Through History Mutants forged the field of genetics, starting with Gregor Mendel's short or tall, yellow or green, round or wrinkled pea plants. Since Mendel's work in the 19th century, geneticists have used white-eyed flies to demonstrate sex linkage and bread mold spore variants to study the recombination of traits that occurs during sexual reproduction. Christiane Nusslein-Volhard and Edward Lewis, two of this year's Lasker award winners, are being recognized for pioneering work in developmental genetics that involved fruit flies with homeotic mutations--striking abnormalities, such as multiple thoraxes or duplicate sets of wings (see story on page 14). Mutants have chronicled environmental change. Over the last century in England, dark variants of the peppered moth Biston betularia have flourished as their lighter-colored cousins were easily spotted by hungry birds on tree bark darkened by pollution from the industrial revolution. And an "ice-minus" deletion mutant of the bacterium Pseudomonas syringae that allows crops to withstand frosts ushered in the era of field tests of genetically engineered organisms. Homo sapiens is itself a model organism when families with inherited diseases provide tissue to researchers, offering chromosomal toeholds for identifying disease-causing genes. Different species are used to answer different types of biological questions, but in general, useful species share certain characteristics. They must be easy to raise or culture, have a plethora of observable traits, and reproduce often and abundantly. Some model organisms pique interest because of unusual properties. This is certainly the case for Dictyostelium discoideum, the cellular slime mold whose dual lifestyle offers a unique peek at development. When its bacterial food is plentiful, Dictyostelium exists as single-celled amoebas--its growth phase. When food is gone, the individual cells emit and follow biochemical signals, amassing to form a motile slug, complete with specialized cell types. This is its developmental phase. The animal can now move to locate food, and begin the cycle anew. "Development and growth are totally separate," says Alan Kimmel of the National Institute of Diabetes and Digestive and Kidney Diseases. "You can get mutations in developmental pathways and, if they do not also block growth, you can maintain the mutated gene forever. Then you can see how the organism is compromised when you let it develop" by starving it. Kimmel has discovered Dictyostelium mutations that disrupt the organism's response to incoming signals. --R.L.
What many of these flies, slime, rodents, and bacteria have in common--besides their occasional transit through the post office--is that they are mutants: genetic variants developed to help researchers figure out how genes work. Mutant strains of organisms generally come either from informal networks of researchers or from government-funded genetic stock centers. Most stock centers send organisms to several hundred researchers per year, amounting to thousands of orders because laboratories tend to request multiple stocks.
Genetic variants have traditionally been used to explain normal biological functioning by showing, often quite strikingly, how processes go awry. An unusual DNA sequence is a mutation; the expression of that variant--a black-bodied fly or curly-eared cat--is the mutant. And although, chemically speaking, all DNA is created equal, growing, feeding, and breeding the model organisms that provide the hereditary molecule for study present a variety of challenges.
Today's favorites include the old standards on which the field of genetics was founded, plus some intriguing newcomers. And all of these organisms, and the innumerable mutant strains that have been developed, are essential tools in many of today's life science laboratories.
Most genetic mutant stock centers trace their origins to private collections belonging to individual scientists. Many collections were taken under the National Science Foundation's wing in the 1980s. Elizabeth Harris, director of the Chlamydomonas (a single-celled green alga) mutant stock center at Duke University in Durham, N.C., wrote the first proposal to NSF to fund a stock center in 1978. "We had fallen heir to a large collection [of Chlamydomonas] and needed support to hire a technician and care for the stocks," she says.
That first grant proposal, submitted as research, fell through the classification cracks--not quite a research proposal, but also not a huge facility request. "It took heavy lobbying by the Genetics Society of America to finally create a section of NSF that would take care of these things," Harris adds. Today, the Living Organisms Culture Collection, NSF's network of stock centers, has an annual budget of $3.5 million, which allows most stocks to be sent free. "Some centers charge a nominal fee, because something like an axolotl (a large salamander) is expensive to send," says director Machi Dilworth.
Unfortunately, overnight delivery can be a problem, says Kathy Matthews, director of the Drosophila mutant stock center at Indiana University in Bloomington. "There is a story about an accident with a Federal Express shipment, when white rats got loose on an airplane. Rumor has it that since then, they and others have instituted a `no living things in the mail' policy." (Federal Express does makes exceptions for special cases, such as circus animals in need of quick transport. In addition, the company accepts certain live animals intended for consumption, such as lobsters.) Shippers typically rely on Uncle Sam, United Parcel Service, or, as Duke's Elizabeth Harris does, asking travelingscientists to hand-deliver living mail.
Politics can also impede mutant travel plans. Harris recalls a disgruntled researcher in Northern Ireland whose healthy-looking shipment would not reproduce. "I suspect the culture was X-rayed at the airport because an inspector thought it was a bomb." The collections under the auspices of NSF vary from year to year, but at present include more than a dozen species housed in stock centers around the U.S. In addition, NSF is a major supporter of the American Type Culture Collection in Rockville, Md., a privately owned, nonprofit collection of bacteria, yeasts, and fungi (The Scientist, Aug. 20, 1990, page 1). Although all of the collections are quite busy, the demand for three in particular--mice, a mustard weed, and a tiny roundworm--is increasing.
Mice are an excellent model organism because their genetic organization closely parallels that of humans. With mice, U.S. scientists can do research using embryos and fetuses that would be prohibited if human tissue were used. Consider the search for the gene behind the childhood kidney cancer Wilms' tumor. Shortly after Katherine Call and David Housman at the Massachusetts Institute of Technology and Gail Bruns and coworkers at Harvard Medical School identified the human gene last year, Nicholas Hastie, at the Medical Research Council in Edinburgh, Scotland, was using the gene to probe developing kidney tissue in normal human embryos. The U.S. researchers are making do with a mouse model.
