Do vertically transmitted symbionts co-existing in a single host compete or cooperate? A modelling approach.

When several symbionts infect simultaneously the same host (multiple infections), the interactions between them affect the dynamics of the symbiotic population. Despite their widespread occurrence, associations with multiple vertically transmitted symbionts have attracted little attention. Vertical transmission tends to homogenize the symbiotic population because of the bottleneck that occurs at transmission. However, fitness advantages conferred on the host by the different symbionts or the induction of reproductive manipulations can make it possible for multiple infections to persist. We used a matrix population model to understand the kind of interactions that can emerge between vertically transmitted symbionts in established multiple infections. Selection acts only to maximize the production of multiply-infected offspring. For a wide range of parameters, this condition allows cooperation between symbionts to be selected for, through their co-transmission, even when it generates additional costs for female fecundity, a reduction in individual transmission, or affects the dependence upon other symbionts.