2012 has started out super well for me professionally. The second chapter of my dissertation was just published!
Budke, Jessica M., Bernard Goffinet, and Cynthia S. Jones. 2012. The cuticle on the gametophyte calyptra matures before the sporophyte cuticle in the moss Funaria hygrometrica (Funariaceae). American Journal of Botany 99(1): 14-22.
The short summary is that the calyptra is a little cap of female gametophyte tissue that covers the moss sporophyte apex during development. If it is removed from the apex early during development the sporophyte apex dries out and the sporophyte may die. I confirmed a long-proposed idea that the calyptra is covered by a waxy cuticle. I found that the calyptra cuticle is significantly thicker than the cuticle on either the leafy gametophyte or sporophyte. These anatomical differences may point to a functional role of the calyptra cuticle in desiccation protection of the underlying sporophyte.
This paper is the next part of that story. The calyptra cuticle develops when the calyptra is quite small and completely surrounds the sporophyte embryo. Even at this early stage of development, the calyptra cuticle is multilayered and thick. At this stage the moss sporophyte cuticle consists of a single thin cuticle layer. The sporophyte cuticle develops as a wave from the bottom up, adding layers and thickening across the nine developmental stages I examined.
I think that this figure ended up being a good summary of the development of the cuticle layers.
Figure 7. (From the AJB paper)
Diagram illustrating cuticle layers on the calyptra and sporophyte for stages 1 to 9 (Table 1), with numbers of the developmental stages identified beneath each diagram. The calyptra is covered by a multilayered cuticle at all nine developmental stages. Numbers inside the sporophytes indicate the different number of cuticle layers present at each region of each developmental stage. Abbreviations: C, calyptra; CL, cuticular layer; CWP, cell wall projections of the cuticular layer; eDCP, electron-dense cuticle proper; eLCP, electron-lucent cuticle proper; S, sporophyte.
The idea is that the calyptra has a complex cuticle even at early developmental stages. This early cuticle development may enable the calyptra to protect the young sporophyte from drying out. During early development the sporophyte cuticle is less complex and thus the sporophyte may require protection. Later in development, the moss sporophyte develops its own cuticle and the protection of the calyptra may no longer be necessary.
Overall, I think that my studies are adding to the scientific evidence that points to the importance of the calyptra for the development of the moss sporophyte. Stay tuned for the final chapter where I experimentally removed the calyptra cuticle to get at its importance for sporophyte development and fitness. Happy New Year!
The formal abstract for the paper is below.
• Premise of the study: In vascular plants, leaf primordia prevent desiccation of the shoot apical meristem. Lacking leaves, the undifferentiated moss sporophyte apex is covered by the calyptra, a cap of maternal gametophyte tissue that is hypothesized to function in desiccation protection. Herein, we compare cuticle development on the calyptra and sporophyte to assess the calyptra’s potential to protect the sporophyte from desiccation. As the first comprehensive study of moss sporophyte cuticle development, this research broadens our perspectives on cuticle development and evolution across embryophytes.
• Methods: Calyptrae and sporophytes at nine developmental stages were collected from a laboratory-grown population of the moss Funaria hygrometrica. Tissues were embedded, sectioned, then examined using transmission electron microscopy. Epidermal cells were measured for thickness of the cuticle layers, cell wall thickness, and lumen size.
• Key results: The calyptra cuticle develops precociously and reaches maturity before the sporophyte cuticle. Calyptrae are covered by a four-layered cuticle at all stages, whereas sporophyte cuticle maturation is delayed until sporangium formation. The development and thickening of the sporophyte cuticle occurs in an acropetal wave.
• Conclusions: A multilayered calyptra cuticle at the earliest developmental stages is consistent with its ability to protect the immature sporophyte from desiccation. Young sporophytes lack a complex cuticle and thus may require protection, whereas in older sporophytes a mature cuticle develops. The moss calyptra is not a vestigial structure, but rather the calyptra’s role in preventing desiccation offers a functional explanation for calyptra retention during the 450 Myr of moss evolution.