Yakushimabryum longissimum (Pylaisiadelphaceae) gen. & sp. nov., from the Yakushima Island, Japan

Yakushimabryum longissimum gen. & sp. nov. (Pylaisiadelphaceae, Musci), is described from Yakushima Island, Japan, on the basis of morphology and rbcL data. It is characterized by more or less glossy, yellowish green living plants, naked dormant branch buds, absence of costa in the leaves, smooth and linear leaf lamina cells, alar groups composed of a number of quadrate cells arranged in a scalariform manner, ascending branches that are often elongate into slender tips, and filamentous gemmae consisting of smooth, long-rectangular cells. Molecular phylogenetic analyses suggest an affinity neither to Clastobryopsis nor Gammiella, both of which grow in similar habitats and share alar morphology and shape of propagules, but to Isocladiella and Isopterygium, both of which have filamentous pseudoparaphyllia and different types of propagules. A key to the Japanese genera of Pylaisiadelphaceae is provided.