Syndinean dinoflagellates of the genus Euduboscquella are paraphyletic.

Syndinean dinoflagellates of the genus Euduboscquella infect marine ciliates and dinoflagellates. Euduboscquella species infecting dinoflagellates are understudied relative to congeners infecting ciliates and their molecular phylogeny remains uncertain. Morphology, development, and rRNA gene sequences of intracellular parasites infecting heterotrophic dinoflagellates from coastal waters of Busan, Republic of Korea in summer to fall of 2019-2021 indicate that Cucumeridinium coeruleum, Gyrodinium cf. ochraceum, and two unidentified species of Gyrodinium were each infected by a different Euduboscquella species. Morphological features including shield structure, shape and color of the mature trophont, and sporogenic process distinguished each of the four parasites from the 10 previously described species of Euduboscquella. Our molecular and phylogenetic analyses showed considerably greater genetic distance of SSU and ITS-LSU rRNA gene regions among Euduboscquella species infecting dinoflagellates than among those infecting ciliates. Rather than clustering as a group with Euduboscquella species infecting ciliates, SSU rRNA sequences of the four novel parasites spread out across the syndinean Group I phylogeny, occurring in two different clades and a new lineage. Placement of our novel parasites in multiple clades that encompass Ichythyodinium chabelardi strongly indicates that the genus Euduboscquella is paraphyletic.

Ultrastructure of selected life history stages of the parasitic dinoflagellate Euduboscquella cachoni.

Euduboscquella species differ from most other syndinean dinoflagellates by having mononucleate trophonts, but resemble species of Amoebophrya and Sphaeripara by episome-hyposome differentiation and cortical complexity. Cytology and development of Euduboscquella species are well characterized, but their ultrastructure remains essentially unexplored. Transmission electron microscopy of Euduboscquella cachoni trophonts, tomont, and sporocytes revealed previously unrecognized structures. Initially dense, fibrous chromosomes uncoiled during early infection, with condensed chromosomes absent over much of the growth cycle recondensing at trophont maturity. The hyposomal amphiesma was two appressed membranes, the episomal cortex was alveolate, and a supraepisomal cavity limited by membrane enclosed the episome. Pseudopod-like extensions of the hyposome during mid infection may facilitate osmotrophic nutrition. The pharyngeal lamina appears to lack ingestatory function; however, transcortical transport of particles occurred via the supraepisomal cavity and episomal micropores. Microtubules originating from the electron-opaque perinema bordering the episome, formed an episomal skeleton hypothesized to function with the pharyngeal lamina, perinema, and the paired membranes of the supraepisomal cavity to effect parasite egress and ingestion of host material. Trichocysts absent during early infection developed during late infection and reached maturity during sporogenesis, suggesting functional importance in spore survival or infection.

Euduboscquella costata n. sp. (Dinoflagellata, Syndinea), an Intracellular Parasite of the Ciliate Schmidingerella arcuata: Morphology, Molecular Phylogeny, Life Cycle, Prevalence, and Infection Intensity.

The syndinean dinoflagellate Euduboscquella costata n. sp., an intracellular parasite of the tintinnid ciliate Schmidingerella arcuata, was discovered from Korean coastal water in November of 2013. Euduboscquella costata parasitized in about 62% of the host population, with infection intensity (= number of trophonts in a single host cell) ranging from 1 to 8. Based on morphology and nuclear 18S ribosomal RNA gene sequences, the parasite is new to science. Euduboscquella costata n. sp. had an infection cycle typical of the genus, but had morphological and developmental features that distinguished it from congeneric species. These features include: (1) episome of the trophont with 25-40 grooves converging toward the center of the shield; (2) a narrow, funnel-shaped lamina pharyngea extending from the margin of the episomal shield to the nucleus; (3) persistence of grooves during extracellular development (sporogenesis); (4) a single food vacuole during sporogenesis; (5) separation of sporocytes early in sporogenesis, regardless of type of spore formed; and (6) dinospore size (ca. 14 µm in length) and shape (bulbous episome with narrower, tapering hyposome). After sporogenesis, E. costata produced four different types of spore that showed completely identical 18S rRNA gene sequences. The gene sequence was completely identical with a previously reported population, Euduboscquella sp. ex S. arcuata, from Assawoman Bay, USA, indicating that the two populations are likely conspecific. Favella ehrenbergii, a widely recorded tintinnid known to host Euduboscquella spp., co-occurred with S. arcuata, but was not infected by E. costata in field samples or during short-term, cross-infection experiments.

