Permanent spore dyads are not a thing of the past': on their occurrence in the liverwort Haplomitrium (Haplomitriopsida)

The liverwort Haplomitrium gibbsiae is shown to regularly produce spores released in the form of permanent dyad pairs. Developmental studies indicate that the dyads are produced via a unique half-lobed configuration of the developing sporocyte. Many fossil cryptophytes of Siluro-Devonian age, which are clearly embryophytes based on their morphology, contain permanent spore dyads in their sporangia, but this is the first demonstration of their occurrence in a living plant, a species belonging to Haplomitriopsida, which resolves in a clade that is considered to be sister to all remaining liverworts. Dispersed spore-like dyads are found in the rock record as far back as the mid-Cambrian, but most researchers still regard the first occurrence of isomorphic, tetrahedral tetrads in the mid-Ordovician as the benchmark age for the origin of land plants. Regardless of the geological antiquity of the embryophytes, it appears that H.gibbsiae has retained a non-simultaneous form of sporogenesis that may ultimately be traced to a charophytic origin. (c) 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 179, 658-669.