Paramecium putrinum (Ciliophora, Protozoa): The first insight into the variation of two DNA fragments – Molecular support for the existence of cryptic species

Study on Analysis of Several Molecular Identification Methods for Ciliates of Colpodea (Protista, Ciliophora)

The application of molecular techniques to accurately identify protozoan species can correct previous misidentifications based on traditional morphological identification. Colpodea ciliates have many toxicological and cytological applications, but their subtle morphological differences and small body size hinder species delineation. Herein, we used Cox I and β-tubulin genes, alongside fluorescence in situ hybridization (FISH), to evaluate each method in delineating Colpodea species. For this analysis, Colpoda harbinensis n. sp., C. reniformis, two populations of C. inflata, Colpoda compare grandis, and five populations of Paracolpoda steinii, from the soil in northeastern China, were used. We determined that (1) the Cox I gene was more suitable than the β-tubulin gene as a molecular marker for defining intra- and interspecific level relationships of Colpoda. (2) FISH probes designed for Colpoda sp., C. inflata, Colpoda compare grandis, and Paracolpoda steinii, provided rapid interspecific differentiation of Colpodea species. (3) Colpoda harbinensis n. sp. was established and mainly characterized by its size in vivo (approximately $80\times 60\mu \text{m}$ ), a reniform body in outline, one macronucleus, its spherical shape, a sometimes nonexistent micronucleus, 11–15 somatic kineties, and five or six postoral kineties. In conclusion, combining oligonucleotide probes, DNA barcoding, and morphology for the first time, we have greatly improved the delineation of Colpodea and confirmed that Cox I gene was a promising DNA barcoding marker for species of Colpodea, and FISH could provide useful morphological information as complementing traditional techniques such as silver carbonate.

Cryptic Diversity in Paramecium multimicronucleatum Revealed with a Polyphasic Approach

Paramecium (Ciliophora) systematics is well studied, and about twenty morphological species have been described. The morphological species may include several genetic species. However, molecular phylogenetic analyses revealed that the species diversity within Paramecium could be even higher and has raised a problem of cryptic species whose statuses remain uncertain. In the present study, we provide the morphological and molecular characterization of two novel Paramecium species. While Paramecium lynni n. sp., although morphologically similar to P. multimicronucleatum, is phylogenetically well separated from all other Paramecium species, Paramecium fokini n. sp. appears to be a cryptic sister species to P. multimicronucleatum. The latter two species can be distinguished only by molecular methods. The number and structure of micronuclei, traditionally utilized to discriminate species in Paramecium, vary not only between but also within each of the three studied species and, thus, cannot be considered a reliable feature for species identification. The geographic distribution of the P. multimicronucleatum and P. fokini n. sp. strains do not show defined patterns, still leaving space for a role of the geographic factor in initial speciation in Paramecium. Future findings of new Paramecium species can be predicted from the molecular data, while morphological characteristics appear to be unstable and overlapping at least in some species.

Paramecium bursaria—A Complex of Five Cryptic Species: Mitochondrial DNA COI Haplotype Variation and Biogeographic Distribution

Ciliates are a diverse protistan group and many consist of cryptic species complexes whose members may be restricted to particular biogeographic locations. Mitochondrial genes, characterized by a high resolution for closely related species, were applied to identify new species and to distinguish closely related morphospecies. In the current study, we analyzed 132 sequences of COI mtDNA fragments obtained from P. bursaria species collected worldwide. The results allowed, for the first time, to generate a network of COI haplotypes and demonstrate the relationships between P. bursaria strains, as well as to confirm the existence of five reproductively isolated haplogroups. The P. bursaria haplogroups identified in the present study correspond to previously reported syngens (R1, R2, R3, R4, and R5), thus we decided to propose the following binominal names for each of them: P. primabursaria, P. bibursaria, P. tribursaria, P. tetrabursaria, and P. pentabursaria, respectively. The phylogeographic distribution of P. bursaria species showed that P. primabursaria and P. bibursaria were strictly Eurasian, except for two South Australian P. bibursaria strains. P. tribursaria was found mainly in Eastern Asia, in two stands in Europe and in North America. In turn, P. tetrabursaria was restricted to the USA territory, whereas P. pentabursaria was found in two European localities.

