SPOKE Edge Directionality

Although many edge types are currently displayed with arrows in the UI, they are not directional in the sense discussed in Daniel's Hetionet v1.0 paper (Himmelstein et al. eLife 2017, see Directionality, p. 17 of PDF). In that paper, only one edge type is described as directional and shown with arrows in the text: Gene→regulates→Gene.

Edges such as Compound-binds-Protein or Disease-causes-Symptom put the two types of nodes in a specific order, but we could just as easily name them the other way, with the nodes in the reverse order; still, the relationship between those two nodes would still be the same.

I believe that the edges where directionality is significant and should be honored for searching have these properties:

1. the nodes on either end are of the same type (e.g., both Gene)
2. swapping them gives a different relationship

The edge types for which this is the most obvious are the hierarchical ones, and since there are two edge types in opposite directions between the same pairs, it would be easy (by setting the edge-type Options) to enable going only one way or the other, or both ways, in a single search:

• Anatomy-contains-Anatomy, Anatomy-isa-Anatomy
• Disease-contains-Disease, Disease-isa-Disease
• Organism-contains-Organism, Organism-isa-Organism
• Pathway-contains-Pathway, Pathway-isa-Pathway

Also directional, but requiring thought as to whether searches in the UI should be restricted to the direction, since they don't have a corresponding opposite-direction type:

• Anatomy-partof-Anatomy
• GeneProduct-downregulates-Gene, GeneProduct-upregulates-Gene
  (despite the “gene product” terminology, both end nodes are of type Gene)
• KGene-downregulates-Gene, KGene-upregulates-Gene
• OGene-downregulates-Gene, OGene-upregulates-Gene