In a previous blogpost, I asked the question how long species are able to form hybrids after their lineages diverged — and I showed that certain species of flowering plants (angiosperms) retain this ability for at least 50 million years (Ma). This question is of particular significance because in many lineages, both plants and others, hybrids may not represent evolutionary dead ends – sometimes a hybrid undergoes whole-genome duplication a new hybrid lineage originates. This process of polyploid speciation has been estimated to account for up to 15% of speciations in flowering plants (angiosperms) and 30% in ferns (Wood et al., 2009).

What is the situation in ferns? Because ferns have slower evolutionary rates than flowering plants, we might expect that they retain their hybridisation abilities for longer. Here, I zoom in on the horsetails, Equisetum (Fig. 1), a small plant lineage with ca 15 species that both displays extensive hybridisation and is often considered a “living fossil” — because it has survived for hundreds of millions of years with remarkably little change. Their lineage (order Equisetales) is an early offshoot of the larger fern lineage (class Polypodiopsida), and dates back to the late Devonian period, over 350 Ma ago. Recently, a fossil was discovered from the Triassic (ca 237 Ma) of Argentina (Gnaedinger et al., 2023), which pushes the origin of Equisetum back in time by ~50 Ma. Morphology places this fossil species, Equisetum laterale (= Equisetites lateralis), as sister to extant Equisetum subg. Paramochaete (Elgorriaga et al., 2018), but its most ancient record has until now not been included in a dating analysis. I therefore present a preliminary dated phylogeny here in this blogpost (Fig. 2).

The dating analysis indicates that Equisetum species have retained the ability to hybridise for ~99 Ma in subg. Equisetum and for ~91 Ma in subg. Hippochaete (Fig. 2) – in other words, for twice as long as in flowering plants! Interspecific hybridisation is quite common within each subgenus, but not between them, further suggesting that the subgenera have become genetically too dissimilar to hybridise since they diverged nearly 200 Ma ago.

Interestingly, the the second-widest hybrid combination in Equisetum, Equisetum scirpoides × E. variegatum (Fig. 3), was discovered as late as in 2016 in northern Sweden (Lubienski & Dörken, 2016), owing to the high morphological similarity of the parental taxa —  despite their deep divergence ~91 Ma ago!

How unique is it to retain the ability to form hybrids after 100 Ma? We don’t know, partly because consensus has not yet been reached about the ages of the different fern sublineages. But there are at least a few other examples of hybridising ferns with a possibly similar divergence age, such as Dryopteris and Polystichium in Dryopteridaceae (Long et al., 2023; Regalado et al., 2023), Woodsia and Physematium in Woodsiaceae (Lu et al., 2019; Nitta et al., 2022), and Cystopteris and Gymnocarpium in Cystopteridaceae (Rothfels et al., 2015). In the last example, the authors proposed a divergence age of ~60 Ma, but the most recently dated phylogeny (Nitta et al., 2022) points to an age perhaps twice as old. This blogpost obviously only scratches the surface, but maybe hybridisation between 100 Ma old lineages is not rare at all in plants – at least not in ferns.

Literature:

Christenhusz, M.J.M., Chase, M.W., Fay, M.F., Hidalgo, O., Leitch, I.J., Pellicer, J. & Viruel, J. 2021. Biogeography and genome size evolution of the oldest extant vascular plant genus, Equisetum (Equisetaceae). Annals of Botany 127:681-695. doi: https://doi.org/10.1093/aob/mcab005

Duckett, J.G. 1979. An experimental study of the reproductive biology and hybridization in the European and North American species of Equisetum. Botanical Journal of the Linnean Society 79:205-229. doi: https://doi.org/10.1111/j.1095-8339.1979.tb01514.x

Elgorriaga, A., Escapa, I.H., Rothwell, G.W., Tomescu, A.M.F. & Rubén Cúneo, N. 2018. Origin of Equisetum: Evolution of horsetails (Equisetales) within the major euphyllophyte clade Sphenopsida. American Journal of Botany 105:1286-1303. doi:https://doi.org/10.1002/ajb2.1125

Gnaedinger, S., Villalva, A.S. & Zavattieri, A.M. 2023. Triassic Equisetites lateralis Phillips with strobilus in organic connection from Patagonia of Argentina and endophytic oviposition insect scars. Review of Palaeobotany and Palynology 317:104964. doi:https://doi.org/10.1016/j.revpalbo.2023.104964

Long, X., Peng, Y., Feng, Q., Engel, M.S., Shi, C. & Wang, S. 2023. A new fossil fern of the Dryopteridaceae (Polypodiales) from the mid-Cretaceous Kachin amber. Palaeobiodiversity and Palaeoenvironments 103:489-494. doi: https://doi.org/10.1007/s12549-023-00572-4

Lu, N.T., Zhou, X.-M., Zhang, L., Knapp, R., Li, C.-X., Fan, X.-P., Zhou, L., Wei, H.-J., Lu, J.-M., Xu, B., Peng, Y.-L., Gao, X.-F. & Zhang, L.-B. 2019. A global plastid phylogeny of the cliff fern family Woodsiaceae and a two-genus classification of Woodsiaceae with the description of ×Woodsimatium nothogen. nov. Taxon 68:1149-1172. doi: https://doi.org/10.1002/tax.12180

Lubienski, M. & Dörken, V.M. 2016. The hybrid between Equisetum scirpoides and E. variegatum in northern Europe. American Fern Journal 106:116-130, 115. doi: https://doi.org/10.1640/0002-8444-106.2.116.1

Marshall, C.R. 2019. Using the fossil record to evaluate timetree timescales. Frontiers in Genetics 10. doi: https://doi.org/10.3389/fgene.2019.01049

Nitta, J.H., Schuettpelz, E., Ramírez-Barahona, S. & Iwasaki, W. 2022. An open and continuously updated fern tree of life. Frontiers in Plant Science 13. doi: https://doi.org/10.3389/fpls.2022.909768

Regalado, L., Schneider, H., Müller, P. & Schmidt, A.R. 2023. Character evolution of modern eupolypods supports the assignment of the fossil fern Cretacifilix fungiformis to Dryopteridaceae. Frontiers in Ecology and Evolution 11. doi: https://doi.org/10.3389/fevo.2023.1162577

Rothfels, C.J., Johnson, A.K., Hovenkamp, P.H., Swofford, D.L., Roskam, H.C., Fraser-Jenkins, C.R., Windham, M.D. & Pryer, K.M. 2015. Natural hybridization between genera that diverged from each other approximately 60 million years ago. The American Naturalist 185:433–442. doi: https://doi.org/10.1086/679662

Wood, T.E., Takebayashi, N., Barker, M.S., Mayrose, I., Greenspoon, P.B. & Rieseberg, L.H. 2009. The frequency of polyploid speciation in vascular plants. Proceedings of the National Academy of Sciences of the United States of America 106:13875–13879. doi: https://doi.org/10.1073/pnas.0811575106