On the frond row : Sporophyte development in the fern Ceratopteris richardii

A major event during land plant evolution was the establishment of a dominant sporophyte generation, which is coupled to the origin of vascular plants approximately 450 million years ago. Many important plant organs like roots, shoots and vasculature only develop within the sporophytic generation. Ferns are unique in having both a free-living gametophytic and a free-living sporophytic generation, providing an excellent experimental model to better understand the developmental necessities of sporophyte development. Therefore, in this thesis, we use and develop the fern Ceratopteris richardii as a model, to better understand sporophyte establishment during embryogenesis and development.From reviewing the literature, it becomes clear that barely any molecular data exists on non-seed plants and non-bryophyte models, whilst those taxa are the most important to understand the evolutionary framework. Subsequently, we compare on a molecular level what the prerequisites are the sporophytic body plan. Thereby, I identify hormonal regulation as a key defining characteristic between the two generations in Ceratopteris. To extend the bioinformatic analysis to experimental work, next, we focus on the plant hormone auxin and its role during development in both the gametophytic and sporophytic generations. We observe that gametophytes respond similarly to bryophytes whilst the sporophytes’ response is similar to that of seed plants. Manipulating auxin accumulation in the gametophyte leads to the ectopic formation of roots and vascular tissue in the gametophyte, suggesting that ontogeny is only correlated to development, and that changes in auxin signalling are an important factor to distinguish the two generations in Ceratopteris.Lastly, we switch gears to embryogenesis as the direct link between the two generations. We compare the reproductive structures in which embryogenesis takes place across all land plants. Embryogenesis takes place in highly divergent structures, and recent advances in Arabidopsis embryo research suggest an important role of mechanical stress in embryogenesis, next to a more canonical form of chemical regulation. Therefore, we characterize embryogenesis in the fern Ceratopteris. Embryogenesis occurs in a very predictable fashion, and patterning appears to happen early. The early patterning is further confirmed by a transcriptome analysis. The polarization of the embryo can be traced back to an asymmetric division in the zygote. We were able to disturb late embryo development by applying exogenous auxin; however, early embryo patterning remained unaffected. Besides, a strong correlation is present between maternal tissue morphology and zygotic division orientation. We explore this correlation in detail with finite-element modelling. The modelling suggests that heterogeneity in cell wall stiffness between the meristem and more differentiated cells generates tissue stresses that we propose to orient the zygotic division in such a way that the future sporophytic root points towards the gametophytic rhizoids, thereby inheriting the axis of development.All these different aspects let us to better understand the developmental principles for sporophyte establishment and development, and view this in a broader evolutionary context.