Originally published 5 September 1988
Like a Newton, a Darwin, or an Einstein, Drosophila, the fruit fly, begins life as a single cell. Within that cell are the genes that will lead, in the fullness of time, to a human of genius, or to an insect with…ah, shall we say, another sort of scientific fame.
Among multi-celled organisms, Drosophila is the subject of choice in genetic research. In Biological Abstracts, the index of biological research, Drosophila has more entries than any other animal except the bacterium Escherichia coli. The science of genetics owes more to this tiny insect than to any other creature.
The fruit fly’s preeminence is mostly due to its long use as a research animal. Drosophila was adopted by T. H. Morgan in his important studies in genetics that began at Columbia University in the early years of the 20th century. These studies led to the classic textbook of Morgan, Sturtevant, Muller, and Bridges, Mechanisms of Genetic Inheritance, which in 1915 established the link between genes and chromosomes. Since that time, much of what we know about mutation, speciation, and other genetic phenomena has been discovered with populations of fruit flies in nature and in the lab.
Fame is spreading
Drosophila is small (about the size of a grain of rice) and has a short life cycle. Both circumstances make the insect an ideal subject for genetic experiments involving large numbers of individuals and many generations. The fruit fly’s full complement of genes is 1/20th that of a white rat or a human, which simplifies genetic analysis. The genes are borne on only four pairs of chromosomes, compared to 23 pairs in humans. But the main reason for Drosophila’s fame is this: As more and more experimental techniques were developed for studying fruit flies, the more likely it became that future researchers would turn to the same subject for their experiments. In biological research, familiarity breeds not contempt but dependence.
So important a service has this little insect rendered to biology that it behooves us to pay homage. Somewhere in the hallowed halls of science a giant statue of Drosophila should be erected, but in lieu of that let me rehearse for a moment the fruit fly’s short, sweet life.
Like ourselves, Drosophila begins life as a mother’s egg fertilized by a father’s sperm. But from that point forward everything moves more quickly. Within a day, the egg hatches to produce a tiny larva, or maggot. The larva typically feeds upon yeasts that it finds in rotting fruit. Six days (and a few molts) later the larva seals itself up in a hard brown capsule, or pupal case. It remains in this enclosure for four days, during which time almost all of the larval organs dissolve into a soupy mix from which the organs of the mature fly form. When the transformation is complete, the adult fly inflates a little bag on its head which forces open a trap door in the pupal case.
The fruit fly generally emerges from the pupal case in the early morning (Drosophila means “lover of dew”). Good timing is essential. The earlier out, the easier it is to be successful in the competition for mates. There is no point in emerging in darkness when a mate can’t be seen, and fatal to emerge in the heat of midday with delicate wet skin. A genetically programmed chemical “clock” ticks in Drosophila’s brain. The clock is set in the embryonic stage by changes in temperature and light. When the “alarm clock” signals that the time is right, out pops Drosophila.
Now comes the moment of truth. A male fruit fly follows a prospective mate and vibrates its wings to produce an exotic love song of clicks and whirrs. The song is amazingly species specific, and genetically determined. Even geneticists can’t tell some species of Drosophila apart except by their song. If the pitch or rhythm is not just right, the female immediately terminates the courtship.
Wonderfully prolific
If the female responds, then a little foreplay begins while the male continues to vibrate. Copulation follows, which can last as long as 15 minutes. From then on it’s all down hill; the female lays her fertilized eggs, hundreds at a time, and the cycle begins again.
Fruit flies are wonderfully prolific. They can produce 25 generations in a year. Let’s assume that a female lays 100 eggs, half male and half female. These hatch, develop into mature adults, and mate, and each of the 50 new females lays 100 eggs, half male and half female. And so on. In the absence of any checks on the growth of population, within 25 generations a single pair of fruit flies would give rise to enough progeny to fill a ball the size of the Earth’s orbit. Obviously, in nature there are ample checks on population growth, but the point is clear. Fruit flies will rapidly fill racks of laboratory bottles with their descendants, and the action of heredity over many generations can be readily observed.
Thousands of genetic mutations of Drosophila have been cataloged. There is a project underway to map the location of all genes on the insect’s four chromosomes. And in addition to genetic studies, Drosophila is used in research on sensory mechanisms, neural networks, biological rhythms, learning and memory, and various behaviors.
Why so much effort devoted to a tiny fly? On the molecular level humans and Drosophila have much in common. Many of our proteins and their way of functioning are virtually the same. It appears that much of the basic molecular machinery of life was in place before the evolutionary divergence of the ancestors of insects and mammals 500 million years ago.
Certainly, humans are more complex than fruit flies, but as biologist Gerald Rubin says in an article in Science on the current state of Drosophila research, “there is no compelling reason not to believe, and much circumstantial evidence to support, the contention that most of this complexity is achieved by reiteration and adaption of common, evolutionarily ancient processes.” In other words, much of what we learn about Drosophila, with its quick life cycle, prodigious progeny, and simple genes, applies to more complex animals, including ourselves.
We salute you, Drosophila, singer of love songs, lover of dew, prolific soldier in the quest for the secrets of life.
