Metchnikovellidae

The type species is A. bufonis Paperna and Lainson, 1995 in Bufo marinus (Amphibia, Anura). It develops in single or aggregated xenomas, each up to 250 µm diameter in the lamina propria of the intestine. The enlarged host cells are bordered by a thin plasma membrane overlain by fibroblasts and the parasites develop at the periphery around a central hypertrophic host cell nucleus. Monomorphic, monokaryotic throughout life cycle. Meiosis unknown. Transmission probably by ingestion of spores or cannibalism by tadpoles. Merogony: multinucleate stout plasmodia with diffuse nuclei differentiate into elongate forms with compact nuclei. Sporogony: ribbon-like plasmodia with a single or double row of compact nuclei fragment into sporoblasts, possibly via binucleate segments. No sporophorous vesicles. Spores: 1.3-1.6 x 0.6-0.8 µm (fixed), ovoid but with anterior end depressed over polar sac. Polaroplast vesicular. Polar tube isofilar with about 5-6 coils and whorled membranes (posterosome?) terminally. Cytoplasm invaginated into the region of whorled membranes. Spores released from xenomas are phagocytised by macrophages, which persist in spleen and liver in newly metamorphosed toads but infection is gradually lost in adults.

The type species is A. canningae Weiser and Purrini, 1980 in the male gonads of Lepidocyrtus lignorum (Collembola, Tomoceridae). Dimorphic (?), diplokaryotic and monokaryotic. Meiosis unknown. Transmission unknown. Merogony: unknown. Sporogony: apparently two developmental sequences. In one sequence, tetra and octonucleate plasmodia give rise to free binucleate sporoblasts and spores. The other sequence, gives rise to several uninucleate sporoblasts, within sporophorous vesicles. Spores: free spores, 45 x 22.5 µm are pyriform and binucleate, the 2 nuclei differing in shape. The endo and exospore are thick, the exospore having an irregular, foamy character. Polaroplast is lamellar. Polar tube is isofilar with 89 coils. No data are available on the second type of spore. Comment: this is an incompletely known genus with no satisfactory illustrations for reproduction.

The type species is B. vesicularum Cali, Takvorian, Lewin, Rendel, Sian, Wittner, Tanowitz, Keohane, and Weiss, 1998 infecting skeletal muscle of Homo sapiens (Primates, Hominidae). Monomorphic, diplokaryotic throughout life cycle. Meiosis unknown. Transmission unknown. Development in direct contact with host cell cytoplasm. All stages with electron dense surface coat. Merogony: binary fission of cells with two diplokarya. Meronts exhibit vesiculo-tubular structures, embedded in surface coat or forming chains extending for a certain distance from the parasite surface. Spores 2.5-2.9 x 1.9-2µm (fixed). Polar tube anisofilar with 7-10 coils, of which the last2-3 are narrower. Coils arranged in 2-3 rows. Comment: By its life cycle and structure this genus is similar to the genus Nosema. However, it differs from typical Nosema spp. by the electron dense coat on the plasma membrane and its extensions.

The type species is C. operophterae (Canning, 1960) Canning, Barker, Nicholas and Page, 1985 in silk glands, gut and other tissues of Operophtera brumata (Lepidoptera, Geometridae). The species was transferred to the new genus by Canning, Barker, Nicholas and Page (1985,a). Monomorphic, monokaryotic throughout life cycle. Meiosis unknown. Transmission is transovarial and per os. Merogony: small plasmodia with several isolated nuclei surrounded by a single close-fitting membrane, possibly of host origin. The membrane divides with the meronts. Sporogony: multinucleate plasmodia, divide within the encasing membrane. The membrane is persistent around the sporoblasts and spores, which may number up to sixty. Spores, 2.5 x 1.2 µm, elongate-ellipsoid and uninucleate, with a corrugated exospore and moderately thick endospore. The polaroplast was not described. Polar tube is isofilar, with 10 - 11 coils in a single rank. Comment: the origin of the membrane surrounding the meronts and persisting around spores was not determined. The genus resembles Endoreticulatus except that Endoreticulatus has a double layer of endoplasmic reticulum surrounding all stages. Cells of the silk glands are infected in a regular pattern and are not hypertrophied, in contrast to the effect of Orthosomella on the same host.

