THE NEW GENUS YONAGUNIA KAWAGUCHI ET MASUDA (HALYMENIACEAE, RHODOPHYTA), BASED ON Y. TENUIFOLIA KAWAGUCHI ET MASUDA SP. NOV. FROM SOUTHERN JAPAN AND INCLUDING Y. FORMOSANA (OKAMURA) KAWAGUCHI ET MASUDA COMB. NOV. FROM SOUTHEAST ASIA

Yonagunia Kawaguchi et Masuda, gen. nov. (Halymeniaceae, Rhodophyta) is proposed to accommodate a new species, Yonagunia tenuifolia Kawaguchi et Masuda and the species currently known as Prionitis formosana (Okamura) Kawaguchi et Nguyen. Based on auxiliary cell ampullar features, Yonagunia is included in the group of genera with the simplest type of ampulla (the Grateloupia type) that comprises Dermocorynus, Grateloupia, Kintokiocolax, Phyllymenia, and Zymurgia. However, Yonagunia differs from these genera in the behavior of cells in the ampullar filaments immediately after diploidization, most cells of the primary and secondary filaments simultaneously dividing to form grape-like clusters of small globular cells that subsequently elongate and produce involucral filaments to laxly surround the maturing carposporophyte. Yonagunia is resolved by our rbcL gene sequence analyses as one of five monophyletic clades within the Halymeniaceae (an Aeodes/Pachymenia, a Polyopes, a Carpopertis /Cryptonemia/Halymenia, a Yonagunia, and a Grateloupia clade) that is positioned as sister to the Grateloupia clade. Carpogonial branch apparatuses are identified as a potential taxonomic significance on the same level as auxiliary cell ampullae.