Yet the mouse can help point the way to obtaining human genes in the first place. Peggy Wallace and Francis Collins at the University of Michigan and Ray White's group at the University of Utah were led last year to the gene behind neurofibromatosis by a mouse cancer gene, provided by Arthur Buchberg at the National Cancer Institute, that highlighted the correct human chromosomal region.
Most mutant mice come from the Jackson Laboratory, in Bar Harbor, Maine, which lost 400,000 animals and a great deal of lab space in a May 1989 fire (The Scientist, June 26, 1989, page 5). Fortunately, the 1,200 mouse variants weren't lost, thanks to a frozen mouse embryo repository.
The new kid on the block in terms of model organisms is the mustard weed Arabidopsis thaliana, a relative of cabbage and horseradish. It neatly fits the bill--it is only 30 centimeters tall, thousands of seeds fit on a petri dish, the life cycle takes less than two months, it grows in tissue culture and can be easily genetically engineered, and it produces 10,000 seeds per plant. And its genetic endowment is ideal--its five chromosomes pack only 70 million base pairs of DNA, and most of its genes are single-copy, "meaningful" sequences. The National Research Council, U.S. Department of Agriculture, and NSF are financially backing studies of the small weed, and a stock center is being set up at Ohio State University, directed by Randall Shoals.
If Arabidopsis defines perfection in a plant genetic system, the tiny nematode Caenorhabditis elegans is surely its animal equivalent. Work on the worm dates to a single investigator, Sydney Brenner, at the MRC laboratory of molecular biology in Cambridge, England, whose 1963 interest in the millimeter-long roundworm has literally spawned today's hundred or so worm labs worldwide. Like Arabidopsis, C. elegans fulfills the usual list of requirements. Big pluses are its versatility--it lives in soil and water and can be frozen--and its simplicity. It is a full-fledged multicellular animal, yet it has a known, small number of cells and a transparent body through which one can watch them divide and develop. As a result, an unprecedented amount of information has already accumulated on the little nematode, including maps of all cell lineages and a nerve map depicting the functional networks of its 302 neurons.
An air of sharing permeates the world's "worm people." Some 450 of them thrice yearly peruse the Worm Breeders Gazette, published by biologist Robert Edgar of the University of California, Santa Cruz. Supplies of C. elegans hail from a 12-year-old stock center at Washington University in St. Louis, under the direction of Don Riddle and funded by NSF's Division of Research Resources. "The facility also serves as a central clearing house to name strains and genes, and for mapping and bibliographical information," says Marty Chalfie, a professor of biology at Columbia University who works with the nematode. Like other stocks, C. elegans can be had for the mere price of postage.
NSF denied support for a 15-year-old tomato stock center that houses a collection stretching back 40 years, according to director Charles Rick at the University of California, Davis. Funding for the tomato genetic stock center today comes from the U.S. Department of Agriculture, private industry, and the university, which supplies a curator. "The collection consists of three main groups--about 1,000 wild species, about 800 monogenic lines, and a miscellaneous category," Rick says. The last group includes stocks bearing multiple mutations, chromosomal abnormalities, stress-tolerant varieties, and primitive and standard cultivars. Tomato seeds are stored in a vault where temperature and humidity are low, and 2,500 to 3,000 samples are sent annually to about 100 research groups. "We steadfastly hold to the free distribution of seeds," Rick says. "It is very difficult to take care of all the bookkeeping that charging would entail, and charging would discourage some people from requesting stocks."
Organisms with smaller followings can rely on informal stock-sharing networks, rather than centralized stock centers. For example, only 50 or so labs worldwide work on the cellular slime mold Dictyostelium discoideum, says Alan Kimmel of the National Institute of Diabetes and Digestive and Kidney Diseases, whose lab is one of the few, and so researchers get by through "passing stocks around," and occasionally ordering from the American Type Culture Collection.
Stock centers also evolve when an informal collection absorbs or duplicates, and augments, an existing collection. Rob Denell, a professor of biology at Kansas State University, found himself in this position when he switched from analyzing homeotic mutations in fruit flies to conducting similar research with the red flour beetle Tribolium castaneum--"trying to turn a small beetle into another Drosophila melanogaster." The flour beetle is more ancient than the fruit fly, and therefore may help to answer compelling questions about the evolution of the homeotic genes that control organization of body parts, Denell says.
Succeeding well beyond their hopes, Denell and his coworkers have added some 80 new variants to the repertoire of this little-studied animal. Although the official stock center resides in the hands of Alexander Sokoloff at California State University at San Bernardino, Denell, who has many of Sokoloff's stocks in addition to his own, now runs an "unofficial stock center."
Although trade in living mutants is brisk, some stock centers are beginning to see a tendency of researchers to zero in on only the gene of interest--like separating the sound of a single instrument from the blend of an orchestra.
"So far, our contacts have been interested in receiving DNA libraries rather than stocks," says Denell of his flour beetle collection. And the molecular state of the biological art is quite advanced for C. elegans--researchers can order a filter paper disk carrying the animal's entire genome from the MRC in Cambridge. So it looks like the vials of slime, flies, and bacteria traversing the mails may be edged out by gene pieces jettisoned from their biological homes, which even the overnight carriers might be persuaded to deliver. But that's another story.
Ricki Lewis teaches biology at the State University of New York, Albany, and is the author of Life, a college biology text (Dubuque, Iowa, Wm. C. Brown Publishing, 1992).