Horsetails are ancient polyploids: Evidence from Equisetum giganteum

Kevin Vanneste, Lieven Sterck, Zander Myburg, Yves Van de Peer and Eshchar Mizrachi

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Horsetails represent an enigmatic clade within the land plants. Despite consisting only of one genus (Equisetum) that contains 15 species, they are thought to represent the oldest extant genus within the vascular plants dating back possibly as far as the Triassic. Horsetails have retained several ancient features but nevertheless exhibited enough plasticity to survive multiple extinction events and geological time scales, and currently have a subcosmopolitan distribution. They are also characterized by a particularly high chromosome count (n = 108). Whole genome duplications (WGDs) have been uncovered in many angiosperm clades, and have been associated with the success of angiosperms both in terms of species richness and biomass dominance, at least under certain ecological conditions, but remain understudied in non-angiosperm clades through lack of available data. Here, we report the first unambiguous evidence of an ancient WGD in the fern linage, based on sequencing and de novo assembly of an expressed gene catalog from the giant horsetail E. giganteum. We demonstrate that horsetails underwent an independent paleopolyploidy during the Late Cretaceous prior to the diversification of the genus, but did not experience any recent polyploidizations that could account for their high chromosome number. This WGD may have contributed to the evolutionary success of horsetails by helping them exploit novel ecological niches generated in a time period when angiosperms rose to dominance and/or to survive past the K-Pg boundary.

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