Genetic Diversity and Phylogeny of the Genus Euplotes (Protozoa, Ciliophora) Revealed by the Mitochondrial CO1 and Nuclear Ribosomal Genes

Nuclear ribosomal and mitochondrial genes have been utilized individually or in combination to identify known species and discriminate closely related species. However, compared with metazoans, genetic diversity within the ciliate order Euplotida is poorly known. The aim of this study is to investigate how much nucleotide sequence divergence occurs within Euplotes. A total of 14 new gene sequences, comprising four SSU rDNA and 10 CO1 (including three species for the first time) were obtained. Phylogenetic analyses were carried out based on sequences of two DNA fragments from the same 27 isolates. We found that CO1 revealed a larger interspecific divergence than the SSU rRNA gene, thus demonstrating a higher resolution for separating congeners. Genetic distances differ significantly at the species level. Euplotes balteatus was revealed to have a large intraspecific variation at two loci, while E. vannus showed different levels of haplotype variability, which appeared as a polyphyletic cluster on the CO1 tree. These high genetic divergences suggest the presence of more cryptic species. By contrast, the CO1 gene showed low variability within E. raikovi, appearing as monophyletic clusters, which indicates that this species could be identified based on this gene. Conclusively, CO1 is a suitable marker for the study of genetic diversity within Euplotes, and increased taxon sampling gives an opportunity to screen relationships among members of this genus. Additionally, current data present no clear biogeographical pattern for Euplotes.

Cryptic host-driven speciation of mobilid ciliates epibiotic on freshwater planarians

Mobilids are among the most taxonomically diverse but morphologically uniform groups of epibiotic ciliates. They attach to their hosts by means of an adhesive disc as harmless commensals such as Urceolaria, or as parasites causing significant economic loss such as some Trichodina species. We investigated the diversity, species boundaries, and phylogenetic relationships of mobilids associated with freshwater planarians, using 114 new sequences of two mitochondrial (16S rRNA gene and cytochrome c oxidase gene) and five nuclear (18S rRNA gene, ITS1-5.8S-ITS2 region, D1/D2 domains of 28S rRNA gene) markers. Although the morphological disparity of the isolated trichodinids and urceolariids was low, Bayesian coalescent analyses revealed the existence of five distinct evolutionary lineages/species given the seven molecular markers. The occurrence of mobilids perfectly correlated with their planarian hosts: Trichodina steinii and two Urceolaria mitra-like taxa were associated exclusively with the planarian Dugesia gonocephala, Trichodina polycelis sp. n. with the planarian Polycelis felina, and Trichodina schmidtea sp. n. with the planarian Schmidtea polychroa. Host organisms thus very likely constitute sharply isolated niches that might permit speciation of their epibiotic ciliates, even though no distinct morphological features appear to be recognizable among ciliates originating from different hosts.

Some morphologically distinguishable hypotrich ciliates share identical 18S rRNA gene sequences – taxonomic insights from a case study on Oxytricha species (Protista, Ciliophora)

Modern taxonomic studies of ciliated protozoa require both morphological and molecular data. One dilemma is how to distinguish morphologically similar species with few nucleotide differences in the widely used marker, the 18S rRNA gene. In the present study, two Oxytricha species were morphologically documented using light and electron microscopy. The mitochondrial cytochrome c oxidase subunit I (COI) gene and a fragment of the rRNA gene covering the 18S-ITS1-5.8S-ITS2-28S rRNA gene regions were sequenced. Phylogenetic analyses of all available Oxytricha granulifera-related populations were performed to reveal the internal relationships of this group. We described a new species, Oxytricha atypica sp. nov., distinguished from its congeners by having seven postoral ventral cirri resulting from the additional fragmentation of anlage V during ontogenesis. Although their 18S rRNA genes differ by only one nucleotide, divergence of the COI gene is as high as 11.8% between O. atypica and the closely related species, O. granulifera. All but one of the COI nucleotide substitutions were synonymous. We documented the highly conserved nature of the 18S rRNA gene in the morphospecies of Oxytricha. Based on these findings, we speculate that O. granulifera contains cryptic species or morphospecies needing further characterization, and new insights for the taxonomy of hypotrich ciliates are also discussed.

Biodiversity of Ciliates and Their Symbionts: A Special Issue

Interests to estimate and assess the diversity of ciliates have a centuries-long history [...]

Genetic diversity of Paramecium species on the basis of multiple loci analysis and ITS secondary structure models

Among ciliates, Paramecium has become a privileged model for the study of "species problem" particularly in the case of the "Paramecium aurelia complex" that has been intensely investigated. Despite extensive studies, the taxonomy of Paramecium is still challenging. The major problem is an uneven sampling of Paramecium with relatively few representatives of each species. To investigate species from the less discovered region (Pakistan), 10 isolates of Paramecium species including a standing-alone FT8 strain previously isolated by some of us were subjected to molecular characterization. Fragments of 18S recombinant DNA (rDNA), ITS1-5.8S-ITS2-5'LSU rDNA, cytochrome c oxidase subunit II, and hsp70 genes were used as molecular markers for phylogenetic analysis of particular isolates. The nucleotide sequences of polymerase chain reaction products of all markers were compared with the available sequences of relevant markers of other Paramecium species from GenBank. Phylogenetic trees based on all molecular markers showed that all the nine strains had a very close relationship with Paramecium primaurelia except for the FT8 strain. FT8 consistently showed its unique position in comparison to all other species in the phylogenetic trees. Available sequences of internal transcribed spacer 1 (ITS1) and ITS2 and some other ciliate sequences from GenBank were used for the construction of secondary models. Two highly conserved helices supported by compensatory base changes among all ciliates of ITS2 secondary structures were found similar to other eukaryotes. Therefore, the most conserved 120 to 180 base pairs regions were identified for their comparative studies. We found that out of the three helices in ITS1 structure, helix B was more conserved in Paramecium species. Despite various substitutions in the primary sequence, it was observed that secondary structures of ITS1 and ITS2 could be helpful in interpreting the phylogenetic relationships both at species as well as at generic level.

