Reproductive isolation by sex pheromones in the Closterium peracerosum-Strigosum-littorale complex (Zygnematales, Charophyceae)

The Closterium peracerosum-strigosum-littorale (C. psl.) complex consists of unicellular algae and is known to be composed of several reproductively isolated mating groups of heterothallic strains. Group I-E is completely isolated from mating groups II-A and II-B, groups II-A and II-B are partially isolated from each other, and only mating-type plus (mt(+)) cells of group II-A and mating-type minus (mt(-)) cells of group II-B form zygotes. Based on the alignment of 1506 group I introns, significant phylogenetic relationships were observed among mating groups II-A and II-B, while mating group I-E was distant from groups II-A and II-B. Sexual cell division in both mating-type cells of group II-A was stimulated in conditioned media in which cells of group II-B had been cultured. When mt(-) cells of group II-B were stimulated in conditioned medium derived from group II-A, mt(+) cells of group II-B did not respond to the conditioned medium. Conditioned media derived from group I-E did not exhibit sexual cell division (SCD)-inducing activity against any strain except those within its own group. From the alignment of deduced amino acid sequences from orthologous protoplast-release-inducing protein (PR-IP) Inducer genes, we detected a significant similarity among groups II-A and II-B, and mating group I-E had low similarity to other mating groups. The existing degree of reproductive isolation can be partially explained by differences in molecular structures and physiological activities of sex pheromones of these heterothallic mating groups.