Pythium intermedium, a species complex consisting of three phylogenetic species found in cool-temperate forest ecosystems.

Pythium intermedium plays a vital role in the carbon cycle of cool-temperate forests and is widely distributed in Japan's forest soils. In this study, we performed a phylogenetic analysis of the P. intermedium species complex using DNA sequences from multiple loci. The study included 35 isolates from cool-temperate forest soils, seven known P. intermedium isolates, and six known Pythium attrantheridium isolates. We also performed morphological observations and mating tests. Our results showed that all the isolates formed one large clade but were divided into three subclades. Furthermore, we observed many mating reactions between isolates from different subclades, including between P. attrantheridium and P. intermedium. Therefore, we suggest that P. intermedium, P. attrantheridium, and another phylogenetic species belong to one species complex. This is the first report of a species complex within P. intermedium and will be helpful in understanding the evolution of Pythium species in natural ecosystems.