Genus Pleistophora Gurley, 1893: An Assemblage of at Least Three Genera1

Until recently, pansporoblastic microsporidia that produce a variable and large number of sporoblasts from a sporont have been included in a single genus, namely Pleistophora Gurley, 1893. Ultrastructural studies have been used to determine whether the resemblance of these species is fundamental or superficial. The results indicated that the multisporous pansporoblastic forms belong to at least three genera and, thus, that Pleistophora is a “composite genus.” The term pansporoblast was originally used for stages in myxosporidian development. The term sporophorous vesicle adopted from Gurley is suggested for the spore-containing vesicle in the Microspora. Three species were studied: Pleistophora typicalis, the type-species; Pleistophora culicis, for which a new genus Vavraia has already been proposed; and Pleistophora simulii. P. typicalis and V. culicis have isolated nuclei throughout their development, and the sporophorous vesicle wall enveloping the sporoblasts is derived from amorphous secretions laid down during merogony external to the plasmalemma. Pleistophora and Vavraia are differentiated principally in terms of the structure of the sporophorous vesicle wall and mode of division of the sporogonial plasmodium. The nuclei of young sporonts of P. simulii are in diplokaryon arrangement and undergo meiosis to give haploid nuclei in the sporoblasts. The sporophorous vesicle wall is membranoid and is laid down external to the plasmalemma at the onset of sporogony. A new genus, Polydispyrenia n. g., is suggested for this species, the affinities of which are closer to the dimorphic species of microsporidia than to Pleistophora or Vavraia. The terms “merontogenetic sporophorous vesicle” and “sporontogenetic sporophorous vesicle” are used to distinguish between the two groups.     

Ultrastructural Study of the Development of Pleistophora schubergi Zwölfer, 1927 (Protozoa,Microsporida) in Larvae of the Spruce Budworm,Choristoneura fumiferana and Its Subsequent Taxonomic Change to the Genus Endoreticulatus

This study demonstrates that Pleistophora schubergi Zwölfer, 1927, a microsporidium originally isolated from the midgut epithelium of Nygmia phaeorrhoea Don (Euproctis chrysorrhoea L.) and Porthetria dispar L., and subsequently reported in several other insects including the spruce budworm, Choristoneura fumiferana (the host used in this investigation), does not belong in the genus Pleistophora Gurley, 1893. Pleistophora schubergi lacks the major features that are characteristic of Pleistophora typicalis, the type species of this genus. A comparison of ultrastructural observations reported for the type species of the genus Pleistophora, P. typicalis, and our observations of P. schubergi revealed significant differences. A thick (0.5 μm) amorphous coat, derived from parasite secretions and deposited external to the parasite plasmalemma, surrounds all developmental stages in P. typicalis. Double membranes, derived from host rough endoplasmic reticulum cisternae encircle the parasite plasmalemma of all developmental stages in P. schubergi. The sporophorous vesicle encases the spores in P. typicalis, and originates from the parasite-secreted coat that is present around meronts. In P. schubergi, the host endoplasmic reticulum cisternae form the envelope that surrounds the meronts. Moreover, the sporophorous vesicle envelope in P. typicalis persists around groups of spores, while in P. schubergi this envelope breaks easily to release the spores in the host cytoplasm. By comparing the characteristics of the microsporidium found in the spruce budworm with those of the recently created polysporous genera that sporulate within a vesicle, we found that P. schubergi does belong in the new genus Endoreticulatus Brooks et al. 1988, and consequently rename it Endoreticulatus schubergi (Zwölfer, 1927) n. comb.     

Napamichum cellatum N. Sp. (Microspora, Thelohaniidae), a New Parasite of Midge Larvae of the Genus Endochironomus (Diptera, Chironomidae) in Sweden