Molecular diversity of the syndinean genus Euduboscquella based on single-cell PCR analysis.

The genus Euduboscquella is one of a few described genera within the syndinean dinoflagellates, an enigmatic lineage with abundant diversity in marine environmental clone libraries based on small subunit (SSU) rRNA. The region composed of the SSU through to the partial large subunit (LSU) rRNA was determined from 40 individual tintinnid ciliate loricae infected with Euduboscquella sampled from eight surface water sites in the Northern Hemisphere, producing seven distinct SSU sequences. The corresponding host SSU rRNA region was also amplified from eight host species. The SSU tree of Euduboscquella and syndinean group I sequences from environmental clones had seven well-supported clades and one poorly supported clade across data sets from 57 to 692 total sequences. The genus Euduboscquella consistently formed a supported monophyletic clade within a single subclade of group I sequences. For most parasites with identical SSU sequences, the more variable internal transcribed spacer (ITS) to LSU rRNA regions were polymorphic at 3 to 10 sites. However, in E. cachoni there was variation between ITS to LSU copies at up to 20 sites within an individual, while in a parasite of Tintinnopsis spp., variation between different individuals ranged up to 19 polymorphic sites. However, applying the compensatory base change model to the ITS2 sequences suggested no compensatory changes within or between individuals with the same SSU sequence, while one to four compensatory changes between individuals with similar but not identical SSU sequences were found. Comparisons between host and parasite phylogenies do not suggest a simple pattern of host or parasite specificity.

Revision of the family Duboscquellidae with description of Euduboscquella crenulata n. gen., n. sp. (Dinoflagellata, Syndinea), an intracellular parasite of the ciliate Favella panamensis Kofoid & Campbell.

Recent recognition that tintinnids are infected by dinophycean as well as syndinean parasites prompts taxonomic revision of dinoflagellate species that parasitize these ciliates. Long overlooked features of the type species Duboscquella tintinnicola are used to emend the genus and family Duboscquellidae, resulting in both taxa being moved from the Syndinea to the Dinophyceae. Syndinean species previously classified as Duboscquella are relocated to Euduboscquella n. gen., with Euduboscquella crenulata n. sp. as the type. As an endoparasitic species, E. crenulata shares with its congeners processes associated with intracellular development and sporogenesis, but differs from closely related species in nuclear and cortical morphology of the trophont, including a distinctively grooved shield (= episome) that imparts a crenulated appearance in optical section. In addition, E. crenulata produces three morphologically distinct spore types, two of which undergo syngamy to form a uninucleate zygote. The zygote undergoes successive division to produce four daughter cells of unequal size, but that resemble the nonmating spore type.

Ultrastructure of the oral apparatus of Mesodinium rubrum from Korea.

Mesodinium rubrum Lohmann is a photosynthetic marine ciliate that has functional chloroplasts of cryptophyte origin. Little is known about the oral ultrastructure of M. rubrum compared with several reports on the sequestration of nuclei and plastids from prey organisms, such as Geminigera cryophila and Teleaulax species. Here, we describe the fine structure of the oral apparatus of a M. rubrum strain from Gomso Bay, Korea. The cytopharynx was cone-shaped and supported by 20-22 ribbons of triplet microtubules. At the anterior end of the cytopharynx, an annulus anchored small cylinders composed of 11 microtubules. The small cylinders were spaced at regular intervals, each reinforced by one set of the triplet microtubules. At the opening of the cytostome, larger 14-membered microtubular cylinders were set adjacent to the small, 11-membered microtubular cylinders, each pair surrounded by separate membranes, however, only the large cylinders extended into the oral tentacles. There were 20-22 oral tentacles each having one to five extrusomes at its tip. At the anterior end of the oral apparatus, microtubular bands supporting the cytostome curved posteriad, extending beneath the cell cortex to the kinetosomes of the somatic cirri. The microtubular bands were connected by striated fibers and originated from kinetosomes anchored by fibers. Each cirrus consisted of eight cilia associated with 16 kinetosomes. The ultrastructure of M. rubrum from Korea provides information useful for taxonomic characterization of the genus Mesodinium and relevant to developing a better understanding of the acquisition of foreign organelles through phagocytosis by M. rubrum.

