A new microsporidian parasite, Potaspora morhaphis n. gen., n. sp. (Microsporidia) infecting the Teleostean fish, Potamorhaphis guianensis from the River Amazon. Morphological, ultrastructural and molecular characterization

A fish-infecting Microsporidia Potaspora morhaphis n. gen., n. sp. found adherent to the wall of the coelomic cavity of the freshwater fish, Potamorhaphis guianensis, from lower Amazon River is described, based on light microscope and ultrastructural characteristics. This microsporidian forms whitish xenomas distinguished by the numerous filiform and anastomosed microvilli. The xenoma was completely filled by several developmental stages. In all of these stages, the nuclei are monokaryotic and develop in direct contact with host cell cytoplasm. The merogonial plasmodium divides by binary fission and the disporoblastic pyriform spores of sporont origin measure 2.8+/-0.3 x 1.5+/-0.2 microm. In mature spores the polar filament was arranged into 9-10 coils in 2 layers. The polaroplast had 2 distinct regions around the manubrium and an electron-dense globule was observed. The small subunit, intergenic space and partial large subunit rRNA gene were sequenced and maximum parsimony analysis placed the microsporidian described here in the clade that includes the genera Kabatana, Microgemma, Spraguea and Tetramicra. The ultrastructural morphology of the xenoma, and the developmental stages including the spores of this microsporidian parasite, as well as the phylogenetic analysis, suggest the erection of a new genus and species.     

Ultrastructural details of the xenoma of Loma myrophis (phylum Microsporidia) and extrusion of the polar tube during autoinfection

Xenomas of the recently described new microsporidian species Loma myrophis parasitizing the gut tissue of the Amazonian fish Myrophis platyrhynchus (family Ophichthidae) were described by light- and transmission-electron microscopy. The xenoma consisted of a thin fibrillar wall that surrounded a hypertrophic host cell cytoplasm containing numerous microsporidian developmental stages and spores. Several spores showed different stages of natural extrusion of the polar tube. Numerous longitudinal and transverse sections of the extruded polar tubes were observed in developing life-cycle stages (spores excepted), the nucleus of hypertrophic host cell, the xenoma wall and surrounding fibroblasts. The extruded polar tubes were projected in all directions with no preferential orientation. These aspects suggested that autoinfection occurred within this xenoma.     

A new microsporidian infecting the musculature of the endangered tidewater goby (Gobiidae)

A previously unrecognized microsporidian (Kabatana newberryi n. sp.) is described from the musculature of Eucyclogobius newberryi (Gobiidae) in Big Lagoon, Humboldt County, California. Spores are ovoid, ranging in size from 2.8 +/- 0.3 microm in total length and 1.9 +/- 0.4 microm in width (measurements of 30 spores made by calculation from micrograph). The polar filament has 9-10 coils in 1-2 rows. Development occurs in direct contact with host muscle cell cytoplasm, without xenoma or sporophorous vesicle. Phylogenetic analysis of the new species and of 35 other microsporidians known to infect fish using 1115 base pairs of aligned 16S rRNA gene indicate the new species is most closely related to Kabatana takedai. However, the new species differs by 11% sequence divergence from K. takedai. Divergence in morphology and genetic data allow for diagnosis from all other fish-infecting microsporidia and supports recognition of a new species of microsporidian, Kabatana newberryi n. sp., presently known only from a suspected specific host, the endangered tidewater goby Eucyclogobius newberryi.     

Phylogenetic Relationships Among Microsporidia Based on rDNA Sequence Data, With Particular Reference to Fish-Infecting Microsporidium Balbiani 1884 Species.

