Valve and ‘Stigma’ Structure and Phylogeny of an Enigmatic Cymbelloid Diatom Karthickia verestigmata Glushchenko, Kulikovskiy & Kociolek

Abstract

The epilithic diatom Karthickia verestigmata Glushchenko et al. was described from North Viet Nam and Laos. Karthickia is a monotypic genus characterized by a unique ‘stigma’ located on the secondary side of the valve with an external circular opening and opening internally by two elongated slits. In 2012 we collected a diatom population consistent with the description of K. verestigmata from the Wang Kaew Waterfall on the upper Wang River in Doi Luang National Park, Thailand. This study details its internal ‘stigma’ structure utilizing sections made with a focused ion beam and observed with a scanning electron microscopy. From the bottom of external circular hole two narrowly flat spaces expanded from the bottom of the external opening down to each inner slit, lying in a trough that is a partial extension of a stria. A phylogenetic hypothesis for Karthickia was created using multiple genes. The taxonomic status of Karthickia is discussed in the context of other genera in the Cymbellales.