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.

Keywords: Dispersal; Eastern Cape; Isoetes; Isoetes wormaldii; Phylogeny; Polyploidy; Speciation; Species distribution.

Fig. 1

Phylogeny of Isoetes (Isoetaceae). Maximum likelihood analysis of plastid (ndhC-ndhK, rbcL, rpoC1, ycf1, ycf66, and trnV^UAC and its subsequent spacer) and nuclear ribosomal data (nrITS), but excluding nrITS data from outgroups and Isoetes wormaldii because of potential problems to infer positional homology (but see also Additional file 1: Fig. S3). Bootstrap support values and Bayesian posterior probabilities (as estimated in a separate analysis in MrBayes) are indicated on the tree as follows: maximum likelihood bootstrap / Bayesian posterior probability. Clade names A–E and their respective subclades are discussed in the text. Geographic distribution of species are indicated in color according to the legend to the left and following the World Geographical Scheme for Recording Plant Distributions [45]. An exception is the Mediterranean distribution, which refers to an occurrence in either one of the 22 sovereign countries in Europe, Africa and Temperate Asia that borders the Mediterranean Sea. Collection localities (country, and state for USA, Canada and Australia) of investigated samples are indicated in parenthesis to the right of the taxon names

Fig. 2

Isoetes wormaldii Sim. a–c Isoetes wormaldii in its natural habitat. Photos: Tony Dold (Rhodes University); d Drawing by Thomas Robertson Sim, reproduced with permission from the original publication by Sim 1905 [54]; e leaves with a single unbranched vein. Note the, for the genus, unusual leaf shape: flattened with a largely constant width to its rounded tip; f Map of southern South Africa. The two red dots indicate the only known populations of Isoetes wormaldii, which are located in the vicinity of the towns Makhanda (left) and East London (right) in the Eastern Cape, South Africa; g South African botanist Thomas Robertson Sim (1858–1938) (photographer unknown; source: Tr sim00.JPG—Wikimedia Commons)

Fig. 3

Spores of Isoetes wormaldii (Pocock 20009 [BM]). a Megaspore, proximal view (slightly to the side); b Megaspore, distal view (slightly to the side); c Microspore, proximal view (slightly to the side); d Microspore, distal view (slightly to the side). Scale bars: a, b 100 µm; c, d 10 µm