Mating frequency, colony size, polyethism and sex ratio in fungus-growing ants (Attini)

The mating frequency of queens was estimated for eight attine ant species, Myrmicocrypta ednaella, Apterostigma mayri, Cyphomyrmex costatus, C. rimosu s (four lower attines), Trachymyrmex isthmicus, Sericomyrmex amabalis, Acro myrmex octospinosus and Atta colombica (four higher attines), and correlate d to colony size, worker polyethism, and sex ratio. Mating frequency was ca lculated from within-colony relatedness estimated by CAP-PCR DNA fingerprin ting. Most queens of lower attines and T. isthmicus mated with only one mal e, while those of the three higher attines mated with multiple males. Matin g frequency was positively correlated with colony size. Polyethism among wo rkers was dependent on worker age in lower attines but on body size in high er attines, suggesting some correlation between mating frequency (i.e., wit hin-colony gene diversity) and caste complexity. The sex ratio was biased t oward females in species where the mating frequency equaled one, but toward males in species where the mating frequency was greater than two. Changing in nest site from ground surface to deep underground may have facilitated the evolution of large colony size in Attini, and this may have resulted in the evolution of polyandry (a queen mates with multiple males). With the e volution of polyandry in higher attines, Atta and Acromyrmex in particular have generated high genetic diversity within their colonies and complex soc ial structures.