Coinfection accelerates transmission to new hosts despite no effects on virulence and parasite growth

One of the fundamental aims of ecological, epidemiological and evolutionary studies of host-–parasite interactions is to unravel which factors affect parasite virulence. Theory predicts that virulence and transmission are correlated by a trade-off, as too much virulence is expected to hamper transmission dueowing to excessive host damage. Coinfections may affect each of these traits and/or their correlation. Here, we used inbred lines of the spider- mite Tetranychus urticae to test how coinfection with T. evansi, impacted virulence-transmission relationships, at different conspecific densities. The presence of T. evansi on a shared host did not change the relationship between virulence (leaf damage) and the number of transmitting stages (i.e., adult daughters). The relationship between these traits was hump-shaped across densities, both in single and coinfections, which corresponds to a trade-off. Moreover, transmission to adjacent hosts increased in coinfection, but only at low T. urticae densities. Finally, we tested whether virulence and the number of daughters were correlated with measures of transmission to adjacent hosts, in single and coinfections at different conspecific densities. Traits were mostly independent, meaning that interspecific competitors may increase transmission without affecting virulence. Thus, coinfections may impact epidemiology and parasite trait evolution, but not necessarily the virulence transmission trade-off.