Like a sneak thief caught holding the goods, a tiny mite stands accused of stealing genes and smuggling them where their rightful owners could never go: across the border that separates one species from another. The smuggling operation adds a whole new dimension to ways that creatures can evolve.
Biologist Marilyn Houck of Texas Tech University believes she’s found enough circumstantial evidence to convict the mite--and at the same time solve a 40-year-old mystery. During the mid-1950s a foreign gene suddenly spread through wild populations of the common fruit fly Drosophila melanogaster. Houck’s colleague, University of Arizona geneticist Margaret Kidwell, saw evidence of the spread in the early 1970s when she compared fruit fly strains that had lived in labs for decades with newcomers collected from the wild. She was searching for the cause of sterility in the flies, which turned out to be a distinctive gene, unnamed then but now called a P element.
The more recently these strains had been collected from the wild, the more likely they were to have P elements, Kidwell says. In fact, when we went back earlier than the 1950s, we didn’t find any strains that had the P element at all.
In 1989 Kidwell helped prove that the P elements in D. melanogaster almost perfectly matched those in D. willistoni, a distantly related species. P elements are jumping genes, drifters with no fixed address on a chromosome. Although researchers knew these genes could leap the length of a chromosome--by producing an enzyme that allows them to cut themselves out of one place and paste themselves back in another--they didn’t know how P elements could hurdle the species barrier. The two fly species overlap in their ranges, but--being separate species--they don’t interbreed, so they couldn’t have shared the gene sexually. Some sort of courier seemed likely.
Houck, who at the time ran a mite lab just down the hall from Kidwell’s lab, thought one of the many mite species that are naturally found with fruit flies could have done the P-element smuggling. In particular, she suspected a species called Proctolaelaps regalis. The geographic range of these mites overlaps that of both fruit flies, they feed on fly eggs and larvae, and their stilettolike mouthparts reminded Houck of the glass needles used by geneticists to transfer P elements experimentally. The mites could, she reasoned, feed on an egg from one species and, with genes and gene fragments still dripping from their mouths, plunge into an egg from another species. P elements, armed with their enzyme for slicing into a chromosome, could insert themselves into the new genome.
With range, anatomy, and behavior all pointing the finger at Proctolaelaps, Houck and two colleagues tried to find out whether the mites were holding P elements that matched those from the flies. The researchers used a DNA probe, a string of lab-designed genetic material that binds only to the DNA making up the P element in question. They added these probes to DNA extracted from Proctolaelaps mites living on flies both with and without these P elements. In 1991 the sleuths reported that the probes stuck only to the DNA of mites living on flies with P elements. Mites living on flies without P elements lacked the jumping gene.
Gene pilfering, Houck believes, is a new and important addition to our understanding of how organisms evolve and acquire new traits, going beyond random mutations and sexual recombination of parental genes. Now, in the hope of actually catching the mite in the act of jumping the species barrier, Kidwell has set up a laboratory sting operation: she’s raising mites with both a P-element-lacking strain of D. melanogaster and D. willistoni, the jumping gene’s suspected source.
Houck feels confident the mite will incriminate itself. All we have shown is bloody mouthparts, she says. But it becomes much more than that when you add in all the corroborating information about the geographic overlap, about their feeding habits, about their behavior, and about their natural occurrence with Drosophila. When you put all of the smoking guns together, it would be really trivializing it to say it’s just bloody mouthparts.