Incompletely described genus; dimorphic, mono and diplokaryotic; meiosis unknown; transmission unknown; merogony not described; sporogony culminates in two types of ovoid spores differing in size, number of nuclei and presence or absence of sporophorous vesicle; monokaryotic, small spores are enclosed in sporophorous vesicles, apparently after disporoblastic sporogony, as each vesicle contains two spores; larger, binucleate spores are free, in direct contact with host cell cytoplasm; type species E. chironomi Voronin and Issi, 1986 in the midge (larvae?) Chironomus plumosus (Diptera, Chironomidae).

The type species is G. gamocystis RÄhl and Korn, 1979 in the cytoplasm of the gregarine Gamocystis ephemerae, parasite of the mayfly Ephemera danica (Ephemeroptera, Ephemeridae). The poor description does not allow a satisfactory description of the genus to be presented. Sporogony is said to occur in a sporophorous vesicle, in which 6 or 8 spores are formed. Spores, 2.63.1 x 1.51.7 µm, are uninucleate. Exospore is thin, endospore moderate. Polar tube is isofilar with 45 coils. No satisfactory illustrations are available for reproduction.

Monomorphic, monokaryotic throughout life cycle; transmission by ingestion of spores; merogonial plasmodia, with a small number of nuclei lie in a parasitophorous vacuole of host cell origin and release daughter cells by plasmotomy; number of merogonial generations unknown, possibly only one; sporogonial plasmodium divides by plasmotomy ultimately to give at least 16 sporoblasts inside a thin-walled sporophorous vesicle; vesicle envelope is formed by duplication of the plasma membrane of the sporont; spores, 2.4-2.7 x 1.1-1.7 µm, slightly asymmetrical, oval to slightly kidney-shaped and uninucleate; exospore has two layers, the external one more dense; polaroplast has two lamellar parts, the anterior lamellae being more closely packed; polar tube is isofilar with 5-8 (usually 6) coils in a single rank; some similarity between this genus and the genera Endoreticulatus and Flabelliforma - Endoreticulatus, however has no sporophorous vesicle and its sporogonial stages lie in a parasitophorous vacuole, Flabelliforma develops inside a sporophorous vesicle but its sporogonial plasmodia divide by fan-shaped budding; type species G. intestinalis (in gut epithelium of Daphnia magna and D. pulex (Crustacea, Phyllopoda).

The type species is G. biformis Voronin, 1986 in the fat body of Ceriodaphnia reticulata (Cladocera, Daphniidae). An inadequately described genus. Apparently dimorphic, nuclear condition not described. Meiosis unknown. Transmission unknown. Merogony: unknown. Sporogony: two types of spores are formed. Single spores described as occurring freely, but actually enclosed in what seem to be individual sporophorous vesicles. The membrane of these vesicles have a foamlike appearance. Also groups of 4 spores enclosed in thinwalled sporophorous vesicles. These spores are permanently joined into doublets by a cementing substance, identical with the exospore material. The spore doublets are thus formed in a manner similar to the genus Norlevinea.

Monomorphic, except that spores of 2 sizes are produced from one sporogonic sequence; monokaryotic; merogony observed, presumed absent; sporonts are multinucleate plasmodia, in young sporonts with only a few nuclei cytoplasmic membranes are organised to form a continuous sheath at the periphery, separating an inner nucleated zone of cytoplasm from a narrow strip beneath the plasma membrane; this outer layer persists as a sac within which spores are formed, sporoblasts are formed in the central region by fusion of vesicles around nucleated islands of cytoplasm; microspores 1.32-1.78 µm diameter outnumber macrospores 1.49-2.33 µm diameter, both occur with the same sac and are of similar organisation; Spores almost spherical with typical exo- and endo-spore layers of the wall, nucleus flattened to one side, the opposite side being occupied by an irregular vacuole; polar sac has no anchoring disc and leads directly into a polar tube coil of decreasing diameter (anisofilar) made up of 2-3 coils in microspores and up to 9 coils in macrospores, tube with central core around which are spirally wound 12 tubules enclosed by a unit membrane, typical polaroplast is absent; type species I. acarivora (Larsson, Steiner & Bjornson, 1997) in the gut epithelium of the forage mite Tyrophagus putrescentiae (Acari, Acaridae).

