Abstract
Cryptic species comprise two or more taxa that are grounded under a single name because they are more-or-less indistinguishable morphologically. These species are potentially important for detailed assessments of biodiversity, but there now appear to be many more cryptic species than previously estimated. One taxonomic group likely to contain many cryptic species is Dicranopteris, a genus of forked ferns that occurs commonly along roadsides in Asia. The genus has a complex taxonomical history, and D. linearis has been particularly challenging with many intra-specific taxa dubiously erected to accommodate morphological variation that lacks clear discontinuities. To resolve species boundaries within Dicranopteris, we applied a molecular phylogenetic approach as complementary to morphology. Specifically, we used five chloroplast gene regions (rbcL, atpB, rps4, matK, and trnL-trnF) to generate a well-resolved phylogeny based on 37 samples representing 13 taxa of Dicranopteris, spanning the major distributional area in Asia. The results showed that Dicranopteris consists of ten highly supported clades, and D. linearis is polyphyletic, suggesting cryptic diversity within the species. Further through morphological comparison, we certainly erected Dicranopteris austrosinensis Y.H. Yan & Z.Y. Wei sp. nov. and Dicranopteris baliensis Y.H. Yan & Z.Y. Wei sp. nov. as distinct species and proposed five new combinations. We also inferred that the extant diversity of the genus Dicranopteris may result from relatively recent diversification in the Miocene based on divergence time dating. Overall, our study not only provided additional insights on the Gleicheniaceae tree of life, but also served as a case of integrating molecular and morphological approaches to elucidate cryptic diversity in taxonomically difficult groups.
Highlights
Cryptic species, a common and increasingly used term, refers to taxa that are erroneously classified as a single species due to the paucity of conspicuous morphological differences (Trontelj and Fišer, 2009; Detwiler et al, 2010; Struck et al, 2018)
The analyses revealed that D. linearis was polyphyletic with several varieties clustered with other species of forked ferns (Figure 2 and Supplementary Figures 1, 2)
Accessions of D. linearis from southern China formed a clade that was strongly supported as a sister to D. linearis from Malaysia and D. taiwanensis (BIPP = 0.99, MLBS = 79%, and MPBS = 77%)
Summary
A common and increasingly used term, refers to taxa that are erroneously classified as a single species due to the paucity of conspicuous morphological differences (Trontelj and Fišer, 2009; Detwiler et al, 2010; Struck et al, 2018). Cryptic species represent a potentially important influence on the accuracy of detailed assessments of biodiversity Trontelj and Fišer, 2009) and can lead to novel insights regarding patterns and processes of biodiversity, including geographic variation in species distributions and species coexistence (Fiser et al, 2018). Cryptic species are seldom considered in biodiversity assessments owing to the lack of affordable and efficient diagnostic methods (Witt et al, 2006). This is compounded by the fact the high rate at which cryptic species are discovered in molecular studies suggests that number of cryptic species is far greater than prior estimates. One practical proposal has been that the taxonomic ranks of species should be reserved for organisms showing observable morphological variation, while resolved cryptic species should be designated as intraspecific ranks (Maxwell and Dekkers, 2019; Maxwell et al, 2021)
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