Species Delimitation and Cryptic Diversity Analysis of Parachlorella-Сlade Microalgae (Chlorophyta)
<b>Abstract</b>—This article discusses the system of green microalgae of the clade <i>Parachlorella</i>, based on morphological, ecological, and molecular genetic data. The diagnostic characteristics of the selected genera and species and the systematic position of some taxa are discussed. Molecular phylogenetic analysis of the <i>Parachlorella</i> clade has shown that, despite the relative simplicity of morphology, it is characterized by high cryptic diversity. The polyphyletic nature of the genus <i>Dictyosphaerium</i> and some species (<i>D. libertatis</i>, <i>D. ehrenbergianum</i>, <i>Compactochlorella kochii</i>, <i>Mucidosphaerium pulchellum</i>, <i>M. palustre,</i> and <i>Closteriopsis acicularis</i>) was shown. The use of the intron as a universal criterion for closely related species was effective for representatives of groups II, IV, VII, and VIII, as well as for strains of the species <i>Compactochlorella kochii</i> and <i>D. libertatis</i>. The application of the classical CBC approach, based on the search for CBC in conservative ITS2 regions, was successful only for the representatives of group II. The results of analysis of genetic distances and the GMYC, PTP, and ABGD species delimitation algorithms are not universal tools for determining species boundaries and need to be confirmed by other characteristics (cell shape, ability to form colonies, the way the cells attach to hyaline interconnecting strands, the type of chloroplast, the number of pyrenoids, the presence of mucilage, the presence/absence of introns, the level of genetic differences, the presence of CBC in ITS1 and ITS2, the habitat). Due to the combined use of morphological, ecological and genetic features, it was possible to characterize 11 groups and 2 phylogenetic lines within the clade <i>Parachlorella</i> and to make assumptions about the division of genera and species within the selected groups.