GATEKEEPERS IN ARMS RACE
An article by Donaldson et al (1) has broad implications for the concept of co-evolutionary arms races between parasites and their hosts. It reports that Bacteroides fragilis determines what other bacteria can or cannot colonize a mammalian gut.
Having one organism in a host act as a gatekeeper, determining what other organisms can or cannot infect that host, means that the major species interactions are not only between parasites and their hosts, but are also between parasites and gatekeepers as well as between gatekeepers and hosts.
The substantial literature on parasite-host arms races now needs to be reconsidered in terms of three species, not two.
Parasite and host species not only have to interact with each other, they both also have to interact with gatekeeper species. This makes the gatekeeper species an integral, if not the main, participant in the co-evolutionary arms race.
Not all parasite-host interrelationships have gatekeepers, but the presence of gatekeepers was probably not considered by authors of arms race articles. From now on it should be.
1. G. P. Donaldson et al., Science 360, 795 (2018).