Recently, large discrepancies have been identified between microsporidian systematics based on molecular and traditional characteristics. In the current study the 530f-580r region of the rRNA gene of eight microsporidian species was cloned and sequenced. Included were two unclassified species of Microsporidium Balbiani, 1884 and an unidentified microsporidian that infects the musculature of different sea bream species. Sequence identities in excess of 98% indicated that these three species almost certainly are members of the same genus. Phylogenetic analyses of all microsporidian sequence data available for this region of the gene (20 species) and for partial small subunit sequences (51 species of 21 genera) revealed these species to be distinct from the family Pleistophoridae Doflein, 1901 and closely related them to the genus Sproguea Weissenberg, 1976. This clade was found to comprise a sister taxon to that containing the vast majority of fish-infecting species. Broad cladistic divisions were found between terrestrial insect-infecting and fish-infecting species, which together are distant from the aquatic insect-infecting microsporidia. The rRNA gene of certain fish-infecting genera was found to be more highly conserved than previously reported. This has implications for its utility in diagnostic assays and phylogenetic studies at, or close to, the species level.     

Integrative Approach for the Reliable Detection and Specific Identification of the Microsporidium Loma morhua in Atlantic Cod (Gadus morhua)

Microsporidia are fungal parasites that infect diverse invertebrate and vertebrate hosts. Finfish aquaculture supports epizootics due to high host density and the high biotic potential of these parasites. Reliable methods for parasite detection and identification are a necessary precursor to empirical assessment of strategies to mitigate the effects of these pathogens during aquaculture. We developed an integrative approach to detect and identify Loma morhua infecting Atlantic cod. We show that the spleen is more reliable than the commonly presumed gills as best organ for parasite detection in spite of substantial morphological plasticity in xenoma complexes. We developed rDNA primers with 100% sensitivity in detecting L. morhua and with utility in distinguishing some congeneric Loma species. ITS sequencing is necessary to distinguish L. morhua from other congeneric microsporidia due to intraspecific nucleotide variation. 64% of L. morhua ITS variants from Atlantic cod have a 9‐nucleotide motif that distinguishes it from Loma spp. infecting non‐Gadus hosts. The remaining 36% of ITS variants from Atlantic cod are distinguished from currently represented Loma spp., particularly those infecting Gadus hosts, based on a 14‐nucleotide motif. This research approach is amenable to developing templates in support of reliable detection and identification of other microsporidian parasites in fishes.     

Resolution of a taxonomic conundrum: an ultrastructural and molecular description of the life cycle of Pleistophora mulleri (Pfeiffer 1895; Georgevitch 1929)

The classification of a microsporidian parasite observed in the abdominal muscles of amphipod hosts has been repeatedly revised but still remains inconclusive. This parasite has variable spore numbers within a sporophorous vesicle and has been assigned to the genera Glugea, Pleistophora, Stempellia, and Thelohania. We used electron microscopy and molecular evidence to resolve the previous taxonomic confusion and confirm its identification as Pleistophora mulleri. The life cycle of P. mulleri is described from the freshwater amphipod host Gammarus duebeni celticus. Infection appeared as white tubular masses within the abdominal muscle of the host. Light and transmission electron microscope examination revealed the presence of an active microsporidian infection that was diffuse within the muscle block with no evidence of xenoma formation. Paucinucleate merogonial plasmodia were surrounded by an amorphous coat immediately external to the plasmalemma. The amorphous coat developed into a merontogenetic sporophorous vesicle that was present throughout sporulation. Sporogony was polysporous resulting in uninucleate spores, with a bipartite polaroplast, an anisofilar polar filament and a large posterior vacuole. SSU rDNA analysis supported the ultrastructural evidence clearly placing this parasite within the genus Pleistophora. This paper indicates that Pleistophora species are not restricted to vertebrate hosts.     

Small subunit ribosomal DNA phylogeny of microsporidia with particular reference to genera that infect fish

Molecular data have proved useful in the study of microsporidia phylogeny. Previous studies have shown that there are several important differences between phylogenies based on rRNA and morphological data. In the present study, small subunit (SSU) rDNA sequences were obtained from 7 different fish-infecting microsporidia from 4 different genera (Glugea Thélohan, 1891, Loma Morrison and Sprague, 1981, Pleistophora Gurley, 1893, and Spraguea Weissenberg, 1976). The lengths of the SSU rDNA genes in these species were between 1,332 and 1,343 base pairs. Phylogenetic analysis was performed using parsimony, maximum likelihood, and Kimura 2-parameter with neighbor joining. The analyses revealed that the microsporidia could be divided into 3 major groups. With the exception of Nucleospora salmonis Hedrick, Groff, and Baxa, 1991, all the microsporidia infecting fishes occurred in the same group. The analysis showed that Pleistophora mirandellae Vaney and Conte, 1901 and Pleistophora aguillarum Hoshina, 1951 are not species of Pleistophora. Furthermore, the analysis showed that Loma is not a member of Glugeidae Thélohan, 1892.     

