Micromitriaceae: A new family of highly reduced mosses

Abstract Morphological complexity tends to increase at a macroevolutionary scale among land plants, but reversals or loss of previously acquired complex traits occurs across various lineages and in particular in bryophytes. In mosses reduction may pertain to the leafy gametophyte and the architecture of the sporophyte, and is often considered linked to shifts in habitats. Reduction in morphological complexity may obscure the phylogenetic affinities of the taxa, in particular when multiple derived characters are lost. Ephemeraceae comprise two genera, Ephemerum and Micromitrium , growing typically on disturbed, seasonally moist soils. The hypothesis of a shared ancestry of these genera is drawn from the similarities in their highly reduced morphologies: their ephemeral gametophytes are tiny, bearing few leaves composed of typically smooth cells, and their capsules lack a differentiated lid and are nearly sessile. The family was historically included in Funariidae, but recent phylogenetic inferences revealed that Ephemeraceae likely arose within Dicranidae and may be of polyphyletic origin. Phylogenetic inferences based on eight loci confirm that both genera belong to Dicranidae, and reconstructions of the familial relationships within this subclass corroborate the independent origins of the ephemeral life history and associated character losses, with Ephemerum diverging from a pottiaceous ancestor and Micromitrium sharing a unique common ancestor with Leucobryaceae (Dicranales). Micromitrium is thus excluded from Ephemeraceae and accommodated within its own newly described family, Micromitriaceae. This study strengthens the hypothesis that homoplasy is recurrent throughout the evolutionary history of bryophytes and that phylogenetic inferences from DNA sequences are essential to test the systematic concepts based on morphological characters, and particularly in cases of taxa with the most simple architectures and life histories.