Two fungal symbioses collide: endophytic fungi are not welcome in leaf-cutting ant gardens

Abstract

Interactions among the component members of different symbioses are not well studied. For example, leaf-cutting ants maintain an obligate symbiosis with their fungal garden, while the leaf material they provide to their garden is usually filled with endophytic fungi. The ants and their cultivar may interact with hundreds of endophytic fungal species, yet little is known about these interactions. Experimental manipulations showed that (i) ants spend more time cutting leaves from a tropical vine, Merremia umbellata, with high versus low endophyte densities, (ii) ants reduce the amount of endophytic fungi in leaves before planting them in their gardens, (iii) the ants' fungal cultivar inhibits the growth of most endophytes tested. Moreover, the inhibition by the ants' cultivar was relatively greater for more rapidly growing endophyte strains that could potentially out-compete or overtake the garden. Our results suggest that endophytes are not welcome in the garden, and that the ants and their cultivar combine ant hygiene behaviour with fungal inhibition to reduce endophyte activity in the nest.

Figure 1

Ants reduce G. cingulata associated with leaves before planting the leaves in their fungal gardens. (a) Mean (±s.e.) proportion of leaf pieces that were colonized by G. cingulata in treatments where leaf pieces were prepared by ants or not prepared by them (without surface sterilization). (b) A photo of plates from assays of (i) prepared and (ii) non-prepared leaf pieces. The large, white halos around leaf pieces are G. cingulata. In (i), two leaf pieces with small white growth show pieces that were colonized by the ants' slower-growing cultivar L. gongylophorus.

Figure 2

Most endophyte species show reduced growth in the presence of the fungal cultivar, with faster-growing endophytic species showing a greater reduction than slower-growing endophytes. (a) Percentage decrease in colony diameter from 32 endophyte strains of 7 species correlates to endophyte growth rate; r²=0.29, p=0.002. (b) Comparison of Colletotrichum sp.1 in the (i) presence and (ii) absence of the fungal cultivar after 48 hours growth. (c) Comparison of Endomelanconiopsis endophytica (Rojas et al. 2008) in the (i) presence and (ii) absence of the fungal cultivar after 72 hours growth.