ABSTRACT The new microsporidium, Napamichum cellatum, a parasite of the adipose tissue of midge larva of the genus Endochironomus in Sweden, is described based on light microscopic and ultrastructural characteristics. Plurinucleate Plasmodia with nuclei arranged as diplokarya divide, probably by plasmotomy, producing a small number of diplokaryotic merozoites. The number of merogonial cycles is unknown. Each diplokaryotic sporont yields eight monokaryotic sporoblasts in a thin-walled, more or less fusiform sporophorous vesicle. A small number of multisporoblastic sporophorous vesicles were observed, in which a part of the sporoblasts were anomalous. The sporogony probably begins with a meiotic division. The mature spores are slightly pyriform. Fixed and stained spores measure 2.1-2.4 × 3.7-4.5 μm. The five-layered spore wall is of the Napamichum type. The polar filament is anisofilar with seven to eight coils (142-156 and 120 nm wide). The angle of tilt is 55-65°. The polaroplast has an anterior lamellar and a posterior tubular part. The granular, tubular and crystal-like inclusions of the episporontal space disappear more or less completely when the spores mature. The crystal-like inclusions are prominent in haematoxylin staining, but not visible with the Giemsa technique. The microsporidium is compared to other octosporoblastic microsporidia of midge larva and to the species of the genera Chapmanium and Napamichum.     

Establishment of Endoreticulatus N. G. for Pleistophora fidelis (Hostounský & Weiser, 1975) (Microsporida: Pleistophoridae) Based on the Ultrastructure of a Microsporidium in the Colorado Potato Beetle,Leptinotarsa decemlineata (Say) (Coleoptera: Chrysomelidae)1,2

A new genus, Endoreticulatus n. g., is described for the microsporidium Pleistophora fidelis (Hostounský & Weiser, 1975) based on light and electron microscopic studies of a microsporidium in the Colorado potato beetle, Leptinotarsa decemlineata (Say). This latter microsporidium is considered to be conspecific with P. fidelis because both isolates have been shown to be infectious for L. decemlineata where infection is limited to the epithelial cells of the midgut; both are haplokaryotic and develop as groups of sporoblasts and spores in subpersistent vacuoles in the host cell. In addition, meronts, sporonts, and spores of each isolate often occur simultaneously in a common cell, and by light microscopy they both appear similar. Ultrastructural studies of the isolate from L. decemlineata revealed that all developmental stages occur in parasitophorous vacuoles derived from cisternae of rough endoplasmic reticulum of the host cell cytoplasm. Based on the unique nature of the parasitophorous vacuole, a new genus, Endoreticulatus, is proposed for P. fidelis. The genus is compared with the other genera whose species undergo multisporous sporogony in sporophorous vesicles. In addition, the nature of the parasitophorous vacuole of Endoreticulatus fidelis (Hostounský & Weiser, 1975) n. comb. is compared with the parasitophorous vacuole known to encase various developmental stages of several other microsporidian species.     

Microsporidia of the Genus Trachipleistophora—Causative Agents of Human Microsporidiosis: Description of Trachipleistophora anthropophthera N. Sp. (Protozoa: Microsporidia)

Trachipleistophora anthropophthera n. sp., was found at autopsy in the brain of one and in the brain, kidneys, pancreas, thyroid, parathyroid, heart, liver, spleen, lymph nodes, and bone marrow of a second patient with AIDS. The parasite is similar to the recently described T. hominis Hollister, Canning, Weidner, Field. Kench and Marriott, 1996, in having isolated nuclei, meronts with a thick layer of electron dense material on the outer face of their plasmalemma and sporogony during which spores are formed inside a thick-walled sporophorous vesicle. In contrast to T. hominis , this species is dimorphic as it forms two kinds of sporophorous vesicles and spores: Type I-round to oval polysporous sporophorous vesicle. 7-10 μm in size, usually with eight spores (3.7 × 2.0 μm), thick endospores, subterminal anchoring disc and anisofilar polar filaments forming seven thicker and two thinner terminal coils. This type of sporophorous vesicle is associated with 25-30 nm filaments extending into the host cell cytoplasm. Type II—smaller, bisporous sporophorous vesicle (4-5 times 2.2-2.5 μm) with two, nearly round, thin-walled spores, 2.2-2.5 × 1.8-2.0 μm in size, having 4-5 isofilar coils. No outside filamentous elements are associated with the bisporous sporophorous vesicle. Both types of sporophorous vesicles were common in the infected brain tissue and could be found within the same cell. The newly described species, together with T. hominis and previously reported Pleistophora -like parasites from human muscle, likely represent a group of closely related human microsporidia.     