Tintinnophagus acutus n. g., n. sp. (Phylum Dinoflagellata), an ectoparasite of the ciliate Tintinnopsis cylindrica Daday 1887, and its relationship to Duboscquodinium collini Grassé 1952.

The dinoflagellate Tintinnophagus acutus n. g., n. sp., an ectoparasite of the ciliate Tintinnopsis cylindrica Daday, superficially resembles Duboscquodinium collini Grassé, a parasite of Eutintinnus fraknoii Daday. Dinospores of T. acutus are small transparent cells having a sharply pointed episome, conspicuous eyespot, posteriorly positioned nucleus with condensed chromosomes, and rigid form that may be supported by delicate thecal plates. Dinospores attach to the host via a feeding tube, losing their flagella, sulcus, and girdle to become spherical or ovoid cells. The trophont of T. acutus feeds on the host for several days, increasing dramatically in size before undergoing sporogenesis. Successive generations of daughter sporocytes are encompassed in an outer membrane or cyst wall, a feature not evident in trophonts. Tintinnophagus acutus differs from D. collini in host species, absence of a second membrane surrounding pre-sporogenic stages, and failure to differentiate into a gonocyte and a trophocyte at the first sporogenic division. Phylogenetic analyses based on small subunit (SSU) ribosomal DNA (rDNA) sequences placed T. acutus and D. collini in the class Dinophyceae, with T. acutus aligned loosely with Pfiesteria piscicida and related species, including Amyloodinium ocellatum, a parasite of fish, and Paulsenella vonstoschii, a parasite of diatoms. Dubosquodinium collini nested in a clade composed of several Scrippsiella species and Peridinium polonicum. Tree construction using longer rDNA sequences (i.e. SSU through partial large subunit) strengthened the placement of T. acutus and D. collini within the Dinophyceae.

Sexual and asexual processes in Protoperidinium steidingerae Balech (Dinophyceae), with observations on life-history stages of Protoperidinium depressum (Bailey) Balech (Dinophyceae).

A suite of morphological, histological, and molecular techniques was used to reveal for the first time division, sexuality, mandatory dormancy period of hypnozygotes, and identity of life-history stages of any Protoperidinium spp. In both Protoperidinium steidingerae and Protoperidinium depressum, asexual division occurred by eleutheroschisis within a temporary cyst, yielding two daughter cells. Daughter cells were initially round and one-half to two-thirds the size of parent cells then rapidly increased in size, forming horns before separating. Gamete production and fusion was constitutive in clonal and non-clonal cultures, indicating that both species may be homothallic. Gametes were isogamous, approximately half the size and lacking the pink pigmentation of the vegetative cells, and were never observed to feed. Gamete fusion resulted in a planozygote with two longitudinal flagella. Planozygotes of P. steidingerae formed hypnozygotes. The fate of planozygotes of P. depressum is unknown. Hypnozygotes of P. steidingerae had a mandatory dormancy period of ca. 70 days. Germination resulted in planomeiocytes with two longitudinal flagella. Nuclear cyclosis occurred in the planozygotes of P. depressum, but in the planomeiocytes of P. steidingerae. The plate tabulation and gross morphology of gametes of P. steidingerae and P. depressum differed markedly from those of vegetative cells. Thus, misidentification of morphologically distinct life-history stages and incomplete examination of thecal plate morphology in field specimens has likely led to taxonomic confusion of Protoperidinium spp. in previous studies.

Morphological characterization and intraspecific variation of Zoothamnium intermedium Precht, 1935 (Ciliophora, Peritrichia) attached to calanoid copepods in the Chesapeake Bay, USA.

A redescription of Zoothamnium intermedium Precht, 1935, a peritrich epibiont on copepods, is provided using specimens colonizing Acartia tonsa and Eurytemora affinis in Chesapeake Bay, USA. Bell-shaped zooids of Z. intermedium ranged in size from 31.2-54.7 microm x 16.7-31.3 microm in vivo. A single contractile vacuole and a "C shaped" macronucleus lie in the upper half of the body. Colonies had up to 30 zooids, but most presented two to four zooids. The single myoneme was continuous from the zooids through the lateral branches and basal stalk, terminating 4-73.2 mm before the attachment point of the colony. The ciliature of Z. intermedium was typical of sessile peritrichs, consisting of an outer haplokinety and an inner polykinety 1 (PK1) that made approximately turns around the peristomial disk before entering the infundibulum. Within the infundibulum, PK 1 was accompanied by polykinetid 2 (PK2) and polykinetid 3 (PK3), each consisting of three kinetosomal rows: PK2 of Z. intermedium terminated adoral to the aboral end of PK1 and had a central row of kinetosomes shorter than the lateral rows. Scanning electron microscopy revealed an annular pattern of pellicular ridges with pellicular pores. Morphological features used in peritrich species identification are quantified for specimens attached to both hosts and compared with other species of Zoothamnium. Statistically significant differences found between specimens attached to A. tonsa and E. affinis may reflect phenotypic plasticity rather than infestation by multiple species of Zoothamnium.

