The inference is based on the homology mappings from Ensembl Compara. Briefly, reactions for which all involved PhysicalEntities (in input, output and catalyst) have a mapped orthologue/paralogue (for complexes at least 1% of components must have a mapping) are inferred to the other species. High level events are also inferred for these events to allow for easier navigation.
More details and caveats of the event inference in Reactome.
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