Induction of sexual reproduction reveals the presence of heterothallic Spirogyra strains (Zygnematophyceae, Streptophyta)

SUMMARY Conjugating algae perform characteristic sexual reproduction through conjugation by gametes without flagella. These algae sexually reproduce through the formation of zygospores from single clones (homothallic) and through the formation of zygospores between different clones (heterothallic). The conjugating algae genus Spirogyra is generally found in freshwater environments, and its conjugation process has been well investigated. However, due to experimental difficulties in inducing conjugation, heterothallism has not been described in Spirogyra . Herein, we reported, to the best of our knowledge, the first discovery of heterothallic Spirogyra strains following the induction of conjugation. The conjugation of the strains of Spirogyra fluviatilis required two clearly genetically differentiated strains. Our mating experiments with these strains also demonstrated that their sexuality was genetically determined and that the F1 strain from germinated zygospores conjugated with only one parental strain. Moreover, phylogenetic analyses showed that these heterothallic strains were phylogenetically related to their homothallic relatives. In this study conducted on Spirogyra , the induction of conjugation in heterothallic strains facilitated our understanding of their diversity.