Nosema helminthorum Moniez, 1887 (Microspora,Nosematidae):A Taxonomic Enigma1

ABSTRACT. The microsporidian parasite known as Nosema helminthorum Moniez, 1887, parasitic in the tapeworm Moniezia expansa (Rudolphi, 1810), has been shown by electron microscopy to have two cycles of development, one with isolated nuclei, the other with paired nuclei (diplokarya). Both merogony and sporogony of the two separate sequences take place in direct contact with the host cell cytoplasm and ultimately give rise to unikaryotic and diplokaryotic sporoblasts. Sporogony is disporoblastic. The nuclear condition of the spores was not seen. The sequences, corresponding to those of the genera Unikaryon and Nosema, may be part of a single dimorphic life cycle and, if so, the species will have to be transferred to a new genus.     

The Light and Electron Microscopic Cytology of Janacekia adipophila N. Sp. (Microspora, Tuzetiidae), a Microsporidian Parasite of Ptychoptera paludosa (Diptera, Ptychopteridae)

ABSTRACT. The microsporidium Janacekia adipophila n. sp., a parasite of Ptychoptera paludosa larvae in Sweden, is described based on light microscopic and ultrastructural characteristics. Merogonial stages and sporonts are diplokaryotic. Merozoites are formed by rosette-like division. Sporonts develop into sporogonial plasmodia with isolated nuclei. These plasmodia give rise to 8–16 sporoblasts by rosette-like budding. A sporophorous vesicle is initiated by the sporogonial plasmodium. Sporoblasts and spores are enclosed in individual sporophorous vesicles. Granular inclusions of the vesicles, visible using light microscopy, discriminate sporogonial stages from stages of the merogony. The monokaryotic, fresh spores are oval with blunt ends, measuring 4.2-6.3 × 9.1-11.2 μm. Macrospores are formed in small numbers. The spore wall has three subdivisions and the exospore is electron-dense. The polaroplast has two parts: closely arranged lamellae anteriorly, wider sac-like compartments posteriorly. The isofilar polar filament, 191–264 nm wide, has 12-13 coils, which are arranged in one layer in the posterior half of the spore. The electron-dense inclusions of the sporophorous vesicle are modified during sporogony, and vesicles with mature spores are traversed by 21–27 nm wide tubules, which connect the exospore with the envelope of the vesicle. The walls of the tubules, the envelope of the vesicles, and the surface layer of the exospore are all identical double-layered structures. The microsporidium is compared to microsporidia of Ptychopteridae and Tipulidae and to related microsporidia of the family Tuzetiidae.     

On the Cytology of Toxoglugea chironomi (Debaisieux, 1931) Jírovec 1936 (Microspora, Thelohaniidae)

ABSTRACT. The light microscopic and ultrastructural characteristics of a microsporidium provisionally identified as Toxoglugea chironomi (Debaiseux, 1931) Jírovec, 1936, is described. It was isolated from oenocytes and adipose tissue of a midge larva of the genus Dicrotendipes . Merozoites are diplokaryotic. The sporogony produces, by fragmentation, eight monokaryotic spores in a sporophorous vesicle. Mature spores are horse-shoe shaped. The total length is about 5.8 μm, the width 0.8-0.9 μm, the external height of the curve 2.3-3.5 μm, and the external width of the curve 3.5-5.2 μm. The polaroplast has lamellar compartments of two types: narrow and closely packed anteriorly, and wider and more loosely arranged posteriorly. The isofilar polar filament is arranged in 8–10 coils in the posterior fourth of the spore. The external nuclear membrane is sometimes continuous with the endoplasmic reticulum. Lamellar and tubular material of exospore construction are present in the episporontal space from the beginning of sporogony. Teratological and normal spores sometimes occur together in the sporophorous vesicle. The identification of the species is discussed and the ultrastructure is compared to Toxoglugea variabilis , the only further species of the genus with known ultrastructural cytology.     

