Social Parasitism In Ants (How Different Individuals In A Single Species Can Be)

quantamagazine |  When the researcher Daniel Kronauer was still a postdoc in 2008, he traveled to Okinawa, Japan, for wild specimens of clonal raider ants (the species Ooceraea biroi). In the first colony he collected, he noticed two ants with a strange appearance. They were small like workers, but they also sported small wing buds, which was striking because usually only ant queens develop wings. What made this even stranger was that clonal raider ants don’t even have queens: In keeping with their name, these ants reproduce asexually, so all the ants in a colony are nearly perfect genetic clones.

Kronauer was intrigued by the miniature queens because they seemed so different from the other clonal raider ants even though he believed them to be the same species. But answers to his questions weren’t forthcoming, so he took some specimens, shot some photos for records and then moved on with his work.

A few years later, Kronauer established a lab at Rockefeller University and set up a colony of clonal raider ants for study. One day, his then-doctoral student Buck Trible found a few more of the odd miniature queens in that colony and decided to characterize them.

Trible found that the wings weren’t the ants’ only unusual characteristic. The strange ants also showed different social behaviors, had larger ovaries and laid twice as many eggs. Using genetic tools, he traced all of these changes to a 2.25-million-base-pair-long stretch of DNA. In the ordinary ants, the DNA on each of the two copies of their chromosome 13 was different. But in the miniature-queen ants, the two copies were identical.

As Trible, Kronauer and their colleagues reported in March in Current Biology, all of the characteristics of the odd ants — the wings, the social behaviors and the reproductive traits — were caused by what geneticists call a supergene, a collection of genes that are inherited as a unit and are highly resistant to being broken up. At some point in their evolution, the ants had acquired a second copy of that supergene, and that chromosomal change had transformed their bodies and behaviors. The findings suggested a new mechanism for how complex combinations of body parts and behaviors can sometimes surface all at once in evolution: through a mutation that duplicates a supergene, toggling on entire suites of traits like strings of lights controlled by a light switch.

Ant researchers are excited by the work, and not just because it seems to solve a decades-old mystery about how at least one form of social parasitism evolves in the insects. The supergene discoveries may also help them pin down long-sought features in ants’ genetic architecture that make their colonies develop as hierarchical castes of queens and workers.

More broadly, the new study also offers insights into a fundamental evolutionary question about how different the individuals in a single species can be.