A hundred-year-old question: is the moss calyptra covered by a cuticle? A case study of Funaria hygrometrica

The maternal gametophytic calyptra is critical for moss sporophyte development and ultimately sporogenesis. The calyptra has been predicted to protect the sporophyte apex, including the undifferentiated sporogenous region and seta meristem, from desiccation. We investigate the hypothesis that this waterproofing ability is due to a waxy cuticle. The idea that moss calyptrae are covered by a cuticle has been present in the literature for over a century, but, until now, neither the presence nor the absence of a cuticle has been documented for any calyptra.

The epidermis of the calyptra, leafy gametophyte and sporophyte sporangia of the moss Funaria hygrometrica were examined using scanning and transmission electron microscopy. Thicknesses of individual cuticle layers were quantified and compared statistically. The immunochemistry antibody (LM19) specific for pectins was used to locate cell wall material within the cuticle.

A multi-layered cuticle is present on the calyptra of F. hygrometrica, including layers analogous to the cuticular layer, cell wall projections, electron-lucent and electron-dense cuticle proper observed in vascular plants. The calyptra rostrum has a cuticle that is significantly thicker than the other tissues examined and differs by specialized thickenings of the cuticular layer (cuticular pegs) at the regions of the anticlinal cell walls. This is the first documentation of cuticular pegs in a moss.

The calyptra and its associated cuticle represent a unique form of maternal care in embryophytes. This organ has the potential to play a critical role in preventing desiccation of immature sporophytes and thereby may have been essential for the evolution of the moss sporophyte.