New Intranuclear Symbiotic Bacteria from Macronucleus of Paramecium putrinum—“Candidatus Gortzia Yakutica”

Holospora-like bacteria (HLB) are obligate intracellular Alphaproteobacteria, inhabiting nuclei of Paramecium and other ciliates such as “Candidatus Hafkinia” is in Frontonia. The HLB clade is comprised of four genera, Holospora, Preeria, “Candidatus Gortzia”, and “Candidatus Hafkinia”. These bacteria have a peculiar life cycle with two morphological forms and some degree of specificity to the host species and the type of nucleus they inhabit. Here we describe a novel species of HLB—“Candidatus Gortzia yakutica” sp. nov.—a symbiont from the macronucleus of Paramecium putrinum, the first described HLB for this Paramecium species. The new endosymbiont shows morphological similarities with other HLB. The phylogenetic analysis of the SSU rRNA gene places it into the “Candidatus Gortzia” clade.

Paramecium Diversity and a New Member of the Paramecium aurelia Species Complex Described from Mexico

Paramecium (Ciliophora) is an ideal model organism to study the biogeography of protists. However, many regions of the world, such as Central America, are still neglected in understanding Paramecium diversity. We combined morphological and molecular approaches to identify paramecia isolated from more than 130 samples collected from different waterbodies in several states of Mexico. We found representatives of six Paramecium morphospecies, including the rare species Paramecium jenningsi, and Paramecium putrinum, which is the first report of this species in tropical regions. We also retrieved five species of the Paramecium aurelia complex, and describe one new member of the complex, Paramecium quindecaurelia n. sp., which appears to be a sister species of Paramecium biaurelia. We discuss criteria currently applied for differentiating between sibling species in Paramecium. Additionally, we detected diverse bacterial symbionts in some of the collected ciliates.

Deciphering phylogenetic relationships and delimiting species boundaries using a Bayesian coalescent approach in protists: A case study of the ciliate genus Spirostomum (Ciliophora, Heterotrichea)

The ciliate genus Spirostomum comprises eight morphospecies, inhabiting diverse aquatic environments worldwide, where they can be used as water quality indicators. Although Spirostomum species are relatively easily identified using morphological methods, the previous nuclear rDNA-based phylogenies indicated several conflicts in morphospecies delineation. Moreover, the single locus phylogenies and previous analytical approaches could not unambiguously resolve phylogenetic relationships among Spirostomum morphospecies. Here, we attempt to investigate species boundaries and evolutionary history of Spirostomum taxa, using 166 new sequences from multiple populations employing one mitochondrial locus (CO1 gene) and two nuclear loci (rRNA operon and alpha-tubulin gene). In accordance with previous studies, relationships among the eight Spirostomum morphospecies were poorly supported statistically in individual gene trees. To overcome this problem, we utilised for the first time in ciliates the Bayesian coalescent approach, which accounts for ancestral polymorphisms, incomplete lineage sorting, and recombination. This strategy enabled us to robustly resolve deep relationships between Spirostomum species and to support the hypothesis that taxa with compact macronucleus and taxa with moniliform macronucleus each form a distinct lineage. Bayesian coalescent-based delimitation analyses strongly statistically supported the traditional morphospecies concept but also indicated that there are two S. minus-like cryptic species and S. teres is non-monophyletic. Spirostomum teres was very likely defined by a set of ancestral features of lineages that also gave rise to S. yagiui and S. dharwarensis. However, molecular data from type populations of the morphospecies S. minus and S. teres are required to unambiguously resolve the taxonomic problems.

