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Paramecium bursaria (Ehrenberg 1831), a freshwater ciliate, typically harbors hundreds of green algal symbionts inside the cell. The aim of present study was the molecular identification of newly analyzed P. bursaria symbionts. The second aspect of the present survey was testing a hypothesis whether endosymbionts prefer the specified syngen of the host, and the specified geographical distribution. Ten strains of endosymbionts isolated from strains of P. bursaria originating from different geographical locations were studied. We analyzed for the first time, both the fragment of plastid genome containing 3'rpl36-5' infA genes and a fragment of a nuclear gene encoding large subunit ribosomal RNA (LSU rDNA). The analysis of the LSU rDNA sequences showed the existence of 3 haplotypes and the haplotype diversity of 0.733, and 8 haplotypes for the 3'rpl36-5' infA gene fragment and haplotype diversity of 0.956. The endosymbionts isolated from P. bursaria strains were identified as Chlorella vulgaris, Ch. variabilis and Micractinium conductrix. There was no correlation between the syngen of P. bursaria and the species of endosymbiont.

Paramecium is one of the most studied genera among ciliates. In particular, it is a model organism for investigation of the sibling species problem (also known as the cryptic species problem), spatial distribution, and its role in speciation. The global distribution of Paramecium species and of sibling species belonging to the P. aurelia species complex (Ciliophora, Protista) still need study, e.g. sampling in some territories has been quite limited, while Europe has been investigated for years with the majority of the P. aurelia species isolated from here. The large territory of Yakutia (republic Sakha in the Russian Federation), known for its climate extremes and continuous permafrost that extended over several glacial and interglacial cycles of the Pleistocene, has not been studied before. In the present study we collected paramecia in the central part of Yakutia. Newly established strains were identified to species according to morphology and, in case of the P. aurelia complex, by crossing with the test strains (the reference strains for the particular species). New stands of P. primaurelia, P. biaurelia and P. novaurelia were described from the territory of Yakutia.

Paramecium bursaria is composed of five syngens that are morphologically indistinguishable but sexually isolated. The aim of the present study was to confirm by molecular methods (analyses of mitochondrial COI) the identification of P. bursaria syngens originating from different geographical locations. Phylograms constructed using both the neighbor-joining and maximum-likelihood methods based on a comparison of 34 sequences of P. bursaria strains and P. multimicronucleatum, P. caudatum and P.calkinsi strains used as outgroups revealed five clusters which correspond to results obtained previously by mating reaction. Our analysis shows the existence of 24 haplotypes for the COI gene sequence in the studied strains. The interspecies haplotype diversity was Hd = 0.967. We confirmed genetic differentiation between strains of P. bursaria and the occurrence of a correlation between geographical distribution and the correspondent syngen.

The occurrence of species of the P. aurelia complex has been studied at a large scale in Europe and the majority of known species of the complex have been found there. However, a different number of habitats were studied in particular zones of Europe, the greatest number in the central zone. Herein new stands of several species of the Paramecium aurelia complex are presented from Europe including P. primaurelia, P. biaurelia, P. triaurelia, P. octaurelia, P. novaurelia, and P. dodecaurelia. In South America, studies concerning the distribution of the P. aurelia species complex were carried out only occasionally and the presence of some cosmopolitan species of the complex has been recorded, i.e. P. primaurelia, P. biaurelia, and P. tetraurelia. Recently, new stands of P. primaurelia and P. septaurelia were found in Ecuador. Ciliate biogeography and distribution is also discussed.

The relevance of geographical distribution and the roles of dispersal and spatial isolation during the speciation of microorganisms are nowadays of great interest. The Paramecium aurelia species complex is a perfect model system to explore these questions given its long history as a study subject and broad distribution. However, the world-wide distribution of the Paramecium aurelia complex (Ciliophora, Protista) still needs study, e.g., sampling in the southern hemisphere has been quite limited, while Europe has been investigated for years, with the majority of aurelia species isolated from here. Recently, new stands of species of the P. aurelia complex were found in southern Europe (Malta, Bulgaria, Cyprus) and in the Czech Republic (P. primaurelia, P. triaurelia, P. octaurelia). In Africa (Republic of South Africa), new stands of P. primaurelia, P. triaurelia, and P. octaurelia were found. Interestingly, the rare species P. triaurelia, and P. octaurelia were found to co-occur both in South Africa (SA 13) and the Czech Republic (CKV 8). Newly established strains were identified to species by crossing with the test strains (the reference strains for the particular species).

New stands of P. primaurelia, P. biaurelia, and P. dodecaurelia were found in Russia. P. primaurelia was recorded in Tulun (Siberia, Irkutsk region) and in three stands situated on the Kamchatka peninsula: in Lake Chalaktyrskoye, in the Valley of Geysers, and Petropavlovsk Kamchatski. P. biaurelia was also found in Tulun and in two stands in the vicinity of Lake Baikal and the Buriatia region. P. dodecaurelia was recorded in Cheboksary in European Russia and in other stands situated in Asian Russia: Novosibirsk, the vicinity of Lake Baikal, Buriatia, Kamchatka (Petropavlovsk Kamchatski, Lake Chalaktyrskoye, and Nalychevo). These data extend the ranges of species of the P. aurelia complex in Russia, however, this large territory remains understudied.