Ultrastructural evidence of autoinfection in the gills of Atlantic cod Gadus morhua infected with Loma sp. (phylum Microsporidia)

Infection by a microsporidian of the genus Loma was found in gills of cod Gadus morhua. Xenomas contained parasites in multiple stages of development. Some spores looked empty and had everted polar tubes, which were either straight or coiled. These polar tubes were scattered throughout the xenoma cytoplasm, and some of them pierced the plasma membrane. Those outside of the xenoma penetrated neighboring cells, including blood cells. These observations suggest that a mechanism of autoinfection could occur in blood cells and gill tissue, perpetuating the disease in the host.     

Luna stain, an improved selective stain for detection of microsporidian spores in histologic sections

Microsporidia in histologic sections are most often diagnosed by observing spores in host tissues. Spores are easy to identify if they occur in large aggregates or xenomas when sections are stained with hematoxylin and eosin (H&E). However, individual spores are not frequently detected in host tissues with conventional H&E staining, particularly if spores are scattered within the tissues, areas of inflammation, or small spores in nuclei (i.e. Nucleospora salmonis). Hence, a variety of selective stains that enhance visualization of spores is recommended. We discovered that the Luna stain, used to highlight eosinophils, red blood cells, and chitin in arthropods and other invertebrates, also stains spores of Pseudoloma neurophilia. We compared this stain to the Gram, Fite's acid fast, Giemsa, and H&E stains on 8 aquatic microsporidian organisms that were readily available in our 2 laboratories: Loma salmonae, Glugea anomala, Pseudoloma neurophilia, Pleistophora hyphessobryconis, Pleistophora vermiformis, Glugea sp., Steinhausia mytilovum, and an unidentified microsporidian from UK mitten crabs Eriocheir sinensis. Based on tinctorial properties and background staining, the Luna stain performed better for detection of 6 of the 8 microsporidia. Gram stain was superior for the 2 microsporidia from invertebrates: S. mytilovum and the unidentified microsporidian from E. sinensis.     

Enterospora canceri n. gen., n. sp., intranuclear within the hepatopancreatocytes of the European edible crab Cancer pagurus

Only 1 genus (Nucleospora) within 1 family (Enterocytozoonidae) of the Microsporidia contains species that are parasitic within the nuclei of their host cells; to date, all described intranuclear Nucleospora spp. parasitise fish. This study describes the first intranuclear microsporidian parasite of an invertebrate, the European edible crab Cancer pagurus L. (Decapoda: Cancridae). Infected crabs displayed no obvious external signs, and maximum apparent prevalence of infection within a monthly sample was 3.45%. Infected hepatopancreatic tubules were characterised by varying numbers of hypertrophic and eosinophilic nuclei within epithelial cells. Parasite stages appeared as eosinophilic granular accumulations causing margination of host chromatin. In advanced cases, the tubule epithelia degenerated, with parasites and sloughed epithelial cells appearing in tubule lumens. All life stages of the parasite were observed within host nuclei. Uninucleate meronts were not detected, although binucleate stages were observed. Multinucleate plasmodia (sporogonal plasmodia) contained up to 22 nuclei in section, and late-stage plasmodia contained multiple copies of apparatus resembling the polar filament and anchoring disk, apparently associated with individual plasmodial nuclei. As such, aggregation and early assembly of sporoblast components took place within the intact sporogonial plasmodium, a feature unique to the Enterocytozoonidae. Liberation of sporoblasts from plasmodia or the presence of liberated sporoblasts was not observed in this study. However, large numbers of maturing and mature spores (measuring 1.3 +/- 0.02 x 0.7 +/- 0.01 microm) were frequently observed in direct contact with the host nucleoplasm. Considering the shared features of this parasite with microsporidians of the family Enterocytozoonidae, and the unique presence of this parasite within the nucleoplasm of decapod crustacean hepatopancreatocytes, this parasite (Enterospora canceri) is proposed as the type species of a new genus (Enterospora) of microsporidian. Molecular taxonomic work is now required, comparing Enterospora to Enterocytozoon and Nucleospora, the 2 other genera within the Enterocytozoonidae.     

