Evolution and morphology of lycophyte root systems

Thesis

The evolution of plant roots transformed the Earth’s surface, engineering new ecosystems and environments, and providing the nutrient and water uptake, as well as anchorage necessary to support the first trees. The lycophytes (clubmosses) were the first group of plants to evolve roots and the study of their morphology and evolution has been a major goal for evolutionary scientists working on both extant and extinct lycopsids. The aim of the research described in this thesis is to increase our understanding of both the morphology and evolution of lycopsid roots. This is achieved by presenting three papers on the theme of root morphology and evolution. First, I report the discovery of root hairs on extinct stigmarian rootlets, highlighting the conserved morphology of all Isoetalean rootlets. Second, in my discovery of the oldest fossilized root meristem, I illustrate how the interpretation of exceptionally well preserved fossils can change the way we think about the evolution of development of living plant roots. Third, I identify that the rootlets of Isoetes echinospora and roots of Selaginella moellendorffii have similar gene expression profiles. The new results reported in this thesis taken together with a review of the literature of extant and extinct lycopsid rooting structures, enabled me to identify two contrasting evolutionary patterns: conservatism of lycopsid roots, and huge disparity in the structures to which roots are attached. The highly conserved nature of lycopsid roots, supported by the new data presented in this thesis, is consistent with the hypothesis that all lycopsid roots are homologous (described as the lycopsid root hypothesis). In recognising the homology of lycopsid roots, and the two contrasting patterns of rooting structure evolution, the research presented in this thesis makes a significant contribution to our understanding of the morphology and evolution of lycopsid roots.

Authors: Hetherington, A

• DOI: 10.5287/ora-qzq5pynbg
• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford