Induction of sexual reproduction in Spirogyra clones - does an universal trigger exist?

Species identification of the ubiquitous green algae genus Spirogyra LINK based on the traditional morphological species concept requires sexual reproduction stages including ripe hypnozygotes. Since these stages are only infrequently observed in nature, an artificial onset of the sexual phase in the laboratory would be most welcome. We therefore tried to induce conjugation in 95 strains of Spirogyra originating from various European sampling localities. A linear discriminant analysis based on a comparison of abiotic field data between sites with vegetative and conjugating filaments indicated that nitrate deprivation together with elevated organic compounds of N and P promotes conjugation. In the laboratory, altogether 681 experimental setups were conducted with a focus on variation of nutrient supply and irradiance conditions. No general trigger could be found promoting sexual reproduction in the genus, but the importance of certain nutrient ratios according to specific ecological demands seems likely. Besides nitrate depletion, also red, green and white light sometimes yielded in sexual reproduction stages, whereas ultraviolet radiation and blue light never resulted in hypnozygote formation. Detailed mechanisms of recognition between the compatible filaments are still unknown; apomixis and/or heterothallism might be an explanation for the low success rate in sex induction.