Molecular phylogeny and ultrastructure of Selenidium serpulae (Apicomplexa, Archigregarinia) from the calcareous tubeworm Serpula vermicularis (Annelida, Polychaeta, Sabellida)

Novel cytoskeletal traits in the intestinal parasites (Squirmida, Platyproteum vivax) of Pacific peanut worms (Sipuncula, Phascolosoma agassizii)

The cytoskeletal organization of a squirmid, namely Platyproteum vivax, was investigated with confocal laser scanning microscopy (CLSM) to refine inferences about convergent evolution among intestinal parasites of marine invertebrates. Platyproteum inhabits Pacific peanut worms (Phascolosoma agassizii) and has traits that are similar to other lineages of myzozoan parasites, namely gregarine apicomplexans within Selenidium, such as conspicuous feeding stages, called "trophozoites," capable of dynamic undulations. SEM and CLSM of P. vivax revealed an inconspicuous flagellar apparatus and a uniform array of longitudinal microtubules organized in bundles (LMBs). Extreme flattening of the trophozoites and a consistently oblique morphology of the anterior end provided a reliable way to distinguish dorsal and ventral surfaces. CLSM revealed a novel system of microtubules oriented in the flattened dorsoventral plane. Most of these dorsoventral microtubule bundles (DVMBs) had a punctate distribution and were evenly spaced along a curved line spanning the longitudinal axis of the trophozoites. This configuration of microtubules is inferred to function in maintaining the flattened shape of the trophozoites and facilitate dynamic undulations. The novel traits in Platyproteum are consistent with phylogenomic data showing that this lineage is only distantly related to Selenidium and other marine gregarine apicomplexans with dynamic intestinal trophozoites.

Nutrient Acquisition and Attachment Strategies in Basal Lineages: A Tough Nut to Crack in the Evolutionary Puzzle of Apicomplexa

Apicomplexa are unicellular eukaryotes that parasitise a wide spectrum of invertebrates and vertebrates, including humans. In their hosts, they occupy a variety of niches, from extracellular cavities (intestine, coelom) to epicellular and intracellular locations, depending on the species and/or developmental stages. During their evolution, Apicomplexa thus developed an exceptionally wide range of unique features to reach these diversified parasitic niches and to survive there, at least long enough to ensure their own transmission or that of their progeny. This review summarises the current state of knowledge on the attachment/invasive and nutrient uptake strategies displayed by apicomplexan parasites, focusing on trophozoite stages of their so far poorly studied basal representatives, which mostly parasitise invertebrate hosts. We describe their most important morphofunctional features, and where applicable, discuss existing major similarities and/or differences in the corresponding mechanisms, incomparably better described at the molecular level in the more advanced Apicomplexa species, of medical and veterinary significance, which mainly occupy intracellular niches in vertebrate hosts.

Parallel functional reduction in the mitochondria of apicomplexan parasites

Gregarines are an early-diverging lineage of apicomplexan parasites that hold many clues into the origin and evolution of the group, a remarkable transition from free-living phototrophic algae into obligate parasites of animals.¹ Using single-cell transcriptomics targeting understudied lineages to complement available sequencing data, we characterized the mitochondrial metabolic repertoire across the tree of apicomplexans. In contrast to the large suite of proteins involved in aerobic respiration in well-studied parasites like Toxoplasma or Plasmodium,² we find that gregarine trophozoites have significantly reduced energy metabolism: most lack respiratory complexes III and IV, and some lack the electron transport chains (ETCs) and tricarboxylic acid (TCA) cycle entirely. Phylogenomic analyses show that these reductions took place several times in parallel, resulting in a functional range from fully aerobic organelles to extremely reduced "mitosomes" restricted to Fe-S cluster biosynthesis. The mitochondrial genome has also been lost repeatedly: in species with severe functional reduction simply by gene loss but in one species with a complete ETC by relocating cox1 to the nuclear genome. Severe functional reduction of mitochondria is generally associated with structural reduction, resulting in small, nondescript mitochondrial-related organelles (MROs).³ By contrast, gregarines retain distinctive mitochondria with tubular cristae, even the most functionally reduced cases that also lack genes associated with cristae formation. Overall, the parallel, severe reduction of gregarine mitochondria expands the diversity of organisms that contain MROs and further emphasizes the role of parallel transitions in apicomplexan evolution.

