Abstract
Using genetic markers 18S V4 rDNA and rbcL and morphological investigation of the diatom genus Placoneis, we described three new species. The new species, Placoneis baikaloelginensis sp. nov., Placoneis subundulata sp. nov., Placoneis neohambergii sp. nov. were isolated from Russia (Lake Baikal) and Vietnam (waterbodies of Cát Tiên National Park (Đồng Nai Province) and Khánh Hòa Province). We examine relationships within the Cymbellales and show that the genera Placoneis, Paraplaconeis and Geissleria are phylogenetically independent. We discuss the importance of careful identification of strains used for phylogenetic analysis and we show the history of identification of several different Placoneis elginensis strains. After careful identification of Placoneis elginensis vouchers, we found that we have a few independent species. The question of cryptic or pseudocryptic species in this context is discussed.
Highlights
We examine relationships within the Cymbellales and discuss the possibility of cryptic species within the P. elginensis species complex based on molecular data
The phylogenetic analysis presented here on the Cymbellales, the most comprehensive to date in terms of taxon sampling, supports the results shown in previous analyses [28,29]
That diatoms with naviculoid symmetry are part of the monophyletic Cymbellales lineage
Summary
The systematic position and richness of the genus Placoneis Mereschkowsky 1903 have been repeatedly revised and studied. On the basis of the symmetrical structure of the valve, the representatives of the genus were attributed to Navicula Bory 1822 [1]. In 1903, on the basis of studying the structure of the chloroplast (which is one organelle consisting of two X-shaped plates connected by an isthmus), Mereschkowsky [2] described the genus Placoneis and suggested, due to the nature of the chloroplast, this genus was more aligned with the “Monoplacatae” a group of taxa, including the Cymbellales, with a structured chloroplast. Members of the genus were again consistently placed within Navicula (e.g., [3,4,5]). Eileen Cox resurrected the genus and included seven species within it and designated a generitype, P. gastrum (Ehrenberg)
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