Phylogeny of Merlin’s grass (Isoetaceae): revealing an “Amborella syndrome” and the importance of geographic distribution for understanding current and historical diversity

Abstract

Background Merlin’s grass (Isoetes, Isoetaceae, Lycopsida), is the extant remnant of the isoetalean wood-producing lycopsids that originated during the Paleozoic, possibly in aquatic or boggy habitats. Modern day species are aquatic, semi-aquatic or terrestrial and occur almost worldwide. They display little morphological variation; the lobed corm has helically arranged leaves with internal air channels and basal sporangia. Genetic variation has also proven limited, which has hampered phylogenetic inference. We investigate evolutionary relationships in Isoetes, using molecular data and an extended sample of species compared to previous work, adding species that have never before been included in a phylogenetic study. Results Our results reveal an unexpected discovery of an “Amborella syndrome” in Isoetaceae: a single poorly known species is sister to the remaining family. The species, Isoetes wormaldii, is a rare endemic to the Eastern Cape of South Africa. Its leaves are flattened with a rounded point, which sharply contrasts with the awl-shaped leaves of most other species of Isoetes. The remaining species of Isoetes are resolved in five major clades, also indicated in previous work. While the phylogeny shows geographic structure, the patterns are complex. For example, tropical-southern African species occur in at least five clades, and Indian, Australian and Mediterranean species in at least three clades each. Conclusion The evolutionary and biogeographical history of Isoetes is not easily explained, and may conceivably include ample extinction and a mixture of ancient and more recent processes. Previously shown difficulties with node age estimation increase the problem. The here demonstrated sister-relationship between the phylogenetically, morphologically and genetically distinct Isoetes wormaldii and the remaining family appears to bridge the morphological gap between Isoetes and its extinct relatives, although further studies are needed. Moreover, it shortens the branch length to its living sister genus Selaginella, and may enhance node age estimation in future studies. Isoetes wormaldii is critically endangered, known only from one (to a few) minor populations. Immediate actions need to be taken if we want to prevent this unique species from going extinct.

No phylogenomic support for a Cenozoic origin of the “living fossil” Isoetes

“…Altogether 66 CDS regions were included in the final data set with a total of 50 taxa and 47,547 nucleotide characters. Data sets are available in the Dryad Digital Repository (https://doi.org/10.5061/dryad.q2bvq83pc; Wikström et al, 2022).…”

“…Altogether, the final rDNA data set comprised 49 taxa and 5665 nucleotide characters. Data sets were deposited in the Dryad Digital Repository (https://doi.org/10.5061/dryad.q2bvq83pc; Wikström, et al, 2022).…”

“…Newly produced sequence data are deposited at GenBank (for accessions, see Appendix S1). All data sets and the supporting information (see below) are permanently archived at the Dryad Digital Repository: https://doi.org/ 10.5061/dryad.q2bvq83pc (Wikström et al, 2022).…”

“…Molecular systematics has addressed this issue in a different way. Using primarily plastid and nuclear ribosomal DNA (rDNA) data, phylogenetic analyses have produced a coherent and well‐supported phylogenetic framework of Isoetes (Rydin and Wikström, 2002 ; Hoot et al, 2004 , 2006 ; Schuettpelz and Hoot, 2006 ; Larsén and Rydin, 2016 ; Pereira et al, 2017 ; Larsén et al, 2022 ). Five major clades within Isoetes have been supported: a Gondwanan (Clade A), a Laurasian (Clade B), an Italian‐Turkish (Clade C), an Austro‐Asian (Clade D), and a New World or American clade (Clade E).…”

Rediscovery of the Presumed Extinct Philippine Quillwort Isoetes philippinensis Merr. & L.M. Perry (Isoetaceae) and New Insights on its Morphology and Ecology

“…Using the combined analysis of plastid and nuclear ribosomal data, I. philippinensis is phylogenetically related to the Asia-Tropical species, viz. I. sinensis (China), I. taiwanensis (Taiwan), I. hopei (Indonesia), and I. stevensii (New Guinea) (Larsén et al 2022).…”