Motility and cytoskeletal organisation in the archigregarine Selenidium pygospionis (Apicomplexa): observations on native and experimentally affected parasites

Representatives of Apicomplexa perform various kinds of movements that are linked to the different stages of their life cycle. Ancestral apicomplexan lineages, including gregarines, represent organisms suitable for research into the evolution and diversification of motility within the group. The vermiform trophozoites and gamonts of the archigregarine Selenidium pygospionis perform a very active type of bending motility. Experimental assays and subsequent light, electron, and confocal microscopic analyses demonstrated the fundamental role of the cytoskeletal proteins actin and tubulin in S. pygospionis motility and allowed us to compare the mechanism of its movement to the gliding machinery (the so-called glideosome concept) described in apicomplexan zoites. Actin-modifying drugs caused a reduction in the movement speed (cytochalasin D) or stopped the motility of archigregarines completely (jasplakinolide). Microtubule-disrupting drugs (oryzalin and colchicine) had an even more noticeable effect on archigregarine motility. The fading and disappearance of microtubules were documented in ultrathin sections, along with the formation of α-tubulin clusters visible after the immunofluorescent labelling of drug-treated archigregarines. The obtained data indicate that subpellicular microtubules most likely constitute the main motor structure involved in S. pygospionis bending motility, while actin has rather a supportive function.

Morphology and molecular systematic of marine gregarines (Apicomplexa) from Southwestern Atlantic spionid polychaetes

Gregarines are a common group of parasites that infect the intestines of marine invertebrates, and particularly polychaetes. Here, we describe for the first time four gregarine species that inhabit the intestines of three spionid species: Dipolydora cf. flava, Spio quadrisetosa and Boccardia proboscidea from the Patagonian coast, Argentina, using light and scanning electron microscopy and molecular phylogenetic analyses of small subunit (SSU) rDNA sequences. Even though the spionid species thrive in the same environments, our results showed a high host specificity of the gregarine species. Selenidium cf. axiferens and Polyrhabdina aff. polydorae were both identified from the intestine of D. cf. flava. The new species, Polyrhabdina madrynense sp. n. and Selenidium patagonica sp. n., were described from the intestines of S. quadrisetosa and the invasive species B. proboscidea, respectively. All specimens of D. cf. flava and S. quadrisetosa were infected by gregarines (P = 100%), recording the highest mean intensity values of infection (MI = 80; 60 respectively), in contrast to B. proboscidea (P = 60%; MI = 38). We associated this finding with the recent invasion of this host. It is expected that in the future, an increase of its population density might favour a rising intensity of this gregarine infection.

Fine structure and Molecular Phylogenetic Position of Two Marine Gregarines, Selenidium pygospionis sp. n. and S. pherusae sp. n., with Notes on the Phylogeny of Archigregarinida (Apicomplexa)

Archigregarines are a key group for understanding the early evolution of Apicomplexa. Here we report morphological, ultrastructural, and molecular phylogenetic evidence from two archigregarine species: Selenidium pygospionis sp. n. and S. pherusae sp. n. They exhibited typical features of archigregarines. Additionally, an axial row of vacuoles of a presumably nutrient distribution system was revealed in S. pygospionis. Intracellular stages of S. pygospionis found in the host intestinal epithelium may point to the initial intracellular localization in the course of parasite development. Available archigregarine SSU (18S) rDNA sequences formed four major lineages fitting the taxonomical affiliations of their hosts, but not the morphological or biological features used for the taxonomical revision by Levine (1971). Consequently, the genus Selenidioides Levine, 1971 should be abolished. The branching order of these lineages was unresolved; topology tests rejected neither para- nor monophyly of archigregarines. We provided phylogenies based on LSU (28S) rDNA and near-complete ribosomal operon (concatenated SSU, 5.8S, LSU rDNAs) sequences including S. pygospionis sequences. Although being preliminary, they nevertheless revealed the monophyly of gregarines previously challenged by many molecular phylogenetic studies. Despite their molecular-phylogenetic heterogeneity, archigregarines exhibit an extremely conservative plesiomorphic structure; their ultrastructural key features appear to be symplesiomorphies rather than synapomorphies.

Archigregarines of the English Channel revisited: New molecular data on Selenidium species including early described and new species and the uncertainties of phylogenetic relationships

Background

Gregarines represent an important transition step from free-living predatory (colpodellids s.l.) and/or photosynthetic (Chromera and Vitrella) apicomplexan lineages to the most important pathogens, obligate intracellular parasites of humans and domestic animals such as coccidians and haemosporidians (Plasmodium, Toxoplasma, Eimeria, Babesia, etc.). While dozens of genomes of other apicomplexan groups are available, gregarines are barely entering the molecular age. Among the gregarines, archigregarines possess a unique mixture of ancestral (myzocytosis) and derived (lack of apicoplast, presence of subpellicular microtubules) features.