Aquatic tetrasporoblastic microsporidia from caddis flies (Insecta, Trichoptera): characterisation, phylogeny and taxonomic reevaluation of the genera Episeptum Larsson, 1986, Pyrotheca Hesse, 1935 and Cougourdella Hesse, 1935

Seven microsporidian species infecting caddis fly larvae, corresponding to conventional genera Episeptum, Pyrotheca and Cougourdella were studied using light and electron microscopy. Parts of their small subunit, ITS and large subunit ribosomal RNA genes were sequenced and compared with sequences of rDNA obtained from syntype slides of Cougourdella polycentropi Weiser 1965 and Pyrotheca sp. from Hydropsyche pellucidula. All studied caddis fly microsporidia form a closely related group. Their developmental stages in trichopteran hosts are restricted to fat body cells and oenocytes and have isolated nuclei. In late merogony, uninucleate meronts and binucleate plasmodia are formed. In sporogony a sporogonial plasmodium with four nuclei gives rise by rosette-like budding to four sporoblasts within a non-persistent sporophorous vesicle. Sporoblasts mature into pyriform to lageniform spores. The shape and size of spores, the number of polar filament coils, the structure of the polaroplast and of the exospore, together with morphometric characters present a set of markers unique for respective species. Four new species are established. The new genus Paraepiseptum is proposed to replace the tetrasporoblastic Pyrotheca and Cougourdella species from caddis flies. The genus Episeptum is redefined. Field and laboratory examinations as well as the phylogenetic position within the aquatic clade of microsporidia suggest that the life cycle of trichopteran microsporidia probably involves an alternate (copepod?) host and (or) transovarial transmission.     

Life cycle,ultrastructure and molecular phylogeny of Hyalinocysta chapmani (Microsporidia: Thelohaniidae), a parasite of Culiseta melanura (Diptera: Culicidae) and Orthocyclops modestus (Copepoda: Cyclopidae)

The complete life cycle of the microsporidium Hyalinocysta chapmani is described from the primary mosquito host Culiseta melanura and the intermediate copepod host Orthocyclops modestus. Infections are initiated in larval C. melanura following the oral ingestion of uninucleate spores from infected copepods. Spores germinate within the lumen of the midgut and directly invade fat body tissue where all development occurs. Uninucleated schizonts undergo binary division (schizogony) followed by karyokinesis (nuclear division) to form diplokaryotic meronts. Merogony is by synchronous binary division. The onset of sporogony is characterized by the simultaneous secretion of a sporophorous vesicle and meiotic division of the diplokaryon resulting in the formation of eight ovoid meiospores enclosed within a sporophorous vesicle. Most infected larvae die during the fourth stadium and there is no evidence of a developmental sequence leading to vertical transmission. Hyalinocysta chapmani is horizontally transmitted to O. modestus via oral ingestion of meiospores. Infections become established within ovarian tissue of females and all parasite development is haplophasic. Uninucleate schizonts divide by binary division during an initial schizogonic cycle. Newly formed uninucleate cells produce a thin sporophorous vesicle and undergo repeated nuclear division during sporogony to produce a rosette-shaped, multinucleated sporogonial plasmodium with up to 18 nuclei. This is followed by cytoplasmic cleavage, sporogenesis, and disintegration of the sporophorous vesicle to form membrane-free uninucleate spores. Infected females eventually die and there is no egg development. The small subunit rDNA sequence of H. chapmani isolated from meiospores from C. melanura was identical to the small subunit rDNA sequence obtained from spores from O. modestus, corroborating the laboratory transmission studies and confirming the intermediary role of O. modestus in the life cycle. Phylogenetic analysis was conducted with closely related microsporidia from mosquitoes. Hyalinocysta chapmani did not cluster within described Amblyospora species and can be considered a sister group, warranting separate genus status.     

Amblyospora sp. (Microspora, Amblyosporidae) infecting nerve ganglia of Culex pipiens (Diptera, Culicidae) from Egypt.

A species of Amblyospora-infecting neurones of Culex pipiens is described. Diplokaryotic meronts, which divided by binary fission, were distinguished at the electron microscope level by their unthickened plasma membranes. Sporonts with an electron-dense surface coat gave rise to eight uninucleate sporoblasts within a sporophorous vesicle, cytoplasmic division occurring at the quadrinucleate or octonucleate stages. Indications that nuclear fusion and chromosome reorganization occurred in merogony and sporogony were obtained by light microscopy but meiosis was not detected at the ultrastructural level. Spores were typical of Amblyospora, being ovoid when fresh, truncate when stained, and having an exospore of two membranous layers subtended by a thick amorphous layer, an electron-lucent endospore, an anisofilar polar filament, and a polaroplast comprised of an anterior region of close-packed lamellae and a posterior region of expanded sacs. The metabolic products in the sporophorous vesicle took the form of large globules, small globules with electron-dense borders, and fine granules. These were depleted in mature sporophorous vesicles, though a surface layer of fine granules on the spores may have been derived from them. Many stages were degenerate and it is suggested that C. pipiens may be an accidental host in which the parasite could develop suboptimally in nervous tissue only. Infections in larvae hatched from eggs in the laboratory indicate that vertical transmission occurs.     