Do All Dinoflagellates have an Extranuclear Spindle?

The syndinean dinoflagellates are a diverse assemblage of alveolate endoparasites that branch basal to the core dinoflagellates. Because of their phylogenetic position, the syndineans are considered key model microorganisms in understanding early evolution in the dinoflagellates. Closed mitosis with an extranuclear spindle that traverses the nucleus in cytoplasmic grooves or tunnels is viewed as one of the morphological features shared by syndinean and core dinoflagellates. Here we describe nuclear morphology and mitosis in the syndinean dinoflagellate Amoebophrya sp. from Akashiwo sanguinea, a member of the A. ceratii complex, as revealed by protargol silver impregnation, DNA specific fluorochromes, and transmission electron microscopy. Our observations show that not all species classified as dinoflagellates have an extranuclear spindle. In Amoebophrya sp. from A. sanguinea, an extranuclear microtubule cylinder located in a depression in the nuclear surface during interphase moves into the nucleoplasm via sequential membrane fusion events and develops into an entirely intranuclear spindle. Results suggest that the intranuclear spindle of Amoebophrya spp. may have evolved from an ancestral extranuclear spindle and indicate the need for taxonomic revision of the Amoebophryidae.

Ultrastructure of Amoebophrya sp. and its changes during the course of infection.

Amoebophrya is a syndinian parasite that kills harmful bloom forming algae. Previously uncharacterized ultrastructural aspects of infection and development were elucidated. The biflagellate dinospore has two mitochondria, electron-dense bodies, striated strips, trichocysts, and a nucleus with peripherally condensed chromatin. After finding an Akashiwo sanguinea host and adhering to its surface, the parasite penetrates the host surface, apparently using a microfilament based motility and electron-dense bodies within a microtubular basket in the process of parasitophorous vacuole membrane formation. After entering the host nucleus, possibly by a similar mechanism used to enter the host cell, the parasite cytosol expanded substantially prior to mitosis. From 12-36 hours mitochondria were inconspicuous but present. Chromatin condensation was variable. By 36 hours post-infection, parasites had multiple nuclei, a microtubule-supported cytopharynx, and were beginning to form a fully internal mastigocoel. By 48 hours, the characteristic "beehive" appearance was apparent with flagella projecting into a fully developed mastigocoel. The cytoplasm contained trichocysts, elongated mitochondria, and nuclei with peripherally condensed chromatin. Although Amoebophrya lacks an apical complex, its electron-dense bodies show functional similarities to apicomplexan rhoptries. Its lack of permanently condensed chromosomes, but compact dinospore chromatin, supports the idea that dinoflagellate permanently condensed chromosomes may be a remnant of a parasitic ancestor with a compact dispersal stage.

DINOPHYSIS CAUDATA (DINOPHYCEAE) SEQUESTERS AND RETAINS PLASTIDS FROM THE MIXOTROPHIC CILIATE PREY MESODINIUM RUBRUM(1).

"Phototrophic"Dinophysis Ehrenberg species are well known to have chloroplasts of a cryptophyte origin, more specifically of the cryptophyte genus complex Teleaulax/Geminigera. Nonetheless, whether chloroplasts of "phototrophic"Dinophysis are permanent plastids or periodically derived kleptoplastids (stolen chloroplasts) has not been confirmed. Indeed, molecular sequence data and ultrastructural data lead to contradictory interpretations about the status of Dinophysis plastids. Here, we used established cultures of D. caudata strain DC-LOHABE01 and M. rubrum strain MR-MAL01 to address the status of Dinophysis plastids. Our approach was to experimentally generate D. caudata with "green" plastids and then follow the ingestion and fate of "reddish-brown" prey plastids using light microscopy, time-lapse videography, and single-cell TEM. Our results for D. caudata resolve the apparent discrepancy between morphological and molecular data by showing that plastids acquired when feeding on M. rubrum are structurally modified and retained as stellate compound chloroplasts characteristic of Dinophysis species.