Methodology/Principal findings

In this study we revisited five of the early-described species of the genus Selenidium including the type species Selenidium pendula, with special focus on surface ultrastructure and molecular data. We were also able to describe three new species within this genus. All species were characterized at morphological (light and scanning electron microscopy data) and molecular (SSU rDNA sequence data) levels. Gregarine specimens were isolated from polychaete hosts collected from the English Channel near the Station Biologique de Roscoff, France: Selenidium pendula from Scolelepis squamata, S. hollandei and S. sabellariae from Sabellaria alveolata, S. sabellae from Sabella pavonina, Selenidium fallax from Cirriformia tentaculata, S. spiralis sp. n. and S. antevariabilis sp. n. from Amphitritides gracilis, and S. opheliae sp. n. from Ophelia roscoffensis. Molecular phylogenetic analyses of these data showed archigregarines clustering into five separate clades and support previous doubts about their monophyly.

Conclusions/Significance

Our phylogenies using the extended gregarine sampling show that the archigregarines are indeed not monophyletic with one strongly supported clade of Selenidium sequences around the type species S. pendula. We suggest the revision of the whole archigregarine taxonomy with only the species within this clade remaining in the genus Selenidium, while the other species should be moved into newly erected genera. However, the SSU rDNA phylogenies show very clearly that the tree topology and therefore the inferred relationships within and in between clades are unstable and such revision would be problematic without additional sequence data.

Motility in blastogregarines (Apicomplexa): Native and drug-induced organisation of Siedleckia nematoides cytoskeletal elements

Recent studies on motility of Apicomplexa concur with the so-called glideosome concept applied for apicomplexan zoites, describing a unique mechanism of substrate-dependent gliding motility facilitated by a conserved form of actomyosin motor and subpellicular microtubules. In contrast, the gregarines and blastogregarines exhibit different modes and mechanisms of motility, correlating with diverse modifications of their cortex. This study focuses on the motility and cytoskeleton of the blastogregarine Siedleckia nematoides Caullery et Mesnil, 1898 parasitising the polychaete Scoloplos cf. armiger (Müller, 1776). The blastogregarine moves independently on a solid substrate without any signs of gliding motility; the motility in a liquid environment (in both the attached and detached forms) rather resembles a sequence of pendular, twisting, undulation, and sometimes spasmodic movements. Despite the presence of key glideosome components such as pellicle consisting of the plasma membrane and the inner membrane complex, actin, myosin, subpellicular microtubules, micronemes and glycocalyx layer, the motility mechanism of S. nematoides differs from the glideosome machinery. Nevertheless, experimental assays using cytoskeletal probes proved that the polymerised forms of actin and tubulin play an essential role in the S. nematoides movement. Similar to Selenidium archigregarines, the subpellicular microtubules organised in several layers seem to be the leading motor structures in blastogregarine motility. The majority of the detected actin was stabilised in a polymerised form and appeared to be located beneath the inner membrane complex. The experimental data suggest the subpellicular microtubules to be associated with filamentous structures (= cross-linking protein complexes), presumably of actin nature.

A new view on the morphology and phylogeny of eugregarines suggested by the evidence from the gregarine Ancora sagittata (Leuckart, 1860) Labbé, 1899 (Apicomplexa: Eugregarinida)

BACKGROUND

Gregarines are a group of early branching Apicomplexa parasitizing invertebrate animals. Despite their wide distribution and relevance to the understanding the phylogenesis of apicomplexans, gregarines remain understudied: light microscopy data are insufficient for classification, and electron microscopy and molecular data are fragmentary and overlap only partially.

METHODS

Scanning and transmission electron microscopy, PCR, DNA cloning and sequencing (Sanger and NGS), molecular phylogenetic analyses using ribosomal RNA genes (18S (SSU), 5.8S, and 28S (LSU) ribosomal DNAs (rDNAs)).

