Homoplasy rather than intergeneric hybridisation explains mammillose leaf cells in Schistidium mammillosum sp. nov. (Grimmiaceae)

We present a peculiar new species in the Grimmiaceae, Schistidium mammillosum, which is distinct in having mammillose leaf cells and pluristratose lamina. These features are rare in mosses but shared by a few Grimmia species including sympatric G. alpestris. Sporophyte characteristics of the new taxon, in contrast, indicated a species of Schistidium and suggested a possible intergeneric hybrid origin. Nuclear ITS and plastid rps4‒trnF sequence data consistently resolved the new taxon within species of Schistidium, thus contradicting the hybridogenic hypothesis. We suggest that the shared feature of mammillose cells evolved independently in the two genera in adaptation to similar environmental conditions, possibly prolonged snow cover. Supported by morphological similarity, our phylogenetic analyses indicate a sister relationship of S. mammillosum and S. marginale in the Confertum clade. Schistidium mammillosum colonises siliceous rocks in the alpine and nival zone and is to date known from the French Alps and a single locality in Switzerland.