Advancements, deficiencies, and future necessities of studying Saprolegniales: A semi-quantitative review of 1073 published papers

Research on the order Saprolegniales (Oomycota) has been an ongoing quest for more than a century. The best studied genera are Saprolegnia and Aphanomyces, known for their pathogenicity on freshwater animals. In this study, we reviewed 1073 papers and 2803 ITS sequences of Saprolegniales to investigate their taxonomy, diversity and potential roles in mainly freshwater ecosytems. We found that, in general, our knowledge on diversity and ecology of Saprolegniales is limited. Neither classic taxonomy nor available molecular techniques have been sufficient to delineate genera and species and show their relative distribution in freshwater-associated habitats. Also, we currently lack a comprehensive understanding of their involvement in carbon turnover and food web dynamics. Finally, due to lack of using high-throughput sequencing techniques, it is not clear how and to what extent communities of Saprolegniales might differ in freshwater econiches. Therefore, we provide a historical perspective on the establishment of Saprolegniales, explain improvements, highlight deficiencies, and finally propose new research avenues for more systematic studies. We conclude that challenges in studying Saprolegniales can be removed by increasing the practicality of classic taxonomy and applying available molecular toolboxes (multi-gene phylogeny and high-throughput sequencing). Additionally, inclusion of Saprolegniales in freshwater carbon cycling should be addressed for their better ecological resolution.