Evolutionary lability of host associations promotes phylogenetic overdispersion of co-infecting blood parasites

1. Co-infections with multiple parasite taxa are ubiquitous in nature and have the potential to impact the co-evolutionary dynamics between host and parasite, though patterns of phylogenetic community structure of co-infecting parasites and the processes that generate these patterns have rarely been studied across diverse host-parasite communities. 2. Here, we tested for the roles of host and parasite evolutionary history as well as environmental variables as drivers of phylogenetic community structure among co-infecting haemosporidian (malaria) parasites and their avian hosts in the North American boreal forest, a region characterized by an extraordinarily high blood parasite co-infection rate. 3. We used multiple methods to identify non-random patterns of co-infection among parasite species and determined whether these patterns were influenced more by co-evolutionary host associations or environmental variables. We used modelbased approaches to test whether parasites that occurred together in a single host individual exhibited phylogenetic clustering or overdispersion. Lastly, we tested whether the observed phylogenetic community structure could be explained by parasites having convergently evolved similar patterns of host associations. 4. We found that haemosporidian parasite co-infections occurred at a high frequency in the boreal forest system and that parasite taxa co-occurred in significantly nonrandom patterns within host individuals and among host species. Parasite taxa that occurred in co-infections tended to be phylogenetically overdispersed. We show that this pattern of phylogenetic overdispersion can be attributed largely to the effect of evolutionarily labile, convergent host associations that have resulted in the pool of parasites that have the potential to infect a given host consisting nearly exclusively of distantly related lineages. 5. Our findings illustrate that environmental filtering of convergent traits can produce phylogenetically overdispersed communities, even at the level of co-infecting parasites within an individual host organism. Broadly, this analysis illustrates how co-evolutionary history can have a strong influence on the modern phylogenetic community assembly of diverse host-symbiont communities.