New stands of species of the Paramecium aurelia complex are presented in the paper, P. primaurelia recorded in Italy (Pisa) and in Morocco (Marrakesh), P. biaurelia in Italy (Calabria), P. triaurelia in Morocco (Ifrane), P. pentaurelia in Greece (Kastorya), and P. dodecaurelia in Italy (Padua).

New stands of Paramecium biaurelia, P. triaurelia, P. tetraurelia, P. pentaurelia, P. novaurelia, and P. dodecaurelia were recorded in Russia. Especially interesting is the record of P. novaurelia in Vladivostok, Russian Far East, as it is a very rare species outside of Europe. The distribution of species of the Paramecium aurelia complex in Eurasia with emphasis on findings in Russia is discussed.

New stands of Paramecium dodecaurelia were found in Russia, Ukraine, Kazahstan, Poland, and Tenerife. Molecular studies (RAPD and sequencing of LSU rRNA and CO1 mtDNA gene fragments) of P. dodecaurelia strains depicted intraspecific polymorphism within the species, revealed as distinct differences between strains from the USA in comparison with strains from different regions of the Palearctic.

The Volga, which is the largest river in Europe (3690 km long), flows from the north (Tver' region) to the south (Caspian Sea), and its extensive basin (1380 km2) includes very different biotopes. Thus, analysis ofthe occurrence of Paramecium species along this large river basin may significantly enhance our understanding of species distribution according to temperature regime, food richness and other possible factors. The present paper concerns the occurrence of species of the P. aurelia complex in the sampling areas of the Upper Volga River, and a comparison with the occurrence of species of the P. aurelia complex in the Lower Volga region. In the Upper Volga basin, P. biaurelia was the most abundant among species of the complex recorded (among P. triaurelia, P. decaurelia, P. dodecaurelia), in the Lower Volga region eight species of the complex were recorded (P. primaurelia, P. biaurelia, P. triaurelia, P. pentaurelia, P. sexaurelia, P. septaurelia, P. novaurelia, P. decaurelia).

The presence of Paramecium sexaurelia from the Paramecium aurelia complex was recorded for the first time in China (Beijing). RAPD fingerprints (band patterns) of P. sexaurelia strains, the new strain from China and others from Asia, as well as from Europe and Puerto Rico, showed polymorphism within the species as several groups of genotypes characterized by different band patterns.

The presence of Paramecium decaurelia from the Paramecium aurelia species complex was demonstrated in Yaroslavl, Russia, (European part, northwestern Russia) and in the Altai Mts (Asiatic part of Russia, western Siberia). RAPD-PCR fingerprints of the newly identified strains of P. decaurelia, rare throughout the world, were compared to those characteristic for the other known strains ofthe species. P. decaurelia strains show some polymorphism within species, strains from Russia have 60% similarity of band patterns, and strains from USA and Japan about 70% similarity of band patterns.

Paramecium strains collected in Central Russia, Western Siberia (the West Siberian Lowland and the Altai Mountains in the south) were studied. The presence of P. caudatum, P. bursaria, P. multimicronucleatum, P. polycaryum, and four species of the P. aurelia complex, i.e. P. primaurelia (in Omsk), P. biaurelia (in Krasnoyarsk and the Altai Mountains), P. triaurelia (in Krasnoyarsk), and P. pentaurelia (in Novosibirsk, Altai Foreland, and Altai Mountains) was revealed. P. triaurelia and P. pentaurelia were found for the first time in Asia.

The presence of P. primaurelia, P. biaurelia, P. triaurelia, and P. novaurelia of the P. aurelia complex was revealed in the studied region of Russia. RAPD-PCR fingerprints (band patterns) of newly identified P. novaurelia strains from Russia were compared to those characteristic for the other chosen European strains of the species. The strains revealed intraspecific polymorphism as several groups of genotypes confirming the existence of polymorphism within P. novaurelia.

New stands of rare species of the Paramecium aurelia complex were found in Europe, i.e. P. pentaurelia and P. dodecaurelia in Italy and P. septaurelia in Germany. The species were identified by mating reactions with the standard strains of each species. Their relationships with some other known strains of particular species were studied by classical strain crosses (survival in F1 and F2 generations) and by comparison of RAPD-PCR fingerprints. The presence of the cosmopolitan species P. tetraurelia in Italy was also recorded.

The presence of several species of the P. aurelia complex was revealed in the studied regions. In the Volgograd region P. primaurelia, P. biaurelia, P. triaurelia, and P. novaurelia were recorded. In the Astrakhan Nature Reserve P. primaurelia, P. pentaurelia, P. sexaurelia, and P. septaurelia were identified. Among these species, P. septaurelia was recorded for the first time in Europe, known before only from the territory of the USA, P. pentaurelia and P. sexaurelia are species rare in Europe. The studied regions are very rich in species of the P. aurelia complex and worthy of future studies.