Identification of 13 Spirogyra species (Zygnemataceae) by traits of sexual reproduction induced under laboratory culture conditions

The genus Spirogyra is abundant in freshwater habitats worldwide, and comprises approximately 380 species. Species assignment is often difficult because identification is based on the characteristics of sexual reproduction in wild-collected samples and spores produced in the field or laboratory culture. We developed an identification procedure based on an improved methodology for inducing sexual conjugation in laboratory-cultivated filaments. We tested the modified procedure on 52 newly established and genetically different strains collected from diverse localities in Japan. We induced conjugation or aplanospore formation under controlled laboratory conditions in 15 of the 52 strains, which allowed us to identify 13 species. Two of the thirteen species were assignable to a related but taxonomically uncertain genus, Temnogyra, based on the unique characteristics of sexual reproduction. Our phylogenetic analysis demonstrated that the two Temnogyra species are included in a large clade comprising many species of Spirogyra. Thus, separation of Temnogyra from Spirogyra may be untenable, much as the separation of Sirogonium from Spirogyra is not supported by molecular analyses.