Multiple paternity,relatedness and genetic diversity in Acromyrmex leaf-cutter ants

Multiple queen-mating occurs in many social insects, but high degrees of multiple paternity have only been found in honeybees and some yellowjacket wasps. Here we report the first case of an ant species where multiple mating reduces relatedness among female offspring to values significantly lower than 0.5. Genetic analysis of a Panamanian population of the leaf-cutter ant Acromyrmex octospinosus showed that queens mate with at least 4 to 10 males. The detected (minimum) genetically effective paternity of nestmate females was 3.9 and estimates of mean relatedness among nestmate females were ca. 0.33. This implies that multiple queen-mating in Acromyrmex octospinosus reduces relatedness to 44% of the value in full-sib colonies (0.75), realizing 84% of the maximum reduction (to 0.25) that would be obtained with an infinite number of matings. Queens of Panamanian Acromyrmex octospinosus mate with more males than sympatric queens of Atta colombica, which is contrary to the positive relationship between queen-mating frequency and colony size found across more distantly related ant species. Possible selective forces that maintain high queen-mating frequencies in leaf-cutter ants are discussed.