Discussion:

An outstanding goal in evolutionary ecology is understanding how reciprocal host and parasite interactions shape distribution of species. For multi-host parasites, parasite eco-evolutionary dynamics depend on many components including environmental tolerance, host compatibility, genetic diversity, and source-sink dynamics (Woolhouse et al. 2001, Dobson 2004, Gandon 2004, Bellis et al. 2020). Although parasite abundance is often constrained by local environment (Wu et al. 2019), it is rarely considered how parasite evolution may also be indirectly affected by host abiotic environmental tolerance.  Here we present a framework to understand broad-scale interactions between S. hermonthica and its common hosts across abiotic gradients. Combining host distributional and empirical data from diverse populations of a parasitic plant, we find that regional abundance of a particular host is associated with local specialization on that crop by parasites. By controlling host distributions, abiotic gradients may indirectly shape patterns of parasite local adaptation to host communities and consequently, distributions of generalist and specialist parasites.