Parting Genomes: UA Biologists Discover Seeds of Speciation

A University of Arizona graduate student may be the first eyewitness to the birth of a new species. Her new findings, appearing in the June 7, 2004 Proceedings of the National Academy of Science, could help biologists identify and understand the precise genetic changes that lead a species to evolve into two separate species.

Laura K. Reed and her advisor, Regents’ Professor Therese Markow, made the discovery by observing breeding patterns of fruitflies that live among rotting cacti in western deserts. Whether the two closely related fruitfly populations, designated Drosophila mojavensis and Drosophila arizonae, represent one species or two is still debatable among biologists, testament to the Arizona researchers’ assertion that they are in the early stages of diverging into separate species.

The seeds of speciation are sown when distinct factions of a species cease reproducing with one another. When the two groups can no longer interbreed, or prefer not to, they stop exchanging genes and eventually go their own evolutionary ways, forming separate species.

While the evolutionary record is brimming with examples of speciation events, Reed says, biologists haven’t been able to put their finger on just what initiates the reproductive isolation. Several researchers have identified mutant forms of certain genes associated with the inability of fruitflies to hybridize with closely related species, but in all cases those genes were discovered long after the two species diverged. Those genetic changes could have caused the speciation or resulted from it, or they might even be incidental changes that occurred long after the species diverged. The difficulty, Reed explains, is that you have to catch the genetic schism while it’s still brewing.

She and her advisor report that they have managed to do just that. In the wild, D. mojavensis and D. arizonae rarely if ever interbreed, even though their ranges overlap along a broad swath along the northern Mexican coastline. In the lab, researchers can coax successful conjugal visits between members of the two groups. But even under laboratory conditions hybrid crosses aren’t always fruitful. D. mojavensis mothers typically produce healthy offspring after mating with D. arizonae males, but when D. arizonae females mate with D. mojavensis males, all of the resulting hybrid sons are sterile. This partial capacity for interbreeding, Reed says, suggests that these flies are on the verge of evolving to become completely separate species.

Another finding adds support to that notion. Researchers had previously reported that for one strain of D. mojavensis, from Catalina Island, off the southern California coast, mothers always produce sterile sons when crossed to D. arizonae males.

Because the hybrid male sterility trait depends on the mother’s genetic heritage, Reed and Markow concluded that the genetic change—polymorphism, in evolutionary biology parlance—responsible for creating sterile sons must not yet be “fixed,” or firmly established in D. mojavensis populations. And that is a telltale sign that the change was recent.

Reed wanted to know just how deeply the polymorphism causing male sterility had suffused Catalina Island D. mojavensis populations. In other words, do all or just some of the Catalina Island mothers produce sterile sons when mated to D. arizonae males? When she did the experiment, she found that only about half the crosses resulted in sterile sons. That result implies that only half the females in the Catalina Island population had the gene (or genes) for hybrid male sterility.

Surprisingly, when she tested D. mojavensis females from other geographic regions, she found that a small fraction of those populations also exhibited the hybrid male sterility polymorphism. “That polymorphism exists in every population I looked at,” Reed said. “It just happens to be that whatever factors are causing sterility are at higher frequencies in the Catalina Island population.”

Further experiments demonstrated that the sterility trait is caused by more than one genetic change. “I think there are many genes—4 or 5 probably, maybe many more,” Reed predicted.

Now that the researchers are hot on the trail of a set of “speciation genes,” their next task will be to identify them. To help toward that endeavor, they plan to take advantage of the newly begun D. mojavensis genome sequencing project, which will provide a complete roadmap of every gene in the species.

Reed reflects upon the implications of the findings. “There’s a huge amount of biodiversity out there, and we don’t know where it comes from. Evolutionary biologists are excited to figure out what causes what we see out there—the relative forces of selection and drift—whether things are adapting to their environment or variation is random.

“Another important component to that is how that variation is partitioned into separate species. Once you’re a separate species, you have an independent evolutionary trajectory to some other species—an independent set of tools, or genetic potential, relative to other species. So this partitioning of genomes is an important cause of the variation we see in nature.”