Comparison of small subunit ribosomal RNA gene and internal transcribed spacer sequences among isolates of the intranuclear microsporidian Nucleospora salmonis

Nucleospora salmonis is an intranuclear microsporidian associated with a proliferative disorder of the lymphoid cells of captive salmonid fish in the northwestern and northeastern regions of North America, in France, and in Chile. Newer diagnostic approaches have used the polymerase chain reaction (PCR) to detect the parasite in fish tissues. The target sequences for these assays lie in the small subunit ribosomal RNA (ssu rRNA) gene or internal transcribed spacer (ITS) as determined from N. salmonis from chinook salmon (Oncorhynchus tshawytscha) from the Pacific Northwest of North America. The lack of sequence data on parasites from diverse geographic origins and hosts led us to compare several isolates of N. salmonis. There was a high degree of similarity in the ssu rDNA sequences (> 98%) among all the isolates of N. salmonis examined, regardless of host or geographic origin. The greatest sequence differences were found between isolates from the Pacific regions of America. Isolates from Chile shared sequences with one or both geographic groups from North America. A similar distribution of sequence types was observed when ITS-1 sequences of selected isolates were analyzed. Sequence data from two N. salmonis-like isolates from marine non-salmonid fish showed one closely related and the second less closely related to N. salmonis isolates from salmonid fish. These results provide evidence for a homogeneous group of aquatic members of the genus Nucleospora found among salmonid fish (N. salmonis) that can be detected using diagnostic PCR assays with ssu rDNA target sequences. The presence of parasites related to N. salmonis among marine fish suggests a potentially broad host and geographic distribution of members of the family Enterocytozoonidae.     

Amazonspora hassar n. gen. and n. sp. (phylum Microsporidia,fam. Glugeidae), a parasite of the Amazonian teleost Hassar orestis (fam. Doradidae)

We describe the microsporidian Amazonspora hassar n. gen., n. sp. from the gill xenomas of the teleost Hassar orestis (Doradidae) collected in the estuarine region of the Amazon River. The parasite appeared as a small whitish xenoma located in the gill filaments near the blood vessels. Each xenoma consisted of a single hypertrophic host cell (HHC) in the cytoplasm of which the microsporidian developed and proliferated. The xenoma wall was composed of up to approximately 22 juxtaposed crossed layers of collagen fibers. The plasmalemma of the HHC presented numerous anastomosed, microvilli-like structures projecting outward through the 1-3 first internal layers of the collagen fibrils. The parasite was in direct contact with host cell cytoplasm in all stages of the cycle (merogony and sporogony). Sporogony appears to divide by plasmotomy, giving rise to 4 uninucleate sporoblasts, which develop into uninucleate spores. The ellipsoidal spores measured 2.69 +/- 0.45 x 1.78 +/- 0.18 microm, and the wall measured approximately 75 nm. The anchoring disk of the polar filament was subterminal, being shifted laterally from the anterior pole. The polar filament was arranged into 7-8 coils in a single layer in the posterior half of the spore, surrounding the posterior vacuole. The polaroplast surrounded the uncoiled portion of the polar filament, and it was exclusively lamellar. The spores and different life-cycle stages were intermingled within the cytoplasm of the HHC, surrounding the central hypertrophic deeply branched nucleus. The ultrastructural morphology of this microsporidian parasite suggests the erection of a new genus and species.     