Genetic diversity of parasitic dinoflagellates in the genus amoebophrya and its relationship to parasite biology and biogeography.

We determined 18S rRNA gene sequences of Amoebophrya strains infecting the thecate dinoflagellates Alexandrium affine and Gonyaulax polygramma from Korean coastal waters and compared those data with previously reported sequences of Amoebophrya from cultures, infected cells concentrated from field samples, and environmental 18S rRNA gene sequences obtained from a variety of marine environments. Further, we used these data to examine genetic diversity in Amoebophrya strains relative to geographic origin, host phylogeny, site of infection, and host specificity. In our analyses of known dinoflagellate taxa, the 13 available Amoebophrya sequences clustered together within the dinoflagellates as three groups forming a monophyletic group with high bootstrap support (maximum likelihood, ML: 100%) or a posterior probability (PP) of 1. When the Amoebophrya sequences were analyzed along with environmental sequences associated with Marine Alveolate Group II, nine subgroups formed a monophyletic group with high bootstrap support (ML: 100%) and PP of 1. Sequences known to be from Amoebophrya spp. infecting dinoflagellate hosts were distributed in seven of those subgroups. Despite differences in host species and geographic origin (Korea, United States, and Europe), Amoebophrya strains (Group II) from Gymnodinium instriatum, A. affine, Ceratium tripos (AY208892), Prorocentrum micans, and Ceratium lineatum grouped together by all of our tree construction methods, even after adding the environmental sequences. By contrast, strains within Groups I and III divided into several lineages following inclusion of environmental sequences. While Amoebophrya strains within Group II mostly developed within the host cytoplasm, strains in Groups I and III formed infections inside the host nucleus, a trait that appeared across several of the subgroups. Host specificity varied from moderately to extremely species-specific within groups, including Group II. Taken together, our results imply that genetic diversity in Amoebophrya strains does not always reflect parasite biology or biogeography.

Spatial and temporal patterns in the occurrence of peritrich ciliates as epibionts on calanoid copepods in the Chesapeake Bay, USA.

We investigated temporal and spatial patterns of distribution in two peritrich ciliates (i.e. Zoothamnium intermedium and Epistylis sp.) living as epibionts on calanoid copepods (i.e. Acartia tonsa and Eurytemora affinis) in Chesapeake Bay. Net tow samples collected along the main axis of the Bay were analyzed to estimate the occurrence of epibionts on copepods and to explore relationships among infestation prevalence, host abundance, and environmental variables. Zoothamnium intermedium and Epistylis sp. colonized populations of A. tonsa during spring and summer months, while only Z. intermedium colonized E. affinis during spring. Occurrence of epibionts on copepods showed high interannual variation, marked seasonality, and geographic heterogeneity. Extensive statistical analyses rejected simple scenarios of interactions between epibiosis, environmental variables, and host density, suggesting a more complex dynamics for the system. Analyses of epibiont colonies and zooids per host area (i.e. the sum of width and length of the body including antennae and swimming legs calculated assuming a cylindrical shape) were also performed. Overall, epibiont infestation prevalence (i.e. colonies/host area) and load (i.e. zooids/host area) were higher on copepodites than on adults for both host species, suggesting a preferential attachment to juveniles, or a higher predation pressure on adult stages. Infestation density and loads of both epibiont species were higher on the cephalothorax and abdomen of A. tonsa and E. affinis in comparison to the antennae and swimming legs, suggesting that ciliates can more easily colonize less active parts of the host.

Ecology of the red-tide dinoflagellate Ceratium furca: distribution, mixotrophy, and grazing impact on ciliate populations of Chesapeake Bay.

