Maternal cuticle protects moss offspring from dehydration

Protection against the desiccation is critical for the survival of terrestrial plants. An array of adaptations has evolved that decrease water loss from plant bodies (e.g. cuticle, bark, trichomes, scales). Of these structures, the plant cuticle is ubiquitous, occurring in taxa from all major lineages of land plants, e.g. mosses, ferns, gymnosperms, Ginkgo and angiosperms. The cuticle consists of layers of the biopolymers cutin/cutan and waxes that cover and permeate the outermost layers of the cell wall, decreasing water loss.

Cuticle function relating to water loss has been studied primarily in angiosperms and less so in other plant lineages. Cuticles on the leafy gametophytes of mosses may also function in decreasing water loss. A new paper in Annals of Botany demonstrates that the cuticle on the maternal moss gametophyte plays a critical role in reducing dehydration stress on developing sporophyte offspring. Without the calyptra cuticle, dehydration during early development disrupts sporophyte maturation, resulting in decreased survival, increased tissue damage, incomplete sporophyte development, more peristome malformations, and decreased spore production. Thus the protection of the calyptra cuticle is crucial for maximizing sporophyte fitness and provides a functional explanation for calyptra retention across mosses. The cuticle on the maternal calyptra is analogous to the cuticle layers provided by the maternal sporophyte of angiosperms on fruits and seeds that protect these organs from dehydration. Thus the moss calyptra may represent the earliest occurrence of maternal protection via structural provisioning of a cuticle in green plants.

Dehydration protection provided by a maternal cuticle improves offspring fitness in the moss Funaria hygrometrica. Ann Bot (2013) 111 (5): 781-789. doi: 10.1093/aob/mct033
In bryophytes the sporophyte offspring are in contact with, nourished from, and partially surrounded by the maternal gametophyte throughout their lifespan. During early development, the moss sporophyte is covered by the calyptra, a cap of maternal gametophyte tissue that has a multilayered cuticle. In this study the effects on sporophyte offspring fitness of removing the maternal calyptra cuticle, in combination with dehydration stress, is experimentally determined. Using the moss Funaria hygrometrica, calyptra cuticle waxes were removed by chemical extraction and individuals were exposed to a short-term dehydration event. Sporophytes were returned to high humidity to complete development and then aspects of sporophyte survival, development, functional morphology, and reproductive output were measured.
It was found that removal of calyptra cuticle under low humidity results in significant negative impacts to moss sporophyte fitness, resulting in decreased survival, increased tissue damage, incomplete sporophyte development, more peristome malformations, and decreased reproductive output. This study represents the strongest evidence to date that the structure of the calyptra cuticle functions in dehydration protection of the immature moss sporophyte. The investment in a maternal calyptra with a multilayered cuticle increases offspring fitness and provides a functional explanation for calyptra retention across mosses. The moss calyptra may represent the earliest occurance of maternal protection via structural provisioning of a cuticle in green plants.