University of California - Santa Barbara Santa Barbara, California, United States
Abstract: Microorganisms have lived in symbiosis with plants for millions of years. Despite the prevalence and persistence of these relationships over evolutionary time, our understanding of what drives plant-associated microbial diversity at broad spatial scales is still limited. Here, we explored how the species richness and composition of foliar fungal endophytes, fungi that live inside plants, varied across a broad latitudinal gradient (9°N-64°N) in North America. We used next-generation sequencing to characterize endophyte communities in 443 plant genera (n = 1,657 plant individuals) that spanned 20 sites.
We asked whether the species richness (alpha diversity) of endophyte communities increased towards the equator in accordance with the latitudinal diversity gradient (LDG), the widely observed pattern where species number increases towards the tropics. Endophyte richness varied bimodally as a function of latitude being greatest in the tropics, consistent with LDG, and was also pronounced, to a lesser degree, at mid-latitudes in temperate conifer forests. Differences in temperature, precipitation, and the leaf environment explained the presence of these two peaks in endophyte richness.
We also explored how endophytes varied in species composition (beta diversity) and among host species (host specificity) as a function of latitude. Differences in endophyte composition among host species, or beta diversity, increased towards temperate regions. Similarly, endophytes in temperate plant communities occupied a lower proportion of the host community, or had greater host specificity, than endophytes in sub-tropical and tropical plant communities. We hypothesize that species-rich host communities in the tropics are barriers to the evolution of host specificity because passively dispersed endophytes cannot reliably colonize the same host species in heterogenous host landscapes.
In conclusion, the alpha diversity of endophytes may respond to climate and the leaf environment, while the beta diversity of endophytes reflects host specificity and plays a major role in driving contrasting patterns of endophyte community structure as a function of latitude.
