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Developmental programmes in the evolution of Equisetum reproductive morphology: a hierarchical modularity hypothesis
Alexandru M F Tomescu1, Ignacio H Escapa2, Gar W Rothwell3,4
1Department of Biological Sciences, Humboldt State University, Arcata, CA 95521, USA.
Annals of Botany
|April 3, 2017
Summary
A new model proposes that the Equisetum strobilus evolved from sporangiophores developing along internodes, not just at nodes. This phytomer-based perspective explains reproductive morphology across fossil and extant species.
Area of Science:
- Plant evolutionary developmental biology
- Paleobotany
- Plant morphology
Background:
- The origin of the Equisetum strobilus has been debated, with traditional views focusing on node-internode alternation and sporangiophore attachment at nodes.
- Excluded fossils like Cruciaetheca and Peltotheca show reproductive structures suggesting sporangiophore attachment along internodes, challenging established paradigms.
- Lack of mechanistic explanations has stalled discussions on Equisetum strobilus evolution.
Purpose of the Study:
- To propose a mechanism-based hypothesis for Equisetum strobilus evolution by shifting focus to the phytomer as a modular unit.
- To challenge traditional node-internode perspectives and explore alternative models for reproductive morphology.
- To provide a framework for understanding the evolutionary trajectory of reproductive structures in Equisetales.
Main Methods:
- Adopting a phytomer-centric view of shoot development, emphasizing modularity.
- Integrating data from developmental anatomy, regulatory gene expression, and the fossil record.
- Developing a hypothesis based on the combined activity of apical and intercalary meristems in shoot growth and reproduction.
Main Results:
- The proposed hypothesis posits that Equisetum shoots grow via apical and intercalary meristems, with the latter producing sporangiophore whorls along internodes.
- Hierarchical expression of regulatory modules (reproductive transition, growth determinacy, node-internode differentiation) explains reproductive morphologies in Cruciaetheca, Peltotheca, and Equisetum.
- Sporangiophores develop independently of node-internode identity, suggesting an ancient developmental module predating leaves.
Conclusions:
- The phytomer-based model offers a mechanistic explanation for Equisetum strobilus evolution, supporting a Cruciaetheca-Peltotheca-Equisetum evolutionary direction.
- This model re-evaluates the homology of the Equisetum strobilus and its developmental origins.
- The findings imply that sporangiophore development utilizes an ancestral euphyllophyte module, independent of leaf evolution.