Fern Family Clade Age and Fungal Pathogen Diversity

The evolutionary causes of contrasting patterns of pathogen diversity in different host clades are poorly understood. This study tested the hypothesis that older host lineages have fewer pathogens than younger lineages by examining the incidence of fungal pathogens in fern families differing in their time of origin over a span of 300 million years. Fern-fungal records were obtained from the United States Department of Agriculture database, and study effort was estimated by the number of citations to each fern species in the Web of Science. Pathogen discovery rate (PDR) within a fern family was measured as the rate at which pathogens were recorded in relation to the number of citations to species within that family. PDR for fungi in the Basidiomycota and Ascomycota showed significant differences among fern families and significant phylogenetic signal in the Basidiomycota. PDR for Basidiomycota was significantly negatively correlated with clade age of the fern family. Generic and species diversity within families affected PDR positively, although the effects of clade age were still significant. Within the Basidiomycota the effects of clade age were largely accounted for by the rust fungi (Pucciniales) which formed the majority of recorded basidiomycete pathogens on ferns. The most parsimonious explanation was that the rust fungi host-shifted onto fern families in the early to mid-Cretaceous, but have rarely moved onto more distantly related and older families. However, several other hypotheses could not be excluded, including loss of specialized pathogens in older clades as a result of resistance evolution.