Figure 2

Gene Ontology terms relevant to three phases of symbiotic nutrient exchange. Processes associated with phases I and II of nutrient exchange are described by GO terms from the "GO: 0008150 biological_process" ontology. Terms at the top of the diagram describe higher level processes, terms in the middle represent symbiont processes, and terms at the bottom characterize host processes. Functions associated with phase III are described with GO terms from the "GO: 0003674 molecular_function" ontology that describe nutrient uptake irrespective of symbiotic partner. In the GO, term relationships take the form of a directed acyclic graph (DAG), similar to a hierarchy, except that a given term can have multiple parent terms or multiple child terms. Here, for simplicity, only selected terms are shown, and only a subset of the parent-child relationships are depicted; arrows symbolize GO "is_a" and "part_of" relationships (for more information on term relationships and other aspects ontology structure, i.e. "is_a", "part_of", and "regulates," see [9]). Some dashed arrows are used to enhance readability. GO terms highlighted by dark ovals represent GO terms also shown in Figure 1, and terms filled with grey can be found in the text.