The type species is L. resinellae Weiser and David, 1997 in the pine resin gall moth, Petnova resinella (Lepidoptera, Tortricidae). It first infects the silk glands and Malpighian tubules, later the adipose and connective tissues, especially around gonads, and spores are present in egg follicles and around bundles of sperm. Monomorphic, monokaryotic throughout life cycle. Transmission unknown probably via ova. Development in silk glands occurs in cavities in host cells, not bounded by membranes and does not progress to spore production. Malpighian tubule cells become packed with spores. Merogony: binary fission of binucleate stages. Some stages with a nipple-like protrusion are suggestive of gametes. Sporogony: division not observed. Uninucleate sporoblasts bear a tuft of 1 µm long, 50 nm diameter tubules eccentrically inserted into the exospore at the posterior pole. Spores: 4.5-5.0 x 1.7 µm, are uninculeate and almost tubular. Polaroplast occupies one third of the spore. Polar tube is isofilar with 10 or 11 coils in a single rank. Nucleus is central within concentric bands of polyribosomes. Prominent posterior vacuole. Posterior tuft of tubules, present at posterior end of immature spores, disappears at maturity.

The type species is M. davidii Pell and Canning, 1993 in gut epithelial cells of larvae of Mansonia africana (Diptera, Culicidae) and in unidentified tissues of adults. Dimorphic (possibly polymorphic), diplokaryotic and monokaryotic and probably involving meiosis in sporogony, heterosporous with one sporulation sequence in adult females and the other in larvae. In larval mosquitoes: Merogony: meronts are multinucleate plasmodia, with the plasma membrane thickened by deposition of a surface coat and surrounded by a flattened cisterna of host endoplasmic reticulum (e.r.), studded with ribosomes. Meronts become ribbon-shaped before division, producing cells, each with a single diplokaryon and still enclosed individually in host endoplasmic reticulum. Sporogony: polysporous, initiated in multi-nucleate plasmodia by dispersal of the surface coat and separation of the two nuclei of the diplokarya, followed by nuclear division. Meiosis was not observed but probably occurs here. Sequential division of the plasmodium into segments with fewer nuclei, accompanied by secretion of a new surface coat, eventually produces uninucleate sporoblasts. Sporoblasts, still bounded by host endoplasmic reticulum, lie in a matrix of granular material. A sporophorous vesicle was not observed. Spores, 2.5 x 1.5 µm, are ellipsoid, slightly flattened terminally and uninucleate. Details of the polaroplast and polar tube are lacking, although it was suggested that the polar tube may have but a single coil. In adult mosquitoes: stages are diplokaryotic giving rise to diplokaryotic, reniform spores, 3.4 x 1.8 µm.

The type species is P. ectospora Issi, Kadyrova, Pushkar, Khodzhaeva and Krylova, 1990 in adipose tissue of larvae of Tetisimulium desertorum and T. alajense (Diptera, Simuliidae). An incompletely described genus. Monomorphic, monokaryotic. Meiosis unknown. Transmission unknown. Merogony: unknown. Sporogony: the single nucleus of the sporont divides synchronously to form a plasmodium with 48 or more (even more than 64) nuclei within a spherical sporophorous vesicle, with a thick persistent membrane. The distinguishing character of the genus is the presence of 12 filiform expansions of the sporophorous vesicle membrane connecting 23 sporophorous vesicles together. Spores, 4.37.5 x 3.15.0 µm, ovoid and uninucleate. are situated in one layer just below the sporophorous vesicle membrane, the lumen of the vesicle being filled with an abundance of small granules.

The type species is P. odagmiae Khodzaeva, 1988 from the fat body, gut and salivary glands of several genera and species of blackflies (Odagmia ferganica, Simulium angustifolium, Montisimulium lithkense, M. quattuordecimfilum) (Diptera, Simuliidae). O. ferganica is the type host. Apparently dimorphic, diplokaryotic and monokaryotic. Meiosis unknown. Transmission unknown. Merogony: unknown. The genus is characterized only by the formation of 4, 8 or 16 uninucleate small spores inside a sporophorous vesicle and, of larger, binucleate free spores. Spores: small, ovoid 1.22.5 x 1.21.8 µm (fixed), and larger, ovoid 3.13.7 x 1.22.5 µm (fixed). Both types of spores have a thick wall. Comment: this is an incomplete and unsatisfactorily described genus. The above description is from Issi et al. (1990) as the original description was in a Ph.D. thesis. No satisfactory illustration of this genus exists in published form.

