

The pupal stage lasts for 4–6 days, during which time metamorphosis occurs. The pupa hardens and darkens during this period. Look for the pupal horns off the anterior end of an early-stage pupa; these are the spiracles (outside openings of the respiratory tubes) turned inside out. Finally the pupa is ready to emerge into the adult stage.

Newly emerged adults look pale and puffy—like the pupa, they darken as they age. Drosophila geneticists often need to identify newly emerged females in order to obtain virgins for use in specific crosses. Adult Drosophila males and females are easily distinguished. Males are smaller, with a rounded, blackened tip to their abdomen (posterior segment) and females have a pointed abdomen which may have some light banding. Males also have sex-combs on their forelegs.

The first cross you will do today is for the recessive sex-linked white-eyed trait. The gene for a sex-linked trait is located on the sex chromosome. As in humans, female fruit flies have two X chromosomes whereas males have one X chromosome and one tiny Y chromosome (which lacks most of the genes found on the X chromosome). Since males have only one X chromosome, they have only one allele for sex-linked genes (and cannot be heterozygous). Thus, the pattern of inheritance of X-linked traits differs from that described by traditional Mendelian genetics.

Thomas Hunt Morgan was the first scientist to observe the white-eye mutation in Drosophila in 1910. He identified a white-eyed male. When he crossed this male with a wild-type female, all the offspring had red eyes, demonstrating that red eyes are dominant over white. When these red-eyed offspring were mated, the results followed a predictable 3:1 Mendelian ratio of red to white, but all the white-eyed flies were males. Half of the males were red-eyed, and all of the females were red-eyed. Morgan realized that these results signified that the gene for red vs. white eye color was located on the X chromosome. You will be observing the reciprocal cross by breeding F_l generation offspring from a white-eyed female and a red-eyed male.

What caused the white-eye mutation? Mutations are changes in the DNA nucleotide sequence. Some mutations are caused by large-scale changes in chromosome arrangement. The more common point mutations are due to single base pair changes in DNA’s nucleotide sequence.

Mutations occur spontaneously (but rarely) in nature and can also be caused by X-rays, gamma rays, and chemical mutagens. Research on flies with specific mutations provides information about a gene’s function. White-eyed flies do not see as well as wild-type redeyed flies; it turns out that the white-eyed mutation is due to a change in the nucleotide sequence of a gene for an eye pigment transporter. The effect of this change is that the fly is unable to produce functional pigment transporters, causing its eyes to appear white (nonpigmented). The white-eyed allele is recessive because as long as a female has one allele for the functional transporter, she can make the transporter and move pigment into her eye. Many recessive alleles (whether autosomal or sex-linked) represent a loss of function for some protein.

The second cross you will do today is unknown. The flies you observed today probably had the wild-type phenotype. This is the normal, winged, red-eyed variety most commonly seen in wild populations. However, heterozygous individuals (those with one dominant and one recessive allele for a given trait) usually show the dominant phenotype, so you can’t always determine the genetic makeup by looking at physical traits. In two weeks, you will observe the offspring from today’s unknown cross and use this data to determine the genotypes of both the F₁ flies observed today and the parental (P) generation flies mated at the supply company. These results will be written up in your final complete paper in scientific format.