Genetic and morphological variation in the genus <i>Zygogonium</i> (Zygnematophyceae, Charophyta) from localities in Europe and North America and description of <i>Z. angustum</i>, sp. nov.

The globally distributed genus Zygogonium exhibits a narrow environmental range, with 19 morphologically described species. Its molecular characterization is poor, based on a single accession of the type species Z. ericetorum from Austria. We examined the genetic variability, morphology, and ultrastructure of field-collected Zygogonium material from different sampling sites in Austria, Norway, Ireland, Scotland, and the United States. Phylogenetic analysis based on partial sequences of the psbC gene distinguished three well-supported groups and one subgroup. AtpB gene sequences collected from a subset of samples also support this result, while rbcL gene data provided lower support. Group 1A contained the type species Z. ericetorum from Obergurgl/Austria and samples from Kühtai/Austria, Norway, and Scotland. The morphology was characterized by wide vegetative filaments (15-31 μm) and the occurrence of aplanospores with purple residue. Group 1B contained Z. cf. ericetorum from Ireland and Ellmau/Austria. Filaments were in a similar size range (12-30 μm) as in group 1A. This group had round unusual akinetes with green or purple content, had one or two chloroplasts, and was surrounded by a thick cell wall; no aplanospores were observed. Group 2 collected from Ireland had narrower filaments (8-12 μm), cells up to six times longer than wide, and contained elongated aplanospores. Therefore, we have described a new species Z. angustum sp. nov. Group 3 contained Z. cf. ericetorum from Norway and the United States, represented by vegetative filaments with an intermediate width (13-20 μm), but no other distinct morphological features. The morphological and genetic variability observed in Zygogonium is possibly related to habitat and ecology.