The type species is S. chironomi Voronin and Issi, 1986 in Chironomus plumosus (Diptera, Chironomidae). Dimorphic, monokaryotic and diplokaryotic. Meiosis unknown. Transmission unknown. Merogony: not described. Sporogony: two sequences, both lying free in direct contact with host cell cytoplasm. One sequence ends with groups of 16 small, uninucleate spores. In the other, larger individual spores with a diplokaryon are produced. Spores: both types are ovoid. The polaroplast of the larger spores was decribed as bipartite, the anterior part consisting of chambers filled with fibrils. The polar tube is isofilar with 1213 coils in the smaller spores and 18 coils in the larger spores. In both types several posterior coils are arranged as a second layer of coils inside the others. an unsatisfactorily known genus. The original description specifically states that the groups of spores are free, without sporophorous vesicle membrane. No explanation has been provided how the spores are held together in the group.

Poorly known genus. No type species has been named. The following description is based of those of S. uzbekistanica Khodzhaeva, Krylova and Issi, 1990 in fat body of larvae of Tetisimulium alajense (Diptera, Simuliidae) and S. turgenica Khodzhaeva, Krylova and Issi, 1990 in adipose tissue of larvae of Odagmia sp. (Diptera, Simuliidae). Dimorphic, diplokaryotic and monokaryotic. Meiosis present. Transmission unknown. Merogony: only diplokaryotic late meronts known. Sporogony: sporogonial plasmodia with unpaired nuclei divide into 632 (usually 1224) sporoblasts within a sporophorous vesicle. At an early stage of sporogony the membrane of the sporophorous vesicle is subtended by perpendicularly arranged tubules which later disappear. Synaptonemal complexes occur in sporoblast nuclei. Spores: two types occur, both enclosed in sporophorous vesicles. One type is wedge or pearshaped, elongate, thinwalled and with a tripartite polaroplast (large chambers, lamellae, fibrils). Polar tube is short, apparently anisofilar, with one wide coil and 12 narrow coils. Although 1-2 narrow coils were stated in the description 4 were shown in the original figure. The other spore type has a shorter pyriform shape and thicker walls. The polaroplast is in the form of compartments filled with rows of granules arranged perpendicularly to the long axis of the spore. The polar tube is short and anisofilar. The nuclear condition of the second type was not described but 2 nuclei were shown in the illustration.

Monomorphic, probably with alternation of diplokaryotic and monokaryotic stages; spores are rodlike (short cylindrical) and uninucleate; endospore is thin, exospore forms small, longitudinal electron dense ridges on the spore surface; polaroplast vesicular or spongelike, without lamellae; polar tube is isofilar and short, only 2/3 of the spore length; type species S. chironomi Issi and Voronin, 1986, in larvae of the midge Chironomus plumosus (Diptera, Chironomidae).

The type species is T. bacillifera Bykova, Sokolova and Issi, 1987 in the fat body and hypodermal cells of larvae of the horsefly Hybomitra arpadi (Diptera, Tabanidae). Another species, T. hybomitrae, occurs in adipose tissue of larvae of Hybomitra lundbecti. Dimorphic possibly with monokaryotic and diplokaryotic stages. Transmission unknown. Merogony: early meronts reported to be uninucleate. Late meronts are diplokaryotic cells in small chains of three to six. The end of merogony is marked by separation of the chains into unicellular fragments and the appearance of synaptonemal complexes in the nuclei. Sporogony: two types of diplokaryotic sporonts. In the first type a coat of short fibres perpendicular to the sporont surface appears on the sporont plasmalemma. These sporonts transform into diplokaryotic sporoblasts and spores which lie freely in the host cell cytoplasm. The second type of sporont develops small blisters on the plasmalemma, which merge to form the envelope of a sporophorous vesicle. One, 2, 4 or rarely 10 sporoblasts and later spores differentiate inside each sporophorous vesicle, the number being somewhat characteristic for the different species. The stages inside the sporophorous vesicle do not have the hairy coat typical of the first type of sporogony. Secretions in the episporontal space are in the form of very thick rodlike strands, consisting of an agglomerate of thin tubules of electron dense material or several thinner, mesh-forming electron dense tubules with a less dense interior. The number of the strands corresponds to the number of sporoblasts inside the sporophorous vesicle. During spore maturation these secretion products become less prominent but are sometimes still present in sporophorous vesicles containing mature spores. Spores: free spores, 5.3 x3.2 µm are ovoid and diplokaryotic. Exospore is ornamented with radially oriented fine tubules. Polaroplast is lamellar. Polar tube tapers gradually toward the distal end and forms 1821 coils in one rank. The proximal end of the filament touching the polar aperture is wide and has a characteristic bifurcated appearance. Spores in sporophorous vesicles are 3.5 x 2.3 µm but the fine structure is unknown for the type species. For T. hybomitrae these spores are uninucleate, the exospore is without ornamentation, polaroplast is of the lamellar type and the polar tube is isofilar making 46 coils. Comment: The original description of this genus is unclear in two respects. The occurrence of monokaryotic meronts followed later by diplokaryotic meronts is unusual. The unclear nuclear state of the spores bound in the sporophorous vesicle is another serious defect of the generic diagnosis. The present diagnosis is based on data published in Bykova and Issi (1991).

