Cues to the evolution of leaf-cutting ants: Effects on Atta mexicana colonies cultivating fungal symbionts from advanced (Atta) or primitive (Trachymyrmex) ants

There are few empirical data, based on observations at the fully functional level, supporting the supposed mutual adaptations between leaf-cutting ants and their especialized and genetically unique fungal garden symbionts. Experimental fungal symbiont switches between Atta (derived) and Trachymyrmex (basal) provide new evidence of tangible coadaptatations between leaf-cutter ants and their fungi at the organismic level.  These switches also outline specific negative effects that possibly impose severe restrictions on switches to novel cultivars and result on symbiont specificity. Striking effects were observed when Atta mexicana queen-right colonies cultivated a Trachymyrmex zeteki symbiotic fungus. Effects included severely restricted fungus garden size and ant colony growth, and concomitant reductions from normal worker sizes and numbers. Switched colonies lived at least two years, but never surpassed about 250 workers despite being reared under conditions leading to much higher size in colonies cultivating the original fungal symbiont. It is proposed that in the evolution of leaf-cutters from functionally saprophagous attines, more advanced fungal symbionts provided essential nutritional and physiological advantages, that allowed efficient production of thousands and even millions of workers per colony. These empirical data on symbiont switches allow critical testing of hyphoteses on the specific contributions of the highly derived fungi in the symbiosis with leaf-cutting ants.