The genus Equisetum, also known as horsetails, is an ancient lineage, recently found to group phylogenetically with ferns. Horsetails have a fossil record stretching as far back as the Carboniferous, but their evolutionary and biogeographic history is not well understood. Their low level of morphological change throughout their evolution has earned them the label of ‘living fossils.’ At present, there are 18 species (in 3 subgenera) in the genus, but this number was much greater at points in the ancient past. Equisetum has its greatest diversity in the Northern Hemisphere, but also occurs in tropical South America and the Galapagos, with one further species occurring in tropical Africa, the Mascarenes, and tropical Asia.
In a new article published in Annals of Botany, lead author Maarten J.M. Christenhusz and colleagues constructed a molecular phylogeny of all living species of Equisetum, calibrated with multiple fossils. The authors then used the phylogeny to evaluate the group’s biogeography and genome size evolution. To fully capture variation in extant genome size, they carried out flow cytometry on 28 horsetail populations, including all 18 species as well as one subspecies.
Equisetum probably diverged from the outgroup Marattia around 342 million years ago, during the early Carboniferous. The most recent ancestor of all living Equisetum existed around 175 million years ago, during the early Jurassic. Most divergences among today’s living Equisetum species occurred more recently, in the middle to late Miocene and early Pliocene.
Biogeographically, the genus may have been widespread across Pangaea, with the breakup of the supercontinent leading to the first divergence of subgenera as Paramochaete continued in Gondwana and all others continued in Laurasia. Later, the Lausasian lineage split into subgenera Equisetum and Hippochaete circa 135 million years ago. “Equisetum is clearly one of the most evolutionarily isolated and possibly the oldest extant vascular plant genus,” write the authors.
Genome sizes in Equisetum vary almost threefold across the genus and show distinct differences between subgenera Equisetum and Hippochaete. The latter’s mean genome size is nearly double that of the former. Though further investigation is needed, the authors speculate that the significant difference in genome size may relate to its effect on the size and therefore the mobility of flagellate sperm cells in the two lineages. Hippochaete, with its larger genome, has a greater number of flagella, and may therefore be under weaker selection for lightweight sperm cells than subgenus Equisetum. “Notwithstanding, it should also be noted that the smaller genomes are found in deciduous subgenus Equisetum, whereas larger genome sizes are generally found in (semi-)evergreen subgenus Hippochaete,” the authors write.