Intra vs extracellular mycorrhizal mutualisms

Pairs of traits have evolved across diverse nutritional mutualisms; among these is the establishment of partnerships that cross cell membranes 

Sets of divergent traits have evolved in the nutritional symbioses between plants and mycorrhizae, invertebrates and algae, and within lichens: 1) obligate vs facultative relationships, 2) intracellular vs extracellular associations, and 3) vertical vs horizontal transmission of symbionts. These traits affect coevolution between symbiotic partners however it is unclear how they relate to nutrient exchange within the holobiont. Further, the genetic underpinnings of these divergent traits are poorly understood. In a greenhouse experiment, we inoculated Populus trichocarpa (California poplar) propagules with four mycorrhizal treatments: no mycorrhizae, arbuscular mycorrhizae (intracellular), ectomycorrhizae (extracellular), or both types of mycorrhizae. We applied isotope tracers to specialized planter boxes to quantify the transfer of nitrogen and phosphorous from associated mycorrhizae to the plant host. Further, we are comparing gene expression across treatments to identify genes involved in the establishment and maintenance of symbiosis with each type of mycorrhizae as well as genes that correlate with nutrient exchange.