The Snowball of Speciation

329_1518_f1Among evolution’s best tricks is the act of turning one species into two. Speciation, the foundation of a new species from an accumulation of small changes in an old one, has given birth to the incredible diversity of life on our planet. But in order for a new species to be founded, a sort of genetic restraining order must be put in place. Within a species, individual organisms can evolve extraordinary differences without spawning a new species – think dog breeds, for example. It’s only when two organisms grow so different that they lose the ability to come together and successfully reproduce, that a new speciation event can be declared – think the lion, the tiger, and the sad, sterile liger.

The nuts and bolts of speciation have given headaches to evolutionary biologists all the way back to the very first one. In his books, Charles Darwin laid out the outlines of how a new species could form from an old one, but left the dirty work of figuring out the actual mechanism to the field he created. Some of the most famed evolutionary biologists of the early 20th century took up the challenge and left the field with the DM model of hybrid dysfunction, named for Theodosius Dobzhansky and HJ Muller. For nearly 100 years, that theory stood as an explanation of how natural selection eventually produces sterile offspring, while being too technically difficult to explicitly test. But this year, in the laboratory of University of Chicago professor Jerry Coyne (and simultaneously and independently in an Indiana laboratory), the theory was finally confirmed.

With his first graduate student Allen Orr, Coyne had figured out the experiments and the mathematics that one could use to test the DM theory. The only problem was finding a set of species in which to do those experiments. Three species were necessary, and those three species had to still be closely related enough that breeding was possible, but not successful. A fast reproductive cycle would also be helpful, so the experiment wouldn’t require decades to execute, and the genetic information of those species would have to be fairly well characterized.

It wasn’t until two years ago that all those tools became available, and in a paper published in Science earlier this month, Coyne’s current graduate student Daniel Matute brings 100 years of theory and research into the end zone. Matute ran his experiments with the noble fruit fly from the Drosophila genus, and his project was made possible by the recent discovery of a new species in that genus, called Drosophila santomea. When Coyne and Matute realized that the new species finally completed the triad of closely-related species they would need to test the DM Theory, it launched Matute into a long series of breeding experiments.

Drosophia melanogaster (from Wikimedia Commons)

Drosophia melanogaster (from Wikimedia Commons)

The central question was how quickly genes responsible for hybrid inviability accumulated. The DM Theory proposes that those genes don’t merely add up at a steady pace as two species split off from a common ancestor, they “snowball” at an exponential pace with time. To test that notion, Matute needed to count the genes responsible for inviability in two different pairs of species, with different divergence times. That meant a lot of time putting two Drosophila species together, allowing them to mate, and then counting their offspring under a microscope – essentially following the instructions Coyne and Orr had laid out 20 years prior.

Fortunately, all that work yielded exciting numbers. For Drosophila santomea and Drosphila melanogaster, estimated to have diverged 12.8 million years ago, 65 genes were found to cause inviability. For the more recently split Drosphila melanogaster and Drosophila simulans (who diverged only 5.4 million years ago), the number was far lower: only 10 genes caused inviability. Some simple math and curve-fitting later, and Matute had determined that the inviability genes were accumulating at an quadratic, faster than linear rate, confirming the central tenet of the DM Theory at long last.

But when Matute and Coyne went to publish their findings, there was a surprise: another laboratory, at the University of Indiana, had also confirmed the DM Theory in a plant genus. A tie was declared, and the two papers were published back to back in Science. Matute said he appreciated that the serendipitous timing made the case even stronger for proving the theory’s contentions.

“One of the really nice things about this is we came to the same conclusion, in different systems, and using different methods,” Matute said. “I think that doesn’t leave much space for doubt.”

Now that Matute has demonstrated the overall principle governing hybrid inviablity, he’s digging even deeper. Using the extensive databases available for Drosphila melanogaster genetics, he has begun tracking down the individual genes that cause inviability, to determine why they would trigger failure in the offspring of two species. The seven genes he’s found so far suggest a new mystery, as they govern seemingly unrelated functions, from the expression of other genes to the pigmentation of a fly’s body.

In the meantime, Matute and his mentor are satisfied at solving a long-running evolutionary puzzle that has tormented generations of scientists. At his blog, Coyne wrote: “The nice thing about this result, from the standpoint of an old fogey, is that my newest student tested and confirmed a population-genetic model of speciation devised by my first student.  The circle is unbroken!”

About Rob Mitchum (525 Articles)
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