PT  - JOURNAL ARTICLE
AU  - Marco Archetti
AU  - J. Benjamin Miller
AU  - Douglas W. Yu
TI  - Evolution of passwords for cost-free, honest signalling between symbionts and hosts
AID  - 10.1101/065755
DP  - 2016 Jan 01
TA  - bioRxiv
PG  - 065755
4099  - http://biorxiv.org/content/early/2016/07/25/065755.short
4100  - http://biorxiv.org/content/early/2016/07/25/065755.full
AB  - Honest communication between potential partners with conflicting interests is generally thought to require costly signals. Costly signalling can explain partner choice when it is possible to link a strategic cost to an individual's quality, like in mate choice. However, in mutualisms, it is usually impossible to link a cost to the likelihood that a potential partner will behave cooperatively in the future. In fact, signals like Nod factors in rhizobial bacteria, which form symbioses with leguminous plants, are evidence of cost-free, honest signals in situations of potential conflict. How can such a signalling system evolve? We use a population-genetics model to show that a cost-free, honest signal can evolve when the receiver is under soft selection, which is when high juvenile mortality does not lead to a corresponding reduction in fitness, a common occurrence in many species. Under soft selection, senders evolve increasingly complex messages of identity, a system akin to a password or a lock and key. Thus, a symbiont can signal that it shares a coevolutionary history with a potential host, and if that history is mutualistic, then the host can believe that the symbiont is mutualistic. Password signalling might also explain the evolution of complex species-recognition signals in mate choice and in the acquisition of defensive symbionts.