The symbiosis between Philidris ants and the ant-plant Dischidia major includes fungal and algal associates

Dischidia major is an epiphyte with pitcher leaves that serve as nests for ants. We investigated this ant-plant symbiosis in two sites in southeastern Thailand, Rayong and Trat, using a morphological and molecular approach. In our study sites, D. major was colonized by one monomorphic ant species of genus Philidris. The inner surface of the pitcher leaves had a black and green lining composed of intermingled coccoid cells and filaments of algae and fungi, reminiscent of a biofilm structure. Microscopic investigation of the algae suggested they belonged to Trebouxia (coccoid cells) and Trentepohliaceae (filaments). Molecular investigation of environmental samples and pure cultures of the fungi revealed five species of Chaetothyriales and four species of Capnodiales, among which two have already been isolated from ant-plant symbioses in Africa and South America and five were described species known from various environments around world. One appears to be an undescribed species. Thus, most fungal associates were likely ubiquitous species. Our study highlights the need to include the identity and functional ecology of microbes in studies of the evolutionary and functional ecology of ant-plant symbioses.