[Pest Spotlight] The Rise of the Brazilian She-Ants

A world without males may be unimaginable for humans: Yoko Ono with no John Lennon? Michelle Obama without Barack? Kate Middleton with no Prince William? But that's not the case for the Mycocepurus smithii ant species.

Unlike any other known ant species, the widespread fungus-farming M. smithii of Latin America is completely male-less, and the female ants can produce queens and workers by asexual reproduction.

That's according to a recent study published in PloS ONE, an international, peer-reviewed Public of Library Science journal. In it, Americans Christian Rabeling and Ulrich Mueller and their Brazilian colleagues detail their efforts to confirm the absence of M. smithii males throughout the species' area of distribution, while adding to the current understanding of how the species reproduces.
 

No Males, No Mating. As part of their research, the team, led by Rabeling, examined Mycocepurus male ants collected in Brazil in the 1960s and thought to be males of the M. smithii species. If males of the species existed, it would suggest that the ants reproduce sexually, at least occasionally.

Rabeling's morphological analysis of the males confirmed that the specimens actually represent males of the Mycocepurus obsoletus species, a closely related species of fungus-farming ant. By ruling out these ants as M. smithii males, the team provided definitive proof for the first time that no known male M. smithii ants exist.

In field research, Rabeling also failed to find males in or around M. smithii nests observed in Brazil during the September through October mating season over the course of a six-year period. This finding supports those of previous field observations of M. smithii throughout the species' Latin American range of distribution.

Additionally, the Rabeling team found that none of the M. smithii ants showed behavior typical of mating season — no workers made modifications to the nest in preparation for nuptial flights, and no nuptial flights occurred. In contrast, nest modifications and nuptial flights were noted in sexual ant species observed during the same mating seasons.
 

Inconclusive Evidence. In laboratory examinations, Rabeling proved that the reproductive physiology of M. smithii queens and workers supports the hypotheses of asexual reproduction. Generally, mated, reproductively active queens are characterized by sperm in the spermatheca (the female reproductive receptacle that receives and stores sperm); fully developed ovaries; and, presence of yellow bodies (corpora lutea) at the base of the ovaries, which suggests past reproductive activity.

Rabeling's examination of M. smithii ants identified queens with empty spermatheca, fully developed ovaries and corpora lutea. This examination provided proof of reproductive activity without the presence of sperm, suggesting reproduction without insemination, or asexual reproduction.

In contrast to the queens, none of the M. smithii workers examined had spermatheca, nor did they exhibit any of the signs of reproductive activity, suggesting their sterility and inability to contribute to colony growth.

While Rabeling's work confirms the M. smithii species is male-less and exhibits asexual reproduction across its Latin American distribution, questions still remain about when the species evolved, and how the ants' asexual reproduction occurs.

According to Rabeling, the asexual M. smithii species evolved within the last one to two million years and then experienced a rapid population expansion. This would make M. smithii a relatively young species, since fungus-farming ants evolved about 50 million years ago. Rabeling argues that the low genetic variability between M. smithii populations in Latin America, and the continued presence of female mating apparatus (like the spermatheca found in queens) serve as proof of the recent evolution of M. smithii.

The genetic mechanism of asexual reproduction in M. smithii also continues to stir debate. Previous research suggests that cloning occurs. Rabeling argues that may not be the case, and that firm evidence capable of identifying the cellular mechanism of asexual reproduction is currently lacking. Stay tuned: More research on the species is sure to come.

E-mail the author at jokeef@giemedia.com.