The influence of host outcrossing on symbiont vertical transmission rates

Background/Question/Methods

Mutualistic heritable symbionts are vertically transmitted from parent to offspring. As a consequence of vertical transmission, symbionts experience few opportunities to sexually reproduce compared to their hosts. The asymmetry in sexual recombination of mutualistic partners presents the possibility of genetic mismatches between host and symbiont. These genetic mismatches could result in a loss of symbiont transmission if symbionts fail to successfully occupy genetically distant host offspring. As transmission rate is one of the main drivers of symbiont persistence, genetic mismatches could ultimately influence symbiont population dynamics. The rate of symbiont transmission is hypothesized to be a function of the genetic distance between parent hosts. However, the shape of the functional relationship between genetic distance of host mating pairs and symbiont transmission rate is currently unknown. To test for the potential non-linear relationship between host mating pair genetic distance and symbiont vertical transmission rates, we conducted experimental gene flow treatments among 14 genetically distant host populations of native North American wild rye grasses (E. virginicus and E. canadensis). Both grasses are host to a vertically transmitted fungal endophyte (genus Epihcloë). Gene flow treatments spanned a wide genetic distance spectrum from self-pollination to interspecific hybridization.

Results/Conclusions

Results suggest that experimental gene flow treatments between hosts of large genetic distances (interspecific gene flow) resulted in lower host fecundity i.e. fewer seeds were generated compared to crosses within host species. In contrast, self-pollination resulted in the highest host fecundity. Reductions in host fecundity as a consequence of extreme outcrossing events presents a possible dead end for vertically transmitted symbionts that rely upon hosts for reproduction. The results also indicate potential for symbiont transmission rate to respond to host out-crossing. These results may be particularly relevant for hosts vulnerable to receiving gene flow from foreign sources such as those that occupy hybrid zones or disturbed non-native communities.