Host preference and specialization in the genus Aphanomyces (Oomycetes) from molecular and interaction network insights

Aphanomyces (Saprolegniales, Oomycetes) includes some of the most economically and environmentally devastating pathogens of plants and freshwater animals. Despite research focused on its pathogenicity and dispersal, key biological questions, such as host range and specialization, remain poorly understood. In this study, we investigated the host preference and specialization within Aphanomyces. First, we delineated species boundaries employing delimitation algorithms based on 261 representative sequences of the nuclear ribosomal internal transcriber spacer (nrITS) from 261 samples of 20 described species and several unidentified taxa. Our results suggested that Aphanomyces diversity was previously underestimated, since we identified 14 putative new species. Furthermore, we compiled an interaction database of 1221 host records. Network analyses revealed Aphanomyces as a highly specialized group mainly targeting two clusters of hosts: freshwater animals or plants in wet terrestrial soils. In addition, pathogenic Aphanomyces species appeared to be highly specialized. This study represents the first investigation encompassing the full diversity of Aphanomyces species, not only economically relevant species. Our results set the basis to better understand the evolutionary history of host specialization of harmful oomycete pathogens.