Ceratium furca is a primarily photosynthetic dinoflagellate also capable of ingesting other protists. During 1995 and 1996, we documented the abundance of C. furca in Chesapeake Bay and determined grazing rates on prey labeled with fluorescent microspheres. Abundance usually remained below 20 cells ml(-1), although the species was capable of localized late-summer blooms (< or = 478 cells ml(-1)) in the more saline lower to mid-Bay region. Feeding rates ranged from 0 to 0.11 prey dinoflagellate(-1) h(-1) or from 0 to 37 pg C dinoflagellate(-1) h(-1) and were highest at lower salinities. Clearance rates averaged 2.5 +/- 0.35 microl dinoflagellate(-1) h(-1). Impact of C. furca feeding on prey populations was higher in the lower Bay, averaging 67% of Strobilidium spp. removed d(-1). Ingestion rates were positively correlated with prey abundance and dissolved inorganic nitrogen, but negatively with salinity, depth, dissolved inorganic phosphorus, and inorganic P:N ratio. Daily consumption of prey biomass by C. furca averaged 4.6% of body carbon, 6.5% of body nitrogen, and 4.0% of body phosphorus. with maximal values of 36, 51, and 32%, respectively. Thus, the ability to exploit an organic nutrient source when inorganic nutrients are limiting may give C. furca a competitive advantage over purely photosynthetic species.

Parasites and phytoplankton, with special emphasis on dinoflagellate infections.

Planktonic members of most algal groups are known to harbor intracellular symbionts, including viruses, bacteria, fungi, and protozoa. Among the dinoflagellates, viral and bacterial associations were recognized a quarter century ago, yet their impact on host populations remains largely unresolved. By contrast, fungal and protozoan infections of dinoflagellates are well documented and generally viewed as playing major roles in host population dynamics. Our understanding of fungal parasites is largely based on studies for freshwater diatoms and dinoflagellates, although fungal infections are known for some marine phytoplankton. In freshwater systems, fungal chytrids have been linked to mass mortalities of host organisms, suppression or retardation of phytoplankton blooms, and selective effects on species composition leading to successional changes in plankton communities. Parasitic dinoflagellates of the genus Amoebophrya and the newly described Perkinsozoa, Parvilucifera infectans, are widely distributed in coastal waters of the world where they commonly infect photosynthetic and heterotrophic dinoflagellates. Recent work indicates that these parasites can have significant impacts on host physiology, behavior, and bloom dynamics. Thus, parasitism needs to be carefully considered in developing concepts about plankton dynamics and the flow of material in marine food webs.

Multiple strains of the parasitic dinoflagellate Amoebophrya exist in Chesapeake Bay.

Small subunit rRNA sequences were amplified from Amoebophrya strains infecting Karlodinium micrum, Gymnodinium instriatum and an unidentified Scrippsiella species in Chesapeake Bay. The alignable parts of the sequences differed from each other and from the previously reported rRNA sequence of the Amoebophrya strain infecting Akashiwo sanguinea in Chesapeake Bay by 4 to 10%. This is a greater degree of difference than sometimes found between sequences from separate genera of free-living dinoflagellates. These sequence differences indicate that the Amoebophrya strains parasitizing dinoflagellates in Chesapeake Bay do not all belong to the same species. In spite of their relative dissimilarity, the sequences do group together into a single clade with high bootstrap support in phylogenetic trees constructed from the sequences.

Molecular phylogeny of phyllopharyngean ciliates and their group I introns.

We analyzed small subunit ribosomal DNA (ssu-rDNA) sequences to evaluate both the monophyly of the ciliate class Phyllopharyngea de Puytorac et al. (1974), and relationships among subclasses. Classifications based on morphology and ultrastructure divide the Phyllopharyngea into four subclasses, the Phyllopharyngia, Chonotrichia, Rhynchodia, and Suctoria. Our analyses of ssu-rDNA genealogies derived from sequence data collected from diverse members representing three of the four subclasses of Phyllopharyngea (Suctoria: Ephelota spp., Prodiscophyra collini, Acineta sp.; Phyllopharyngia: Chlamydodon exocellatus, Chlamydodon triquetrus, Dysteria sp.; and Chonotrichia: Isochona sp.) provide strong support for the monophyly of the Phyllopharyngea, and show that the Chonotrichia emerge from within the Phyllopharyngia. Based on this initial sampling, suctorian budding types are monophyletic, and exogenous budding appears to be basal to evaginative and endogenous budding. Further, we report the discovery of a group I intron at position 891 in the Suctoria Acineta sp. and Tokophrya lemnarum, and a second group I intron at position 1506 in T. lemnarum. These introns represent only the second examples of group I introns in a ciliate ribosomal gene, since the discovery of ribozymes in the LSU rRNA gene of Tetrahymena thermophila. Phylogenetic analyses of Group I introns suggest a complex evolutionary history involving either multiple loses or gains of introns within endogenously budding Suctoria.