Biology subjects

Hetherington, A. J.

Publications and source records attributed to Hetherington, A. J..

5 recordsLinked to original sources

bioRxiv2026

Phloem evolved gradually and asynchronously to xylem in early vascular plants

The evolution of the water-conducting xylem and sugar-conducting phloem tissues were key innovations in land plant evolution, enabling the origin of long-distance transport networks1. In extant vascular plants, phloem and xylem are linked functionally and always occur together2, though their evolutionary origin is unclear. This uncertainty is owed to the greater fossilisation potential of lignified xylem tracheids compared to thin-walled phloem cells3, 4, 5. Therefore, the fossil record of xylem is far more extensive than that of phloem, with the first definitive record of xylem being around 40 million years earlier6 than phloem7. This bias in the fossil record obscures characterisation of the origins of plant vasculature. In this study, this limitation is overcome by re-describing the "phloem-like" tissues of exceptionally preserved plants from the 407-million-year-old Rhynie chert8-15. We report that this tissue differs markedly from the phloem of extant plants, and propose its identification as a tissue of food-conducting cells (FCCs). Major histological differences were observed in the fossil plants, including no evidence for a pericycle, which in extant species delimits vascular from ground tissues, and the FCCs of the Rhynie chert plants were significantly larger in diameter than phloem cells. These differences suggest that early vascular plants lacked true phloem. However, putative sieve pores in the FCCs of Asteroxylon mackiei were identified. This represents to our knowledge the earliest record of sieve pores in the fossil record. Our results suggest an evolutionary scenario in which phloem features assembled gradually within FCCs, asynchronous to the evolution of xylem.

paleontology↗

bioRxiv2025

Prototaxites was an extinct lineage of multicellular terrestrial eukaryotes

Prototaxites was the first giant organism to live on the terrestrial surface, reaching sizes of 8 metres in the Early Devonian. However, its taxonomic assignment has been debated for over 165 years1-7. Tentative assignments to groups of multicellular algae or land plants1,2,8-11 have been repeatedly ruled out based on anatomy and chemistry5,7,11-16 resulting in two major alternatives: Prototaxites was either a fungus5,6,17-22 or a now entirely extinct lineage 7,16,23. Recent studies have converged on a fungal affinity5-7,17-20,22. Here we test this by contrasting the anatomy and molecular composition of Prototaxites with contemporary fungi from the 407-million-year-old Rhynie chert. We report that Prototaxites taiti was the largest organism in the Rhynie ecosystem and its anatomy was fundamentally distinct from all known extant or extinct fungi. Furthermore, our molecular composition analysis indicates that cell walls of P. taiti include aliphatic, aromatic, and phenolic components most similar to fossilisation products of lignin, but no fossilisation products characteristic of chitin or chitosan, which are diagnostic of all groups of extant and extinct fungi, including those preserved in the Rhynie chert. We therefore conclude that Prototaxites was not a fungus, and instead propose it is best assigned to a now entirely extinct terrestrial lineage.

paleontology↗

bioRxiv2021

An evidence-based 3D reconstruction of Asteroxylon mackiei the most complex plant preserved from the Rhynie chert

The 407-million-year-old Rhynie chert preserves the earliest terrestrial ecosystem and informs our understanding of early life on land. However, our knowledge of the 3D structure, and development of these plants is still rudimentary. Here we used digital 3D reconstruction techniques to produce the first complete reconstruction of the lycopsid Asteroxylon mackiei, the most complex plant in the Rhynie chert. The reconstruction reveals the organisation of the three distinct axes types - leafy shoot axes, root-bearing axes and rooting axes - in the body plan. Combining this reconstruction with developmental data from fossilised meristems, we demonstrate that the A. mackiei rooting axis - a transitional lycophyte organ between the rootless ancestral state and true roots - developed from root-bearing axes by anisotomous dichotomy. Our discovery demonstrates how this unique organ developed, and highlights the value of evidence-based reconstructions for understanding the development and evolution of the first complex plants on Earth.

paleontology↗

bioRxiv2019

Gene expression data support the hypothesis that Isoetes rootlets are true roots and not modified leaves

Rhizomorphic lycopsids are the land plant group that includes the first giant trees to grow on Earth and extant species in the genus Isoetes. Two mutually exclusive hypotheses account for the evolution of terminal rooting axes called rootlets among the rhizomorphic lycopsids. One hypothesis states that rootlets are true roots, like roots in other lycopsids. The other states that rootlets are modified leaves. Here we test predictions of each hypothesis by investigating gene expression in the leaves and rootlets of Isoetes echinospora. We assembled the de-novo transcriptome of axenically cultured I. echinospora. Gene expression signatures of I. echinospora rootlets and leaves were different. Furthermore, gene expression signatures of I. echinospora rootlets were similar to gene expression signatures of true roots of Selaginella moellendorffii and Arabidopsis thaliana. RSL genes which positively regulate cell differentiation in roots were either exclusively or preferentially expressed in the I. echinospora rootlets, S. moellendorffii roots and A. thaliana roots compared to the leaves of each respective species. Taken together, gene expression data from the de-novo transcriptome of I. echinospora are consistent with the hypothesis that Isoetes rootlets are true roots and not modified leaves.

plant biology↗

bioRxiv2019

Neofunctionalisation of basic helix loop helix proteins occurred when plants colonised the land

ROOT HAIR DEFECTIVE SIX-LIKE (RSL) genes control the development of structures - rhizoids, root hairs, gemmae, mucilage papillae - that develop from single cells at the surface of diverse groups of land plants. RSL proteins constitute a subclass (VIIIc) of the basic helix loop helix (bHLH) class VIII transcription factor family. We set out to determine if the function of RSL genes in the control of cell differentiation in land plants was inherited from streptophyte algal ancestor. The Charophyceae are a monophyletic class of streptophyte algae with tissue-like structures and rhizoids. We identified the single class VIII bHLH gene from the charophyceaen alga Chara braunii (CbbHLHVIII). Phylogenetic analysis suggests that this protein is sister to the RSL (bHLH subclass VIIIc) proteins and together they constitute a monophyletic group. Expression of CbbHLHVIII does not compensate for loss of the RSL function in either Marchantia polymorpha or Arabidopsis thaliana. Furthermore, CbbHLHVIII is expressed at sites of morphogenesis in C. braunii - the apices, nodes and gametangia - but not in rhizoids. This indicates that C. braunii class VIII protein is functionally different from land plant RSL proteins; they control rhizoid development in land plants but not in the charophycean algae. These data are consistent with the hypothesis that RSL proteins and their function in the differentiation of cells at the plant surface evolved in the lineage leading to land plants after the divergence of the land plants and C. braunii from their last common ancestor. This may have occurred by neofunctionalisation at or before the colonisation of the land by streptophytes.

evolutionary biology↗