Description of a new antibiotic producing caste in leafcutter ants

Leafcutter ants live in a very specific symbiosis with some fungi they harvest and different bacteria protecting those fungi from pathogens by producing antibiotics. Some of the ants´ workers are completely covered with such white, exosymbiotic, filamentous bacteria. These workers are not well investigated and there is no prove if they represent an own caste within the leafcutter ants´ division of labour. In order to obtain a more precise picture in that respect three kinds of experiments were done with a colony of Acromyrmex octospinosus. In the first behavioural experiment the preference of location of these workers was distinguished from their sisters by scan sampling. Furthermore the distribution of the ants’ body size was investigated to get results concerning the homogeneity within the castes. Last, the composition of the ants biofilm was investigated by next generation sequencing to find out if bacterial taxa capable of producing antibiotics are present or not. Antibiotic producing bacteria would indicate a special function for this caste. All our findings could show that these workers covered with bacteria differ from their sisters in behaviour as well as in body size. Furthermore we could show that in the biofilm of A. octospinosus a high level of Pseudonocaria exists, known to be able to produce antibiotics. All these results together lead to the conclusion that those workers covered with white bacteria can be described as an own caste. Because of their function one would describe them as medica caste.