RESULTS AND DISCUSSION

We present the results of an ultrastructural and molecular phylogenetic study on the marine gregarine Ancora sagittata from the polychaete Capitella capitata followed by evolutionary and taxonomic synthesis of the morphological and molecular phylogenetic evidence on eugregarines. The ultrastructure of Ancora sagittata generally corresponds to that of other eugregarines, but reveals some differences in epicytic folds (crests) and attachment apparatus to gregarines in the family Lecudinidae, where Ancora sagittata has been classified. Molecular phylogenetic trees based on SSU (18S) rDNA reveal several robust clades (superfamilies) of eugregarines, including Ancoroidea superfam. nov., which comprises two families (Ancoridae fam. nov. and Polyplicariidae) and branches separately from the Lecudinidae; thus, all representatives of Ancoroidea are here officially removed from the Lecudinidae. Analysis of sequence data also points to possible cryptic species within Ancora sagittata and the inclusion of numerous environmental sequences from anoxic habitats within the Ancoroidea. LSU (28S) rDNA phylogenies, unlike the analysis of SSU rDNA alone, recover a well-supported monophyly of the gregarines involved (eugregarines), although this conclusion is currently limited by sparse taxon sampling and the presence of fast-evolving sequences in some species. Comparative morphological analyses of gregarine teguments and attachment organelles lead us to revise their terminology. The terms "longitudinal folds" and "mucron" are restricted to archigregarines, whereas the terms "epicystic crests" and "epimerite" are proposed to describe the candidate synapomorphies of eugregarines, which, consequently, are considered as a monophyletic group. Abolishing the suborders Aseptata and Septata, incorporating neogregarines into the Eugregarinida, and treating the major molecular phylogenetic lineages of eugregarines as superfamilies appear as the best way of reconciling recent morphological and molecular evidence. Accordingly, the diagnosis of the order Eugregarinida Léger, 1900 is updated.

It's official - Cryptosporidium is a gregarine: What are the implications for the water industry?

Parasites of the genus Cryptosporidium are a major cause of diarrhoea and ill-health in humans and animals and are frequent causes of waterborne outbreaks. Until recently, it was thought that Cryptosporidium was an obligate intracellular parasite that only replicated within a suitable host, and that faecally shed oocysts could survive in the environment but could not multiply. In light of extensive biological and molecular data, including the ability of Cryptosporidium to complete its life cycle in the absence of a host and the production of novel extracellular stages, Cryptosporidium has been formally transferred from the Coccidia, to a new subclass, Cryptogregaria, with gregarine parasites. In this review, we discuss the close relationship between Cryptosporidium and gregarines and discuss the implications for the water industry.

Ultrastructure of Selenidium pendula, the Type Species of Archigregarines, and Phylogenetic Relations to Other Marine Apicomplexa

Archigregarines, an early branching lineage within Apicomplexa, are a poorly-known group of invertebrate parasites. By their phylogenetic position, archigregarines are an important lineage to understand the functional transition that occurred between free-living flagellated predators to obligatory parasites in Apicomplexa. In this study, we provide new ultrastructural data and phylogenies based on SSU rDNA sequences using the type species of archigregarines, the Selenidiidae Selenidium pendulaGiard, 1884. We describe for the first time the syzygy and early gamogony at the ultrastructural level, revealing a characteristic nuclear multiplication with centrocones, cryptomitosis, filamentous network of chromatin, a cyst wall secretion and a 9+0 flagellar axoneme of the male gamete. S. pendula belongs to a monophyletic lineage that includes several other related species, all infecting Sedentaria Polychaeta (Spionidae, Sabellaridae, Sabellidae and Cirratulidae). All of these Selenidium species exhibit similar biological characters: a cell cortex with the plasma membrane - inner membrane complex - subpellicular microtubule sets, an apical complex with the conoid, numerous rhoptries and micronemes, a myzocytosis with large food vacuoles, a nuclear multiplication during syzygy and young gamonts. Two other distantly related Selenidium-like lineages infect Terebellidae and Sipunculida, underlying the ability of archigregarines to parasite a wide range of marine hosts.

Extracellular excystation and development of Cryptosporidium: tracing the fate of oocysts within Pseudomonas aquatic biofilm systems

Background

Aquatic biofilms often serve as environmental reservoirs for microorganisms and provide them with a nutrient-rich growth environment under harsh conditions. With regard to Cryptosporidium, biofilms can serve as environmental reservoirs for oocysts, but may also support the growth of additional Cryptosporidium stages.

