Nematode associates and bacteria in ant-tree symbioses

Ant-plant mutualisms are a prominent phenomenon in tropical rainforest ecosystems. In South East Asia the most important ant-tree symbiosis is the Macaranga complex. Until now it has been known to consist of four obligate and specific partner organisms, trees, ants, endophytic trophobiotic scale insects and endophytic fungi. Here we show that two more organisms are integrated in the system. In the stem domatia, masses of bacteria were cultivated in piles consisting of plant food body remains and ant faeces. They were consumed by masses of specific bacteriovorous nematodes. The nematodes could be identified as rhabditids, mainly Diploscapter sp., as well as a few Sclerorhabditis specimens belonging to two new species. These genera are known to be myrmecophilous and suspected of being consumed and digested by the ants. We hypothesize that with this new type of two-step association the ants obtain and recycle scarce nutrients. This is important in a system where the specific ants are highly adapted to the association, and depend exclusively on plant resources as they do not hunt and forage off the plant. The nematodes are even vertically transferred by the swarming queens and therefore present in new claustral colony foundations. Nematode breeding was found in three further, but not in all investigated tree-ant associations. These observations underline the still largely unexplored role of micro-symbionts in insects, and a potential new source of complexity in the trophic interactions in tropical ecosystems.