The morphology and divisional morphogenesis in a Chinese population of Neobakuella aenigmatica Moon et al., 2019 are reinvestigated. The body size, number and arrangement of parabuccal cirri and development of nuclear nodules in this population are discussed in comparison with closely related genera. The 18S rRNA gene sequence of the Chinese population is identical to that of a Korean population, supporting a distant relationship between N. aenigmatica and the cluster of N. flava and Apobakuella fusca. We deduce that this may be caused by the differences in the pattern of the parabuccal cirri and the number of parabuccal rows which are important for the phylogeny of Bakuella-like species.

One of the most diverse clades of ciliated protozoa, the class Spirotrichea, displays a series of unique characters in terms of eukaryotic macronuclear (MAC) genome, including high fragmentation that produces nanochromosomes. However, the genomic diversity and evolution of nanochromosomes and gene families for spirotrich MAC genomes are poorly understood. In this study, we assemble the MAC genome of a representative euplotid (a new model organism in Spirotrichea) species, Euplotes aediculatus. Our results indicate that: (a) the MAC genome includes 35,465 contigs with a total length of 97.3 Mb and a contig N50 of 3.4 kb, and contains 13,145 complete nanochromosomes and 43,194 predicted genes, with the majority of these nanochromosomes containing tiny introns and harboring only one gene; (b) genomic comparisons between E. aediculatus and other reported spirotrichs indicate that average GC content and genome fragmentation levels exhibit interspecific variation, and chromosome breaking sites (CBSs) might be lost during evolution, resulting in the increase of multi-gene nanochromosome; (c) gene families associated with chitin metabolism and FoxO signaling pathway are expanded in E. aediculatus, suggesting their potential roles in environment adaptation and survival strategies of E. aediculatus; and (d) a programmed ribosomal frameshift (PRF) with a conservative motif 5′-AAATAR-3′ tends to occur in longer genes with more exons, and PRF genes play an important role in many cellular regulation processes.

Hypotrich ciliates, a large group of mainly free-living protists with highly diverse cortical structures, are commonly found in a broad variety of habitats worldwide. Methodological difficulties and insufficient faunistic studies have limited our understanding of their true biodiversity and phylogeny. In this report, two new Strongylidium species from China are investigated using live observation, protargol staining and 18S rRNA gene sequencing, and the evolution of strongylidiid ciliates is investigated based on morphological, morphogenetic and molecular data. Recently, the genera Strongylidium Sterki, 1878 and Pseudouroleptus Hemberger, 1985 were assigned to the family Spirofilidae Gelei, 1929. Given that these genera differ morphologically and phylogenetically from the type genus of Spirofilidae, Hypotrichidium Ilowaisky, 1921, their familial classification should be reconsidered. Moreover, the highly characteristic formation pattern of the mixed left ventral cirral row and other morphological features shared by Strongylidium, Pseudouroleptus and Hemiamphisiella Foissner, 1988, together with their 18S rRNA gene sequences, suggest that the three genera form a group closely related to Dorsomarginalia Berger, 2006. The family Strongylidiidae Fauré-Fremiet, 1961 is reactivated for these three genera.