Can endosymbiotic microbes modulate natural selection in plant populations? An example with Lolium perenne and its fungal endophyte

Many plant species are symbiotic with systemic microbes. For example, many grasses are inhabited by fungal endophytes that affect aspects of their host's physiology, morphology, and reproduction. However, there have not been any analyses of the potential effect of endophytes on the strength of phenotypic selection on quantitative traits. Here, a previously published data set on several life history traits measured for two years in a field population of 12-13 Lolium perenne genotypes, each replicated as symbiotic and non-symbiotic plants, was analyzed using the standard Lande-Arnold method of selection analysis. In one year, endophytic symbionts reduced the strength of selection on the number of reproductive tillers when relative fitness was expressed as seed yield. Also, symbionts selected for reduced tiller production when fitness was expressed as mean seed mass. These changes in the strength of selection only occurred when fitness of genotypes when symbiotic was unrelated to fitness of the same genotypes when non-symbiotic. In a second year, when fitness of symbiotic and non-symbiotic groups were significantly correlated, there was no detectable selection on reproductive tiller production. Because the effects of microbial endosymbionts were only shown for one year in a single host population, additional research is needed to better assess how endosymbionts might mediate selective pressures in other natural plant populations.