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A poorly known and critically endangered species of Merlin’s grass surprises researchers

A poorly known and critically endangered species of Merlin’s grass (Isoetaceae) endemic to the Eastern Cape of South Africa is demonstrated to be of substantial evolutionary significance. The species, Isoetes wormaldii, is sister to the rest of the family and genetically and morphologically distinct.

Unexpected results

These are the highly unexpected results of a recent study conducted at Stockholm University. “I never imagined we would discover anything remotely similar to this”, says Eva Larsén, PhD student and main author of the study. There are several hundred species of Merlin’s grass in the world and they are otherwise very similar to each other, genetically as well as morphologically. “That is why it is so unexpected to find that this particular species differs spectacularly from the remaining species in the family”, says Eva Larsén.

soetes wormaldii (Isoetaceae, Merlin’s grass) from the Eastern Cape in South Africa, a lycopod with long and narrow leaves that float on the water surface. Photo: Tony Dold (Rhodes University).

The species, Isoetes wormaldii, was described more than 100 years ago but has not been much noticed and has not been included in any modern evolutionary study. “The reason it was included here is simply that Eva plans her work very carefully", says Eva’s supervisor Catarina Rydin and continues: “Eva realized that African species of Isoetes are particularly poorly investigated”.

Isoetes wormaldii is genetically very different from other species of Merlin’s grass, so different that comparison to equivalent gene regions in other species of the family was sometimes difficult. It is also morphologically unique with its long narrow and flat leaves that grow to the water surface and rests floating on it. Otherwise, leaves and general habit in Isoetes are typically of a “porcupine-type”, with awl-shaped upright leaves, says Eva.

Isoetes wormaldii with flat and floating leaves. Photo: Tony Dold (Rhodes University).

An ancient group of land plants

To the right: an example of the ”porcupine style” common in Isoetes, upright awl-shaped leaves. Credit: Coloured lithograph of Isoetes echinospora by W. Fitch, c. 1863. Public domain.

Merlin’s grass are not grasses but lycopods, ancient vascular plants that have survived to the present. The air-channels in the leaves and the presence of wood in the tiny corm reveal their ancestry and close relationship to the extinct tree lycopods that were common in the swamp forests of the Carboniferous. However, despite their ancient history modern species may be much younger. Several studies have used molecular dating to investigate the age of the living clade but the results have diverged enormously, differing so much that the overall conclusion is that results are meaningless. 

A conceivable explanation is that it becomes methodologically difficult to handle datasets where all living species of Merlin’s grass appear very closely related with little genetic divergence between species, whereas the genetic and evolutionary distance to other living plants (e.g., the spikemosses of Selaginella) is huge. Isoetes wormaldii, the unique sister of the remaining species of Merlin’s grass, will have substantial impact on the results of future studies of the age of the family. “I think you can easily guess what kind of analyses are currently running on our computers”, says Catarina Rydin with a smile.

Critically endangered
Tragically, this newfound sister species to the remaining Merlin’s grasses, is at the brink of extinction. It is extremely rare and decreasing, known only from one-two populations in small wetlands close to the towns Makhanda and East London in the Eastern Cape region of South Africa. It is listed as “critically endangered” in the red list of The South African National Biodiversity Institute (SANBI). ”To save this evolutionary, genetically and morphologically unique species is of highest possible priority”, says Eva Larsén and Catarina Rydin.

Publication details
Eva Larsén, Niklas Wikström, Anbar Khodabandeh and Catarina Rydin: "Phylogeny of Merlin’s grass (Isoetaceae): revealing an “Amborella syndrome” and the importance of geographic distribution for understanding current and historical diversity", in BMC Ecology and Evolution (2022) 22:32.

Contact information
Eva Larsén,
Catarina Rydin,

Stockholm University
Department of Ecology, Environment and Plant Sciences