Results

Here we used confocal laser scanning microscopy, scanning electron microscopy (SEM), and flow cytometry to identify and describe various Cryptosporidium developmental stages present within aquatic biofilm systems, and to directly compare these to stages produced in cell culture. We also show that Cryptosporidium has the ability to form a parasitophorous vacuole independently, in a host-free biofilm environment, potentially allowing them to complete an extracellular life cycle. Correlative data from confocal and SEM imaging of the same cells confirmed that the observed developmental stages (including trophozoites, meronts, and merozoites) were Cryptosporidium. These microscopy observations were further supported by flow cytometric analyses, where excysted oocyst populations were detected in 1, 3 and 6 day-old Cryptosporidium-exposed biofilms, but not in biofilm-free controls.

Conclusions

These observations not only highlight the risk that aquatic biofilms pose in regards to Cryptosporidium outbreaks from water distribution systems, but further indicate that even simple biofilms are able to stimulate oocyst excystation and support the extracellular multiplication and development of Cryptosporidium within aquatic environments.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-014-0281-8) contains supplementary material, which is available to authorized users.

Comparative ultrastructure and molecular phylogeny of Selenidium melongena n. sp. and S. terebellae Ray 1930 demonstrate niche partitioning in marine gregarine parasites (apicomplexa)

Gregarine apicomplexans are a diverse group of single-celled parasites that have feeding stages (trophozoites) and gamonts that generally inhabit the extracellular spaces of invertebrate hosts living in marine, freshwater, and terrestrial environments. Inferences about the evolutionary morphology of gregarine apicomplexans are being incrementally refined by molecular phylogenetic data, which suggest that several traits associated with the feeding cells of gregarines arose by convergent evolution. The study reported here supports these inferences by showing how molecular data reveals traits that are phylogenetically misleading within the context of comparative morphology alone. We examined the ultrastructure and molecular phylogenetic positions of two gregarine species isolated from the spaghetti worm Thelepus japonicus: Selenidium terebellaeRay 1930 and S. melongena n. sp. The ultrastructural traits of S. terebellae were very similar to other species of Selenidium sensu stricto, such as having vermiform trophozoites with an apical complex, few epicytic folds, and a dense array of microtubules underlying the trilayered pellicle. By contrast, S. melongena n. sp. lacked a comparably discrete assembly of subpellicular microtubules, instead employing a system of fibrils beneath the cell surface that supported a relatively dense array of helically arranged epicytic folds. Molecular phylogenetic analyses of small subunit rDNA sequences derived from single-cell PCR unexpectedly demonstrated that these two gregarines are close sister species. The ultrastructural differences between these two species were consistent with the fact that S. terebellae infects the inner lining of the host intestines, and S. melongena n. sp. primarily inhabits the coelom, infecting the outside wall of the host intestine. Altogether, these data demonstrate a compelling case of niche partitioning and associated morphological divergence in marine gregarine apicomplexans.

Phase contrast without phase plates and phase rings--optical solutions for improved imaging of phase structures

Using the optical methods described, phase specimens can be observed with a modified light microscope in enhanced clarity, purified from typical artifacts which are apparent in standard phase contrast illumination. In particular, haloing and shade-off are absent, lateral and vertical resolution are maximized and the image quality remains constant even in problematic preparations which cannot be well examined in normal phase contrast, such as specimens beyond a critical thickness or covered by obliquely situated cover slips. The background brightness and thus the range of contrast can be continuously modulated and specimens can be illuminated in concentric-peripheral, axial or paraxial light. Additional contrast effects can be achieved by spectral color separation. Normal glass or mirror lenses can be used; they do not need to be fitted with a phase plate or a phase ring. The methods described should be of general interest for all disciplines using phase microscopy.

Species boundaries in gregarine apicomplexan parasites: a case study-comparison of morphometric and molecular variability in Lecudina cf. tuzetae (Eugregarinorida, Lecudinidae)

Trophozoites of gregarine apicomplexans are large feeding cells with diverse morphologies that have played a prominent role in gregarine systematics. The range of variability in trophozoite shapes and sizes can be very high even within a single species depending on developmental stages and host environmental conditions; this makes the delimitation of different species of gregarines based on morphological criteria alone very difficult. Accordingly, comparisons of morphological variability and molecular variability in gregarines are necessary to provide a pragmatic framework for establishing species boundaries within this diverse and poorly understood group of parasites. We investigated the morphological and molecular variability present in the gregarine Lecudina cf. tuzetae from the intestines of Nereis vexillosa (Polychaeta) collected in two different locations in Canada. Three distinct morphotypes of trophozoites were identified and the small subunit (SSU) rDNA was sequenced either from multicell isolates of the same morphotype or from single cells. The aim of this investigation was to determine whether the different morphotypes and localities reflected phylogenetic relatedness as inferred from the SSU rDNA sequence data. Phylogenetic analyses of the SSU rDNA demonstrated that the new sequences did not cluster according to morphotype or locality and instead were intermingled within a strongly supported clade. A comparison of 1,657 bp from 45 new sequences demonstrated divergences between 0% and 3.9%. These data suggest that it is necessary to acquire both morphological and molecular data in order to effectively delimit the "clouds" of variation associated with each gregarine species and to unambiguously reidentify these species in the future.

