Plant-associate interactions and diversification across trophic levels

Interactions between species are widely understood to have promoted the
diversification of life on Earth, but how interactions spur the formation
of new species remains unclear. Interacting species often become locally
adapted to each other, but they may also be subject to shared dispersal
limitations and environmental conditions. Moreover, theory predicts that
different kinds of interactions have different effects on diversification.
To better understand how species interactions promote diversification, we
compiled population genetic studies of host plants and intimately
associated herbivores, parasites, and mutualists. We used Bayesian
multiple regressions and the BEDASSLE modeling framework to test whether
host and associate population structures were correlated over and above
the potentially confounding effects of geography and shared environmental
variation. We found that associates' population structure often
paralleled their hosts' population structure, and that this effect is
robust to accounting for geographic distance and climate. Associate
genetic structure was significantly explained by plant genetic structure
somewhat more often in antagonistic interactions than in mutualistic ones.
This aligns with a key prediction of coevolutionary theory, that
antagonistic interactions promote diversity through local adaptation of
antagonists to hosts, while mutualistic interactions more often promote
diversity via the effect of hosts' geographic distribution on
mutualists' dispersal.