Hierarchical modularity hypothesis for Equisetum reproductive morphology

Sporangiophore development and Equisetum strobilus: how fossils and reproductive morphologies illuminate horsetail origins and homology

Equisetum is the sole living representative of a family (Equisetaceae) with a rich fossil record. Widespread extinction in the group and the prevailing view of the horsetail shoot as patterned by node-internode alternation have fuelled a century of debates concerning the origin of the Equisetum strobilus.

Growth habit of Equisetum emphasizing the below- and above-ground stems with long internodes, nodes bearing whorled appendages, adventitious roots and terminal strobilus consisting of whorls of sporangiophores.
Growth habit of Equisetum emphasizing the below- and above-ground stems with long internodes, nodes bearing whorled appendages, adventitious roots and terminal strobilus consisting of whorls of sporangiophores. Drawing by Megan Bishop.

Tomescu et al. adopt a perspective of the shoot as a succession of phytomers, integrating development and comparative morphology of fossil equisetaleans to show that reproductive structures arise from phytomer development rather than directly from apical growth. As well as bringing to light the homology of the Equisetum strobilus, this model implies that sporangiophore development is independent of node-internode identity, representing the expression of an ancestral euphyllophyte developmental module that pre-dates the evolution of leaves.

Editor’s Note: It’s not just the paper and illustrations that grab the eye. There’s also a very helpful glossary at the end that you can refer back to. If you’ve ever needed a short sentence to explain what a Merophyte is, a group of clonally related cells resulting from sequential cell divisions that originate in a single derivative of the apical cell of a meristem, then this is for you. It runs from Acropetal (maturation) to Upward outlook (in morphological evolution).

Reference List

Tomescu, A. M. F., Escapa, I. H., Rothwell, G. W., Elgorriaga, A., & Cúneo, N. R. (2017). Developmental programmes in the evolution of Equisetum reproductive morphology: a hierarchical modularity hypothesis. Annals of Botany, 119(4), 489–505. https://doi.org/10.1093/aob/mcw273