Morphology and phylogenetic position of a mat-forming green plant from acidic rivers in Japan.

A green plant, which we have named "Misuzugoke", was found in acidic rivers in Nagano Prefecture, Japan, where it forms macroscopic mats or aggregates. The field-collected aggregates were nearly hemispherical and were composed of compact branched filaments radiating from a central base. The cells of the plant contained a single nucleus and numerous discoid chloroplasts lacking pyrenoids. Due to the plant's simple filamentous organization and absence of reproductive organs, its classification as an alga or a higher plant could not be determined from the field-collected material. When grown for more than 3 months on neutral agar medium (pH 7.0) containing hormones, leafy shoot buds characteristic of bryophytes appeared on the plant. A partial sequence of the plant's rbcL gene, which encodes the large subunit of ribulose-1, 5-bisphosphate carboxylase/oxygenase was 98% similar to that of Dicranella heteromalla (Hedw.) Schimp. (Haplolepideae, Bryopsida). Phylogenetic analyses based on rbcL gene sequences strongly indicate that Misuzugoke is positioned within the Haplolepideae. This mat-forming green plant is therefore considered to be a reduced form of moss in the Haplolepideae.