A newly-discovered species of ant appears to have branched off from the family tree without leaving home.
Researchers say the new type of parasitic ant—only found in a single patch of eucalyptus trees on the São Paulo State University campus in Brazil—supports a controversial theory of species formation.
“Most new species come about in geographic isolation,” says Christian Rabeling, assistant professor of biology at the University of Rochester. “We now have evidence that speciation can take place within a single colony.”
In discovering the parasitic Mycocepurus castrator, researchers uncovered an example of sympatric speciation, which occurs when a new species develops while sharing the same geographic area with its parent species, yet reproducing on its own.
“While sympatric speciation is more difficult to prove,” Rabeling says, “we believe we are in the process of actually documenting a particular kind of evolution-in-progress.”
A common ancestor
New species are formed when its members are no longer able to reproduce with members of the parent species. The commonly-accepted mechanism is called allopatric speciation, in which geographic barriers—such as mountains—separate members of a group, causing them to evolve independently.
“Since Darwin’s Origin of Species, evolutionary biologists have long debated whether two species can evolve from a common ancestor without being geographically isolated from each other,” says Ted Schultz, curator of ants at the Smithsonian’s National Museum of Natural History and coauthor of the study that is published in the journal Current Biology.
“With this study, we offer a compelling case for sympatric evolution that will open new conversations in the debate about speciation in these ants, social insects, and evolutionary biology more generally.”
Unique species of ant
M. castrator is not simply another ant in the colony; it’s a parasite that lives with—and off of—its host, Mycocepurus goeldii. The host cultivates fungus for its nutritional value, both for itself and, indirectly, for its parasite that doesn’t do any of the work.
That led the researchers to study the genetic relationships of all fungus-growing ants in South America, including all five known and six newly-discovered species of the genus Mycocepurus, to determine whether the parasite did evolve from its presumed host. They found that the parasitic ants were, indeed, genetically very close to M. goeldii, but not to the other ant species.
They also determined that the parasitic ants were no longer reproductively compatible with the host ants—making them a unique species—and had stopped reproducing with their host a mere 37,000 years ago—a very short period on the evolutionary scale.
The researchers found a big clue when they compared the ants’ genes, both in the cell’s nucleus and in the mitochondria.
Sequencing units of genes called nucleotides in the mitochondria of the parasitic ants have begun to reveal differences from what is found in the host ants, though traces of the relationship between host and parasite are still present in the genes of the nuclei. These findings lead Rabeling to conclude that M. castrator has begun to evolve away from its host.
Just comparing some nuclear and mitochondrial genes may not be enough to demonstrate that the parasitic ants are a completely new species, Rabeling says. “We are now sequencing the entire mitochondrial and nuclear genomes of these parasitic ants and their host in an effort to confirm speciation and the underlying genetic mechanism.”
The parasitic ants need to exercise discretion because taking advantage of the host species is considered taboo in ant society. Offending ants have been known to be killed by worker mobs. As a result, the parasitic queen of the new species has evolved into a smaller size, making them difficult to distinguish from a host worker.
Host queens and males reproduce in an aerial ceremony, in the wet tropics only during a particular season when it begins to rain. The parasitic queens and males, needing to be more discreet about their reproductive activities, diverge from the host’s mating pattern.
By needing to hide their parasitic identity, M. castrator males and females lost their special adaptations that allowed them to reproduce in flight, and mate inside the host nest, which makes it impossible for them to sexually interact with their host species.
Other researchers on the project are from the Smithsonian Institution’s National Museum of Natural History, Harvard University, and the Center for the Study of Social Insects at São State University, Rio Claro, Brazil.
Source: University of Rochester