Integrative taxonomy of the Selaginella helvetica group based on morphological, molecular and ecological data

Abstract

The Eurasian and Mediterranean Selaginella helvetica group is one of the taxonomically challenging groups in the cosmopolitan lycophyte genus Selaginella. Species of the S. helvetica group are all small plants with lax strobili composed of more or less isomorphic sporophylls (isosporophylls) that are basically non-resupinate. Owing to the similar and minute morphological characteristics, the inapplicability of chloroplast DNA markers for species delimitation and the lack of range-wide sampling, no comprehensive study has been conducted on these species so far. In this study, we performed phylogenetic analyses based on populationscale sampling of 126 individuals, covering the entire geographical distribution and morphological variation of this group. Five chloroplast (rbcL, psbA, atpI, atpF, atpF-H) and two nuclear DNA regions (26S rDNA, pgiC) were used to explore species diversity of this group. The phylogeny inferred from nuclear DNA datasets revealed nine well-supported monophyletic groups, which provided important evidence for species delimitation and called for taxonomic revision of some taxa. However, in the chloroplast phylogeny, individuals of only three species (S. denticulata, S. helvetica, S. tamamontana) formed monophyletic groups, respectively. We further investigated genome size for each species by flow cytometry and evaluated 21 morphological characteristics of 79 herbarium specimens. MaxEnt analysis was performed to predict the potential suitable distribution of each species. As a result of total evidence, 10 taxa were recognized in the S. helvetica group. The delimitation of S. denticulata, S. laxistrobila, S. nipponica, S. pallidissima, S. prostrata, and S. tamamontana was congruent with previous taxonomical treatments. We reinstated S. shensiensis as an independent species and treated S. longistrobilina as its synonym. Selaginella helvetica (s.str.) is confined to Europe and North Asia, former records of it from the Himalaya-Hengduan mountains represent an independent species (S. jiulongensis comb. nov.). The nuclear pgiC phylogeny and flow cytometry evidence indicated that S. pseudonipponica is an allotetraploid with S. shensiensis and S. nipponica as its parents. Our results highlight that nuclear DNA markers rather than chloroplast DNA markers are more informative in delimitation of closely related species in Selaginella.

Selaginella ophioderma (Selaginellaceae–Lycopodiophyta), a tropical rainforests new species from the Eastern slopes of the Andes mountains of northwestern South America in Colombia, Ecuador and Peru

“…It is adapted to live in diverse ecosystems and comprises species with terrestrial, epipetric, hemiepipythic and epiphytic habits with creeping, ascending, climbing and erect stems (Jermy 1990, Valdespino 1993), which may have sobols and stolons at their bases and, occasionally flagelliform aerial axes. The monophylly of Selaginella has been confirmed by morphological and molecular studies (Arrigo et al 2013, Zhou et al 2015, Weststrand and Korall 2016a, Zhang et al 2020, 2021). However, its infrageneric classification remains contentious with a widely used morphology‐based system (Jermy 1986, 1990) and two competing mostly molecular‐based ones (Zhou and Zhang 2015, Weststrand and Korall 2016b).…”

Integrative species delimitation of Selaginella labordei and closely related species: Uncovering the mysterious identity of S. jugorum and S. tibetica, and description of a new species

“…750 species worldwide (Jermy, 1986; Zhang & al., 2013), and is currently divided into seven subgenera (Weststrand & Korall, 2016a,b; Zhang & al., 2020). The taxonomic work on Selaginella has historically been problematic due to the minute microphyll, morphological diversity, wide distribution, and suspected hybridization events, especially in closely related species (Shalimov & Zhang, 2021; Zhang & al., 2021).…”

“…Based on morphological evidence (leaf margins white, ciliolate and denticulate, and dorsal leaves carinate), we speculate that these two species should be closely related to S. labordei and S. chrysocaulos . In the phylogenetic trees of Zhou & Zhang (2015), “ S. jugorum ” (GenBank accession number KT161494) was nested within the S. helvetica group, and we have shown that this sample was an incorrectly identified specimen of S. laxistrobila K.H.Shing (Zhang & al., 2021). In the absence of intensive morphological and molecular studies, the taxonomic status of S. jugorum and S. tibetica based on known morphological characters alone is not clear‐cut.…”

“…Although nuclear genes have been considered informative for species delimitation in lycophytes (Zhang & al., 2021), ferns (Liang & al., 2019, 2021), and seed plants (Bratzel & al., 2020), low‐copy nuclear gene sequences have always remained difficult to obtain, especially from historical specimens (Staats & al., 2013). With the rapid development of high‐throughput sequencing technology, plastid genomes can be effectively obtained by genome skimming sequencing, and can greatly increase resolution in plant phylogenetics at familial, generic, or species level (Li & al., 2015; Ye & al., 2020; Zhang & al., 2020; Qin & al., 2022; Wei & Zhang, 2022).…”

“…Due to the typical mode of uniparental inheritance and high content of informative loci, an increasing number of studies pointed out that plastid genomes may provide more variation to discriminate closely related species, among which hybridization and polyploidization is less frequently reported (Parks & al., 2009; Li & al., 2015; Magwé‐Tindo & al., 2018). Previous studies on the taxonomy of Selaginella , however, were mainly based on morphological evidence alone or in combination with only a few short loci (Irfan & al., 2021; Shalimov & Zhang, 2021; Zhang & al., 2021), and relationships of closely related species were often poorly resolved. Up to the present, phylogenetic analysis and species identification based on plastid genomes have not been applied to lower taxonomic level of Selaginella .…”