Observations on Ichthyosporidium giganteum (Microsporida) with particular reference to the host-parasite relations during merogony

Connective tissue cells, particularly fibroblasts, of the fish Leiostomus xanthurus Lacépède respond to the invading microsporidian parasite Ichthyosporidium sp. [assumed to be identical with Ichthyosporidium giganteum (Thélohan)] by proliferating themselves, coalescing into a syncytium, synthesizing copious amounts of cytoplasm around the parasites, and walling off the parasitized islands of cytoplasm with fibrous capsules. The resulting cysts are xenoparasitic complexes of the syncytial xenoma type, clearly different from the cell hypertrophy tumor (xenoma sensu Weissenberg) exemplified by the Glugea cyst. These findings involve a new concept of the structure and host-parasite relations of Ichthyosporidium. Formerly, the parasitized masses of cytoplasm were interpreted as extracellular plasmodial stages of the parasite (stages uncharacteristic of the microsporidia), while the parasites themselves were interpreted as nuclei of the "plasmodia." Actually, the parasite undergoes merogony in parasitophorous vacuoles which coalesce before sporogony begins. The nuclei of the mermonts are very small chromatin granules, becoming transformed into large basophilic diplokarya of the sporonts. Sporulation is diplokaryotic throughout, the diplokarya becoming reduced in size through 2 steps during sporogony.     

The Genome of Spraguea lophii and the Basis of Host-Microsporidian Interactions

Microsporidia are obligate intracellular parasites with the smallest known eukaryotic genomes. Although they are increasingly recognized as economically and medically important parasites, the molecular basis of microsporidian pathogenicity is almost completely unknown and no genetic manipulation system is currently available. The fish-infecting microsporidian Spraguea lophii shows one of the most striking host cell manipulations known for these parasites, converting host nervous tissue into swollen spore factories known as xenomas. In order to investigate the basis of these interactions between microsporidian and host, we sequenced and analyzed the S. lophii genome. Although, like other microsporidia, S. lophii has lost many of the protein families typical of model eukaryotes, we identified a number of gene family expansions including a family of leucine-rich repeat proteins that may represent pathogenicity factors. Building on our comparative genomic analyses, we exploited the large numbers of spores that can be obtained from xenomas to identify potential effector proteins experimentally. We used complex-mix proteomics to identify proteins released by the parasite upon germination, resulting in the first experimental isolation of putative secreted effector proteins in a microsporidian. Many of these proteins are not related to characterized pathogenicity factors or indeed any other sequences from outside the Microsporidia. However, two of the secreted proteins are members of a family of RICIN B-lectin-like proteins broadly conserved across the phylum. These proteins form syntenic clusters arising from tandem duplications in several microsporidian genomes and may represent a novel family of conserved effector proteins. These computational and experimental analyses establish S. lophii as an attractive model system for understanding the evolution of host-parasite interactions in microsporidia and suggest an important role for lineage-specific innovations and fast evolving proteins in the evolution of the parasitic microsporidian lifecycle.     

Description of a xenoma-inducing microsporidian, Microgemma tincae n. sp., parasite of the teleost fish Symphodus tinca from Tunisian coasts

A xenoma-inducing microsporidian species was found to infect the liver of the teleost fish, peacock wrasse Symphodus (Crenilabrus) tinca. Minimal estimates of the prevalence of the parasite in fishes caught along Tunisian coasts were as high as 43 % for Bizerte samples (over 2 yr) and 72% for Monastir samples (over 3 yr). Developmental stages were dispersed within a xenoma structure that was bounded only by the plasma membrane of the hypertrophic host cell. Ultrastructural features support allocation to the genus Microgemma Ralphs and Matthews, 1986. Meronts were multinucleate plasmodia and were surrounded by rough endoplasmic reticulum (RER) of the host cell. Merogonic plasmodia developed into sporogonic plasmodia, with loss of the RER interface. Sporogony was polysporoblastic. Ovocylindrical spores (3.6 x 1.2 microm) harbored a lamellar polaroplast and a polar tube that was coiled 9 times. Spore features and host specificity led us to propose a new species, Microgemma tincae. The conversion of M. tincae xenomas into well-visible cyst structures or granulomas reflected an efficient host response involving the infiltration of phagocytic cells, degradation of various parasite stages and formation of a thick fibrous wall. The small subunit rDNA gene of M. tincae was partially sequenced. Phylogenetic analysis confirms the placement within the family Tetramicriidae represented by the genera Tetramicra and Microgemma.     

