Morphological Variability amid Genetic Homogeneity and Vice Versa: A Complicated Case with (Bacillariophyceae) from Tropical Forest Soils of Vietnam with the Description of Four New Species.

A total of 18 strains isolated from soil samples from Cát Tiên National Park have been studied. Based on morphometric analysis and molecular data for the V4 18S rDNA and L regions, we proposed the presence of four new species: , , , and . This is the first study that provides molecular data for such a large number of strains. Furthermore, we encountered some strains with clear morphological differences (which we assigned to several morphotypes) that cannot be separated using the selected genetic markers and cannot be attributed to phenotypic variations in one species; these require further study of their genetic structure. We also observed the opposite case, where in the absence of morphological differences, clear genetic differentiation is shown, which demonstrates the presence of cryptic taxa in our sample. The maximum differences for these strains were observed in the V4 18S rDNA region. Our results show that the effectiveness of commonly used genetic markers V4 18S rDNA and L for separating species can vary greatly. Our study highlights the need to research different genetic markers and their use for proper species separation, as well as the genetic diversity of diatoms, and the need for further studies of intra- and interspecific genetic distances.