Ciliate Environmental Diversity Can Be Underestimated by the V4 Region of SSU rDNA: Insights from Species Delimitation and Multilocus Phylogeny of Pseudokeronopsis (Protist, Ciliophora)

Metabarcoding and high-throughput sequencing methods have greatly improved our understanding of protist diversity. Although the V4 region of small subunit ribosomal DNA (SSU-V4 rDNA) is the most widely used marker in DNA metabarcoding of eukaryotic microorganisms, doubts have recently been raised about its suitability. Here, using the widely distributed ciliate genus Pseudokeronopsis as an example, we assessed the potential of SSU-V4 rDNA and four other nuclear and mitochondrial markers for species delimitation and phylogenetic reconstruction. Our studies revealed that SSU-V4 rDNA is too conservative to distinguish species, and a threshold of 97% and 99% sequence similarity detected only one and three OTUs, respectively, from seven species. On the basis of the comparative analysis of the present and previously published data, we proposed the multilocus marker including the nuclear 5.8S rDNA combining the internal transcribed spacer regions (ITS1-5.8S-ITS2) and the hypervariable D2 region of large subunit rDNA (LSU-D2) as an ideal barcode rather than the mitochondrial cytochrome c oxidase subunit 1 gene, and the ITS1-5.8S-ITS2 as a candidate metabarcoding marker for ciliates. Furthermore, the compensating base change and tree-based criteria of ITS2 and LSU-D2 were useful in complementing the DNA barcoding and metabarcoding methods by giving second structure and phylogenetic evidence.

Evaluation of the molecular variability and characteristics of Paramecium polycaryum and Paramecium nephridiatum, within subgenus Cypriostomum (Ciliophora, Protista)

Although some Paramecium species are suitable research objects in many areas of life sciences, the biodiversity structure of other species is almost unknown. In the current survey, we present a molecular analysis of 60 Cypriostomum strains, which for the first time allows for the study of intra- and interspecific relationships within that subgenus, as well as the assessment of the biogeography patterns of its morphospecies. Analysis of COI mtDNA variation revealed three main clades (separated from each other by approximately 130 nucleotide substitutions), each one with internal sub-clusters (differing by 30 to 70 substitutions - a similar range found between P. aurelia cryptic species and P. bursaria syngens). The first clade is represented exclusively by P. polycaryum; the second one includes only four strains identified as P. calkinsi. The third cluster seems to be paraphyletic, as it includes P. nephridiatum, P. woodruffi, and Eucandidatus P. hungarianum. Some strains, previously identified as P. calkinsi, had COI sequences identical or very similar to P. nephridiatum ones. Morphological reinvestigation of several such strains revealed common morphological features with P. nephridiatum. The paper contains new information concerning speciation within particular species, i.e. existence of cryptic species within P. polycaryum (three) and in P. nephridiatum (six).

Utility of mitochondrial CO1 sequences for species discrimination of Spirotrichea ciliates (Protozoa, Ciliophora)

Ciliates are a diverse species group of the Protozoa, and nuclear and mitochondrial genes have been utilized to discover new species and discriminate closely related species. The mitochondrial cytochrome c oxidase subunit 1 (CO1) gene has been used to discriminate metazoan species and has also been applied for some groups in the phylum Ciliophora. However, it is difficult to produce a universal primer as a standard barcode, because unlike metazoans, mitochondrial DNA sequences of ciliates are long and highly variable. Therefore, to design the new primer set, we sequenced the mitochondrial genomes of two pseudokeronopsids in the class Spirotrichea using next-generation sequencing technology (HiSeq™ 2000). Based on putative CO1 gene fragments of the pseudokeronopsids, we designed the new primer set and successfully sequenced the CO1 of 69 populations representing 47 species (five orders, 14 families, and 27 genera). We found that CO1 showed higher resolution for separating congeneric species than did nuclear SSU rRNA gene sequences, and we identified some putative cryptic species.

Host and symbiont intraspecific variability: The case of Paramecium calkinsi and "Candidatus Trichorickettsia mobilis"

Newly isolated strains of the ciliate Paramecium calkinsi and their cytoplasmic bacterial endosymbionts were characterized by a multidisciplinary approach, including live observation, ultrastructural investigation, and molecular analysis. Despite morphological resemblance, the characterized P. calkinsi strains showed a significant molecular divergence compared to conspecifics, possibly hinting for a cryptic speciation. The endosymbionts were clearly found to be affiliated to the species "Candidatus Trichorickettsia mobilis" (Rickettsiales, Rickettsiaceae), currently encompassing only bacteria retrieved in an obligate intracellular association with other ciliates. However, a relatively high degree of intraspecific divergence was observed as well, thus it was possible to split "Candidatus Trichorickettsia" into three subspecies, one of which represented so far only by the newly characterized endosymbionts of P. calkinsi. Other features distinguished the members of each different subspecies. In particular, the endosymbionts of P. calkinsi resided in the cytoplasm and possessed numerous peritrichous flagella, although no motility was evidenced, whereas their conspecifics in other hosts were either cytoplasmic and devoid of flagella, or macronuclear, displaying flagellar-driven motility. Moreover, contrarily to previously analyzed "Candidatus Trichorickettsia" hosts, infected P. calkinsi cells frequently became amicronucleate and demonstrated abnormal cell division, eventually leading to decline of the laboratory culture.