A mutualistic interaction between a fungivorous nematode and a fungus within the endophytic community of Bromus tectorum

Abstract In its invaded range in western North America, Bromus tectorum (cheatgrass) can host more than 100 sequence-based, operational taxonomic units of endophytic fungi, of which an individual plant hosts a subset. Research suggests that the specific subset is determined by plant genotype, environment, dispersal of locally available endophytes, and mycorrhizal associates. But, interactions among members of the endophyte community could also be important. In a sampling of 63 sites throughout the invaded range of B. tectorum, a fungivorous nematode, Paraphelenchus acontioides , and an endophyte, Fusarium cf. torulosum , were found together in two sites. This positive co-occurrence in the field led to an experimental investigation of their interaction and its effects on relative abundances within the endophyte community. In greenhouse and laboratory experiments, we determined first that P. acontioides preferred F. cf. torulosum to other endophytes, and secondly that the relative abundance of F. cf. torulosum within the endophyte community was increased by the nematode in experimental plants. Taken together our results suggest that the fungivorous P. acontioides uses living plants to cultivate or increase the relative abundance of its preferred fungus. Surprisingly, host plant growth was unaffected by this endophytic, cultivation-based mutualism between a nematode and a fungus.