Revision of Batrachospermum section Macrospora (Batrachospermales, Rhodophyta) with the establishment of the new genus Montagnia

To resolve the paraphyly of Batrachospermum, the sections of the genus have been methodically investigated using DNA sequence data and morphology; this has resulted in the raising of many sections to genus status. Phylogenetic analyses of combined rbcL and COI-5P showed Batrachospermum section Macrospora to be a well-supported clade. We propose Montagnia gen. nov. Montagnia is distinguished from other sections or genera of the Batrachospermales by the following characters: plants irregularly branched; carpogonial branches long and straight and well differentiated from fascicles, bearing involucral filaments in a crown pattern; enlarged pit connections in the cells of carpogonial branches; and pedunculate carposporophytes with large carposporangia. Within the new genus, we re-evaluated the characters used to circumscribe species by examining type specimens as well as samples from North and South America. Of the three currently accepted species attributable to Montagnia, we recognised two species: M. macrospora-type species (proposed synonyms Batrachospermum equisetifolium, B. hypogynum and B. macrosporum) and M. australis (synonym Batrachospermum australe). Morphological characters did not distinguish the two species due to variation within and among samples. The two species can be differentiated only using DNA sequence data and geographic distribution. Montagnia australis is restricted to North America; whereas, M. macrospora is a widespread pantropical species occurring in South America and Asia. Although the intraspecific divergence observed in M. macrospora is the highest reported for members of Batrachospermales, species delimitation methods did not suggest that more than one species is justified.