Dispersion and colonisation by fungus-growing termites: Vertical transmission of the symbiont helps, but then...?

The fungus-growing termites (Macrotermitinae) have developed an obligate mutualistic symbiosis with fungi (Termitomyces) and, in most cases, the symbiotic partner is collected from the environment upon establishment of a new colony (horizontal transmission). The requirement that partners are able to find and recognize each other after independent reproduction is likely to severely constrain long distance dispersal. In support of this hypothesis, we have recently shown that a single colonisation of Madagascar by fungus-growing termites has occurred. The successful colonizers belong to the genus Microtermes, known to inherit their symbiont from the parental colony (vertical transmission). However, the fungal symbionts of Madagascar were not monophyletic, as expected under strict vertical transmission. Here we further discuss these findings, and we suggest further bottlenecks to dispersion and propose a transient window for horizontal transmission for the otherwise vertically transmitted Termitomyces strains.