Thecamoeba astrologa n. sp. – A new species of the genus Thecamoeba (Amoebozoa, Discosea, Thecamoebida) with an unusually polymorphic nuclear structure

Abstract

Thecamoeba astrologa n. sp. was isolated from a plant litter sample. This species has a complex locomotive morphology, to a certain extent intermediate between the striate and the rugose morphotype. The shape of the actively moving cell is similar to that of striate thecamoebians, such as T. quadrilineata. However, in a slow movement, they can be easily confused with rugose species, like T. similis. Thecamoeba astrologa normally has peripheral asterisk-like nucleoli, which are unique among known thecamoebids. However, the structure of the nucleus is unusually variable during the growth of the culture and in some cases may become almost a vesicular one. The phylogenetic analysis based on the 18S rRNA gene sequence shows that this species belongs to the clade of thecamoebids, consisting of species possessing vesicular nuclei. This finding indicates that the structure of the nucleus derived from a single or few observations (as it usually happens in faunistic or ecological studies) may not be a reliable character of a thecamoebid amoeba. In some species nuclei may be highly polymorphic and dominating nuclear structure may depend on the age of the culture. Nuclei with constantly or temporarily peripheral nucleoli are now known in both major phylogenetic branches of the genus Thecamoeba.

Introduction

Amoebae of the genus Thecamoeba Fromentel, 1874 are widespread in the environment and can be isolated from freshwater and marine habitats, soil, leaf litter, and plant surfaces, especially from dead grass remnants (Kudryavtsev and Hausmann, 2009, Mesentsev and Smirnov, 2019, Mesentsev et al., 2020, Page, 1977, Page, 1983, Page, 1988). Members of this amoeba genus are relatively easy to recognize because of their remarkably folded and/or wrinkled dorsal surface (Page, 1977, Page, 1988). During locomotion, amoebae of the genus Thecamoeba move as a whole and do not form discrete pseudopodia or subpseudopodia. They are flattened in locomotion; the outline of the locomotive cell varies from ovoid, elongated in the direction of movement to broad, fan-shaped. By the morphology of the locomotive form, they belong to the rugose or striate morphotype (Smirnov and Goodkov, 1999, Smirnov and Brown, 2004).

In the modern system of Amoebozoa, the genus Thecamoeba belongs to class Discosea, subclass Flabellinia, order Thecamoebida, family Thecamoebidae (Smirnov et al. 2011). The same position it occupies in the system by Adl et al. (2019), except for the absence of ranks in the latter one. At the moment, the genus includes 11 well-described species and 14 less studied species (Kudryavtsev and Hausmann, 2009, Mesentsev and Smirnov, 2019, Mesentsev et al., 2020, Page, 1971, Page, 1977, Smirnov, 1999a, Smirnov, 1999b). The morphology of the locomotive form, cell size, the size of the nucleus, and the organization of nucleolar material are the main characters used for Thecamoeba identification (Page, 1977, Page, 1983, Page, 1988, Page, 1991). Detailed data on the ultrastructure are available only for some species (Dyková et al., 2008, Dyková and Kostka, 2013, Houssay and Prenant, 1970, Kamyshatskaya et al., 2018, Kudryavtsev and Hausmann, 2009, Mesentsev et al., 2020). For other species, ultrastructural data are limited to the glycocalyx images (Page and Blakey, 1979, Smirnov, 1999a, Smirnov, 1999b, Smirnov et al., 2011). Based on these data, new genera were established in the family, but almost no new characters useful for species distinction were found (Page and Blakey 1979).

Several recent studies described sibling species of the genus Thecamoeba, which can hardly be distinguished by light-microscopic morphology. These species can be reliably identified only by molecular methods (Kudryavtsev and Hausmann, 2009, Mesentsev and Smirnov, 2019, Mesentsev et al., 2020). However, molecular data are still available for a minor part of Thecamoeba species. There are eight sequences of the 18s rRNA gene in GenBank, and they belong to five Thecamoeba species only (Dyková and Kostka, 2013, Fahrni et al., 2003, Mesentsev and Smirnov, 2019, Mesentsev et al., 2020, Michel et al., 2006, Smirnov et al., 2011). Getting more SSU sequences may become a problem; in our experiments, the DNA of some species of Thecamoeba does not reply to any of a very wide set of eukaryotic primers. Among the thecamoebids, there are species where even deep next-generation sequencing did not provide any sufficiently long sequence of the 18s rRNA gene (Melton et al. 2019).

During the study of the diversity of amoebae from terrestrial habitats, we isolated the strain marked as Ta117. Amoebae of this strain usually have peripherally located asterisks-like nucleoli of a complex organization. However, detailed studies show that the nuclear structure undergoes drastic changes depending on the age of the culture. By phylogenetic analysis, the strain Ta117 appears to be a relative of Thecamoeba species, normally demonstrating a vesicular nucleus. Here we present the morphological description of this new species and the results of the molecular phylogenetic analysis.