Construction of chimaeric gardens through fungal intercropping: a symbiont choice experiment in the leafcutter ant Atta texana (Attini, Formicidae)

Abstract

Interspecies or intraspecies cooperation can be stabilized evolutionarily if choosing partners favor benefi- cial partners and discriminate against non-beneficial part- ners. We quantified such partner choice (symbiont choice) in the leafcutter ant Atta texana (Attini, Formicidae) by presenting the ants in a cafeteria-style preference assay with genotypically distinct fungal cultivars from A. texana and Acromyrmex versicolor. Symbiont choice was measured as the ants' tendency to choose one or more cultivar(s) from several pure (axenic) cultivar fragments and convert a given fungal fragment into a garden. Microsatellite DNA finger- printing enabled us to identify the cultivars chosen by the ants for their gardens. In 91% of the choice tests, A. texana workers combined multiple cultivars into a single inter- cropped, chimaeric garden, and the cultivars coexisted in such chimaeric gardens for as long as 4 months. Coexis- tence of distinct fungal genotypes in chimaeric gardens appears to contradict a recent model of cultivar competition postulating that each cultivar secretes incompatibility compounds harming other cultivars, which presumably would preclude the intercropped polyculture observed in our experiments. Although we found no clear evidence of novel, recombinant genotypes in the experimental chimae- ric gardens, the intercropping of cultivar genotypes may occasionally lead under natural conditions to exchange of genetic material between coexisting cultivars, thus intro- ducing novel cultivar genotypes into the leafcutter symbiosis. Symbiont choice by ants and any competition between coexisting cultivar strains in chimaeric gardens do not appear to operate fast enough in our laboratory assay to convert chimaeric gardens into the monocultures observed for A. texana under natural conditions.