Obligate asexuality in fungus-growing ants

The general prevalence of sexual reproduction over asexual reproduction among organisms testifies to the evolutionary benefits of genetic recombination. Cases of asexual reproduction in groups otherwise dominated by sexual reproduction challenge evolutionary biologists to explain the special circumstances that confer an advantage to asexual reproduction. Here we report one such instance of asexual reproduction in the ants. We present evidence for obligate thelytoky in the fungus-gardening ant, Mycocepurus smithii, in which queens produce female offspring from unfertilized eggs, workers are sterile, and males appear to be completely absent. Obligate thelytoky is implicated by population genetics, reproductive physiology of queens, lack of males, vestigial mating behavior, and natural history observations. Potential conflicting evidence for sexual reproduction in this species derives from three Mycocepurus males reported in the literature, previously regarded as possible males of M. smithii. However, we show here that these specimens represent males of the congeneric species M. obsoletus, and not males of M. smithii. Mycocepurus smithii is unique among ants, in that males seem to be completely absent and only queens (and not workers) produce diploid offspring via thelytoky.