Two distinct genetic lineages of Drunella trispina (Uéno, 1928) (Ephemeroptera: Ephemerellidae) cohabit but are reproductively isolated in Kyoto, Japan

Based on the analyses of mitochondrial cytochrome c oxidase ­subunit I (cox1) and nuclear 18S rRNA genes, we found two ­distinct genetic lineages of Drunella trispina (Uéno, 1928), which are ­morphologically indistinguishable at the nymphal stage. These two groups seem to have different habitat niches: one prefers the uppermost reaches, of which Strahler’s stream order is three or less, and the other inhabits relatively lower reaches with a stream order of three or larger. However, they coexist in Kibune Stream (called Kibunegawa), a tributary of the Kamo River, in Kyoto, central Honshu, Japan. The sampling site of Kibunegawa originally had a mountain–stream environment with step-pool configuration (at the beginning of the third order). Over the past 60 years, several small check dams have been constructed along Kibunegawa to prevent sediment erosion, resulting in the formation of larger pools and slow riffles, and the emergence of various secondary microhabitats. The results of the DNA analyses indicated that the two genetic groups in Kibunegawa rarely interbred. Preliminary observation of egg surface morphology suggested a potential diagnosis to distinguish between these two distinct genetic groups.