The fine structure of the developing oocyst of Pterospora floridiensis (Apicomplexa, Urosporidae)

Developing oocysts of the gregarine Pterospora floridiensis Landers 2001 were examined by transmission electron microscopy. Each oocyst had an outer capsule and an inner capsule that contained 8 sporozoites. In early stages of development the inner capsular wall was separated from the developing sporozoites and residual mass, and was not appressed to the sporozoites. Early stage sporozoites were connected to a residual mass and were filled with endoplasmic reticulum, golgi and numerous developing secretory vesicles. In late stages of oocyst and sporozoite development, the inner capsular wall was closely appressed to the sporozoite surface. The inner capsular wall was approximately 60-100 nm thick and the outer capsular wall was approximately 160-320 nm thick. There were no extensions on the outer wall for which the genus was named. Late stage sporozoites had no residual mass connection, were more electron dense, and contained three distinct types of dense secretory structures: 1) small oval/spherical dense vesicles, 2) large (350-400 nm) vesicles near the anterior end, and 3) elongated dense tubular bodies that converged at the apex. Few ultrastructural reports exist of developing gregarine oocysts and sporozoites, and as more studies are completed these morphological characteristics may be important in interpreting molecular phylogenetic analyses.

Molecular phylogeny and surface morphology of marine archigregarines (Apicomplexa), Selenidium spp., Filipodium phascolosomae n. sp., and Platyproteum n. g. and comb. from North-Eastern Pacific peanut worms (Sipuncula)

The trophozoites of two novel archigregarines, Selenidium pisinnus n. sp. and Filipodium phascolosomae n. sp., were described from the sipunculid Phascolosoma agassizii. The trophozoites of S. pisinnus n. sp. were relatively small (64-100 microm long and 9-25 microm wide), had rounded ends, and had about 21 epicytic folds per side. The trophozoites of F. phascolosomae n. sp. were highly irregular in shape and possessed hair-like surface projections. The trophozoites of this species were 85-142 microm long and 40-72 microm wide and possessed a distinct longitudinal ridge that extended from the mucron to the posterior end of the cell. In addition to the small subunit (SSU) rDNA sequences of these two species, we also characterized the surface morphology and SSU rDNA sequence of Selenidium orientale, isolated from the sipunculid Themiste pyroides. Molecular phylogenetic analyses demonstrated that S. pisinnus n. sp. and S. orientale formed a strongly supported clade within other Selenidium and archigregarine-like environmental sequences. Filipodium phascolosomae n. sp. formed the nearest sister lineage to the dynamic, tape-like gregarine Selenidium vivax. Overall, these data enabled us to reassess the molecular systematics of archigregarines within sipunculid hosts and make the following revisions: (1) Filipodium was transferred from the Lecudinidae (eugregarines) to the Selenidiidae (archigregarines), and (2) Platyproteum n. g. was established for Platyproteum vivax n. comb. (ex. S. vivax) in order to account for the highly divergent morphological features and better resolved phylogenetic position of this lineage.

Marine gregarines: evolutionary prelude to the apicomplexan radiation?

Gregarine apicomplexans inhabit the intestines, coeloms and reproductive vesicles of invertebrates. An emphasis on specific ancestral characteristics in marine gregarines has given the group a reputation of being 'primitive.' Although some lineages have retained characteristics inferred to be ancestral for the group, and perhaps apicomplexans as a whole, most gregarines represent highly derived parasites with novel ultrastructural and behavioral adaptations. Many marine gregarines have become giants among single-celled organisms and have evolved ornate surface structures. A comparison of gregarine morphology, placed in a modern phylogenetic context, helps clarify the earliest stages of apicomplexan evolution, the origin of Cryptosporidium, and specific cases of convergent evolution within the group and beyond.