Mycocepurus smithii in the fungus garden

This week’s PNAS contains detailed follow-up research on an intriguing story that emerged a couple years ago about a purportedly asexual, all-female ant species. A 2009 report looked for, and failed to find, any genetic signal for sexual reproduction in a population of the fungus-growing ant Mycocepurus smithii in Panama. This data, along with a lack of observed males over the extensive range of the species, prompted researchers to conclude Mycocepurus smithii may be the first documented entirely non-sexual ant species.

A lack of sex is interesting, of course. The genetic mixing inherent in sexual reproduction is thought to be extremely important in creating novel combinations of alleles, providing the raw variation allowing populations to adapt to changing conditions or stay ahead of rapidly evolving parasites.

Myrmecologist Christian Rabeling decided to probe the M. smithii problem in greater depth by sampling ants from a broader distribution. He found what researchers often discover when gathering additional data: the system is messier than thought.

Distribution of sexual (starred) and asexual (circled) Mycocepurus smithii. Adapted from Figure 1 of Rabeling et al (2011).

Rabeling confirmed with a combination of genetic and physiological data that indeed, most populations of M. smithii are asexual. But several sexual populations occur along the Amazon river in northern Brazil, and the genetic data suggest that each is more closely related to some of the asexual lineages than to each other.

Thus Mycocepurus smithii is only mostly asexual, rather than entirely so, and the transition among reproductive modes happens repeatedly.

This is an exciting find indeed. Mycocepurus is one of those rare animals that apparently does both, rendering it a suitable model for studying why sex evolves. And being a fully social ant, it is primed to become a model species for studying sexual evolution in social contexts.

One final note, and a challenge for Brazilian collectors: male specimens of this species remain unknown.


sources:

Christian Rabeling, Omar Gonzales, Ted R. Schultz, Maurício Bacci, Jr. Marcos V. B., Garcia, Manfred Verhaagh, Heather D. Ishak, and Ulrich G. Mueller. 2011. Cryptic sexual populations account for genetic diversity and ecological success in a widely distributed, asexual fungus-growing ant. PNAS Published online before print July 18, 2011, doi: 10.1073/pnas.1105467108

Himler AG, Caldera EJ, Baer BC, Fernández-Marín H, Mueller UG (2009) No sex in fungus-farming ants or their crops. Proc Biol Sci 276:2611–2616.