The type species is T. hominis Hollister, Canning, Weidner, Field, Kench and Marriott, 1996, reported from the skeletal muscle of an AIDS patient, Homo sapiens (Primates, Hominidae). Transmissable to athymic mice in which viscera were infected as well as striated muscle (Hollister et al., 1996). Monomorphic, monokaryotic throughout life cycle. Meiosis unknown. Transmission has been achieved per os in athalmic mice. All stages lie in a zone of disorganised host cell cytoplasm within intact myofibrils. This zone contains bundles and irregular arrays of double walled tubules. Merogony: uninucleate, binucleate or tetranucleate cells bounded by an electron dense surface coat, overlying the plasma membrane and extending into the lysed host cell cytoplasm as prominent branching and anastomosing processes. Vesicles, which probably originate from host sarcoplasmic reticulum, are aligned with these electron dense extensions. Division is by binary fission. Sporogony: the electron dense coat, now smoother in outline, separates as a sporophorous vesicle around a mononucleate sporont. Repeated binary fissions occur within the vesicle, which enlarges to accommodate the growing number of potential sporoblasts, the final number ranging from 2 to >32, in vesicles measuring 5 µm diameter to 14.0 x 11.0 µm. Spores, 4.0 x 2.4 µm, are pyriform with a distinct posterior vacuole. Monokaryotic. Polar tube is isofilar with about 11.5 coils measuring up to 75 µm when extruded. Polaroplast is lamellar.

Spore with three points, nuclei diplokaryotic and monokaryotic, probably involving meiosis in sporogony; sporogony is octosporous within a sporophorous vesicle; spores 3.1 x 2.1 µm are ovoid with truncated anterior end and uninucleate; exospore thin; wall is extended as an anterior rim (appearing as 2 knobs in section) and as a crest running longitudinally around the posterior end (appearing as a single knob in section; type species T. muhezae Pell and Canning, 1992 in adipose tissue of larvae of Mansonia africana (Diptera, Culicidae).

The type species is V. corneae (Shadduck, Meccoli, Davis and Font, 1990) Silveira and Canning, 1994 in corneal stroma of Homo sapiens (Primates, Hominidae). Monomorphic, diplokaryotic throughout life cycle with all stages including spores surrounded by a complete cisterna of host cell endoplasmic reticulum (e.r). Meiosis unknown. Transmission unknown. Merogony: binary fission of diplokaryotic stages, the encasing e.r. cisterna dividing with the meront. Sporogony: ribbon-shaped sporonts with up to eight diplokarya divide into linear arrays of sporoblasts. The division is effected by means of membranous paramural bodies into which the host endoplasmic reticulum penetrates to facilitate the enclosure of the sporoblasts by host endoplasmic reticulum. Spores: 3.7 x 1.0 µm are elongate cylindrical and diplokaryotic, each still surrounded by host e.r. Polar sac not well described. Polar tube is isofilar with about 6 coils. Exospore is about half as thick as endospore. Polaroplast is composed of tightly packed lamellae around the straight part of the polar tube and a few posterior lamellae in groups of 4.