Description of five new Loma (Microsporidia) species in pacific fishes with redesignation of the type species Loma morhua Morrison & Sprague, 1981, based on morphological and molecular species-boundaries tests

Five new species of Loma were described from five Pacific fishes using light-microscopic and ultrastructural features along with phylogenetic analysis of the gene sequences of ribosomal RNA (rRNA) and elongation factor 1-alpha. Morphological data revealed both qualitative and quantitative differences in developmental stages and timing, vesicles, xenoma features, and spore sizes with statistical support that differentiated Loma pacificodae n. sp. in Pacific cod, Loma wallae n. sp. in walleye pollock, Loma kenti n. sp. in Pacific tomcod, Loma lingcodae n. sp. in lingcod, and Loma richardi n. sp. in sablefish from each other and other species in the genus. Phylogenetic analyses combined with monophyly tests supported species designations, but with low resolution in two cases perhaps due to rRNA paralogs or recent speciation. Loma branchialis in haddock was shown to be separate from Loma morhua in Atlantic cod, thereby making L. morhua, and not L. branchialis, the type species. A species from brook trout was shown to be a separate species from Loma salmonae, not a variant strain selected in the laboratory. By comparison with gadid host phylogeny, these Loma species appear to have coevolved with their hosts, first colonizing the Pacific basin about 12 million years ago.     

Ultrastructural characterization and comparative phylogenetic analysis of new microsporidia from Siberian mosquitoes: evidence for coevolution and host switching

A survey of mosquito larvae infected with microsporidia was conducted from 2005 to 2008 in the Tomsk, Kemerovo and Novosibirsk regions of western Siberia, Russia. Twenty-one morphologically and genetically unique species of microsporidia were isolated from nine species of Anopheles, Aedes, Culex and Ochlerotatus mosquitoes including: (1) 14 proposed new species of Amblyospora (A. bakcharia, A. baritia, A. bogashovia, A. chulymia, A. hristinia, A. jurginia, A. kazankia, A. mavlukevia, A. mocrushinia, A. modestium, A. salairia, A. severinia, A. shegaria, and A. timirasia); (2) a newly proposed genus and species, Novothelohaniaovalae and; (3) six species of Amblyospora (A. flavescens, A. kolarovi, A. rugosa), Parathelohania (P. divulgata and P. tomski) and Trichoctosporea (T. pygopellita) from which gene sequences had not been previously obtained. Detailed ultrastructure of meiospores revealed unique cytological features associated with the length, arrangement and ratio of broad to narrow coils of the polar filament, comparative thickness of the exospore and endospore, and overall size of each species reaffirming their value in distinguishing taxonomic relationships. SSU rDNA sequences obtained from each species of microsporidia were unique when compared with GenBank entries. Phylogenetic trees constructed by Maximum Parsimony, Maximum Likelihood and Neighbor Joining analyses yielded similar topologies with a high degree of congruence between parasite and host at the generic level. Species that parasitize Aedes/Ochlerotatus and Culex mosquitoes segregate into distinct monophyletic groupings mirroring their host phylogeny, while species from Anopheles mosquitoes group as a sister clade basal to the entire group of mosquito-parasitic microsporidia as their Anopheles hosts cluster as a sister clade to the entire group of culicine mosquitoes. This provides strong evidence for host-parasite coevolution by descent at the generic level and limited host lineage switching between unrelated taxa. Among parasites of Aedes/Ochlerotatus and Anopheles mosquitoes, we found several instances where a single mosquito species serves as a host for two or more related species of microsporidia, an observation consistent with host switching and independent parasite speciation. Among the microsporidian parasites of Culex mosquitoes, we found only one parasite per host indicating a higher degree of host specificity and less host switching among parasites of this genus. Findings suggest a degree of host-parasite co-speciation with host switching occurring occasionally when the "normal" host is unavailable in the aquatic ecosystem. Frequency of host switching seems to be occurring in proportion to host relatedness and does not cross generic boundaries in this system.     

