Morphology, taxonomy and phylogenetic positions of two Spirogyra species (Zygnematophyceae, Streptophyta) with replicate and semi-replicate transverse walls from Japan

Characteristics of transverse walls in filaments, and modes of conjugation are essential for species identification in the genus Spirogyra. Although most Spirogyra species have plane transverse walls, species with other types of transverse walls (replicate, semi-replicate and colligate types) have also been described. Recent molecular phylogenetic studies grouped species with replicate transverse walls in a single clade. However, phylogenetic positions of Spirogyra species with semi-replicate and colligate transverse walls have not yet been clarified. We examined vegetative and reproductive traits of two Spirogyra species with replicate and semi-replicate transverse walls collected in Japan, under culture conditions. One of them resembled most S. crassispina in having replicate transverse walls and both scalariform and lateral conjugation, with conjugation tubes separated from the filaments in lateral conjugation. However, these two species differed in their vegetative filaments, zygospores and mesospores; and the molecular phylogenetic analyses placed the Japanese strain with replicate transverse walls in a lineage separate from other previously known species with replicate transverse walls. Vegetative filaments of the other Japanese strain had semi-replicate transverse walls resembling those of previously known species S. unduliseptum and S. narcissiana. This strain differs from S. unduliseptum by the variation in zygospore ornamentation and from S. narcissiana by the presence of three types of conjugation; and it formed an independent lineage that was phylogenetically separated from two Spirogyra lineages characterized by having replicate transverse walls. The two Japanese strains reported on are described herein as S. minuticrassispina sp. nov. and S. tertia sp. nov.

Automated summary

Characteristics of transverse walls and modes of conjugation are crucial for species identification in the genus Spirogyra. This study examined two Spirogyra species with different types of transverse walls collected in Japan and found their distinct vegetative and reproductive traits. Molecular phylogenetic analyses also revealed their separate lineages in the tree of Spirogyra species. The two Japanese strains are described as new species, S. minuticrassispina sp. nov. and S. tertia sp. nov.