Accumulation and exchange of parasites during adaptive radiation in an ancient lake

In the ancient Lake Baikal, Russia, amphipod crustaceans have undergone a spectacular adaptive radiation, resulting in a diverse community of species. A survey of microsporidian parasites inhabiting endemic and non-endemic amphipod host species at the margins of Lake Baikal indicates that the endemic amphipods harbour many microsporidian parasite groups associated with amphipods elsewhere in Eurasia. While these parasites may have undergone a degree of adaptive radiation within the lake, there is little evidence of host specificity. Furthermore, a lack of reciprocal monophyly indicates that exchanges of microsporidia between Baikalian and non-Baikalian hosts have occurred frequently in the past and may be ongoing. Conversely, limitations to parasite exchange between Baikalian and non-Baikalian host populations at the margins of the lake are implied by differences in parasite prevalence and lack of shared microsporidian haplotypes between the two host communities. While amphipod hosts have speciated sympatrically within Lake Baikal, the parasites appear instead to have accumulated, moving into the lake from external amphipod populations on multiple occasions to exploit the large and diverse community of endemic amphipods in Lake Baikal.     

A three-genome phylogeny of malaria parasites (Plasmodium and closely related genera): evolution of life-history traits and host switches

Phylogenetic analysis of genomic data allows insights into the evolutionary history of pathogens, especially the events leading to host switching and diversification, as well as alterations of the life cycle (life-history traits). Hundreds, perhaps thousands, of malaria parasite species exploit squamate reptiles, birds, and mammals as vertebrate hosts as well as many genera of dipteran vectors, but the evolutionary and ecological events that led to this diversification and success remain unresolved. For a century, systematic parasitologists classified malaria parasites into genera based on morphology, life cycle, and vertebrate and insect host taxa. Molecular systematic studies based on single genes challenged the phylogenetic significance of these characters, but several significant nodes were not well supported. We recovered the first well resolved large phylogeny of Plasmodium and related haemosporidian parasites using sequence data for four genes from the parasites' three genomes by combining all data, correcting for variable rates of substitution by gene and site, and using both Bayesian and maximum parsimony analyses. Major clades are associated with vector shifts into different dipteran families, with other characters used in traditional parasitological studies, such as morphology and life-history traits, having variable phylogenetic significance. The common parasites of birds now placed into the genus Haemoproteus are found in two divergent clades, and the genus Plasmodium is paraphyletic with respect to Hepatocystis, a group of species with very different life history and morphology. The Plasmodium of mammal hosts form a well supported clade (including Plasmodium falciparum, the most important human malaria parasite), and this clade is associated with specialization to Anopheles mosquito vectors. The Plasmodium of birds and squamate reptiles all fall within a single clade, with evidence for repeated switching between birds and squamate hosts.     

The dynamics of preferential host switching: Host phylogeny as a key predictor of parasite distribution*

Parasites often jump to and become established in a new host species. There is much evidence that the probability of such host shifts decreases with increasing phylogenetic distance between donor and recipient hosts, but the consequences of such preferential host switching remain little explored. We develop a computational model to investigate the dynamics of parasite host shifts in the presence of this phylogenetic distance effect. In this model, a clade of parasites evolves on an evolving clade of host species where parasites can cospeciate with their hosts, switch to new hosts, speciate within hosts or become extinct. Our model predicts that host phylogenies are major determinants of parasite distributions across trees. In particular, we predict that trees consisting of few large clades of host species and those with fast species turnover should harbor more parasites than trees with many small clades and those that diversify more slowly. Within trees, large clades are predicted to exhibit a higher fraction of infected species than small clades. We discuss our results in the light of recent cophylogenetic studies in a wide range of host–parasite systems.