Norlevinea n. g., a New Genus for Glugea daphniae (Protozoa: Microspore), a Parasite of Daphnia longispina (Crustacea: Phyllopoda)1

ABSTRACT. Norlevinea n. g. is established for microsporidia in which a uninucleate meront changes into a sporont by secreting a thin, membranous, sporontogcnetic and fragile sporophorous vesicle (pansporoblast membrane) in which four uninucleate sporoblasts are formed. In contrast to the genus Gurleya, the sporoblasts and later the spores are permanently joined into doublets, being laterally cemented by an electron-dense substance structurally identical to and continuous with the exospore layer. The polar filament is of the anisofilar type. The type species is Norlevinea daphniae (Weiser, 1947) n. comb., a parasite of the ovaries of Daphnia longispina occurring in several carp ponds in Czechoslovakia.     

Pleistophora finisterrensis n. sp., a microsporidian parasite of blue whiting Micromesistius poutassou

Pleistophora finisterrensis n. sp. is a microsporidian parasite of the hypoaxial musculature of the blue whiting Micromesistius poutassou (Risso). Foci of infection are between 3 and 6 mm in length and have no evident effects on adjacent muscle fibres. We found only a single type of spore (uninucleate, with mean dimensions of 4×2 µm in fresh preparations), contained within sporophorous vesicles (mean diameter 19 µm in fresh preparations; 150–250 spores per vesicle). All of the development stages of this microsporidian are monokaryotic. The meronts are initially uninucleate and bounded by a plasmalemma. Towards the end of merogony, meronts are multinucleate plasmodia with a well-defined surface coat. Sporogony is polysporous, with multinucleate sporonts, which likewise have a well-defined surface coat (about 130 nm thick), dividing by plasmotomy to give rise to uninucleate sporoblasts. The polar tube is isofilar and consists of 8–9 turns in the posterior half of spore. The polaroplast is made up of an anterior lamellar part and a posterior vesicular part.     

Life Cycle of Culicosporella lunata (Hazard & Savage, 1970) Weiser, 1977 (Microspora) as Revealed in the Light Microscope with a Redescription of the Genus and Species1

Light microscopy studies of Culicosporella lunata (Hazard & Savage), a parasite of the mosquito Culex pilosus (Dyar & Knab), revealed two sporogonial sequences. One sequence begins with diplokaryotic meronts that undergo repeated nuclear divisions to produce sporogonial plasmodia with nuclei in diplokaryotic arrangement. These plasmodia form rosette-like clusters of sporoblasts during incomplete cytokinesis and, eventually, binucleate spores. These spores initiate infections in healthy larvae when they ingest spores. The second sequence begins with diplokaryotic meronts that undergo karyogamy and meiosis to form Thelohania-like sporonts and haploid spores. Anomalies are often observed in these sporonts which result in aberrant spores, usually fewer than eight, in an accessory (pansporoblastic) membrane. Normal haploid spores are morphologically similar to those of species of Amblyospora. The genus and the type species are redefined based on new information presented here and it and the type species are placed in the family Amblyosporidae.     

Ultrastructure and Description of a New Species of Telomyxa (Microspora: Telomyxidae) from the Semiaquatic Beetle,Ora texana Champ. (Coleoptera: Helodidae)1

A new species of the uncommon microsporidian genus Telomyxa (Microspora: Telomyxidae) has been found parasitizing the larval fat body of the semiaquatic beetle, Ora texana. In this species, the sporogonic sequence results in the formation of sporocysts measuring 7.7 times 6.5 μm that contain two crested uninucleate spores (averaging 5.7 times 2.2 μm). The spores are essentially oblong/ovate, tapering toward the anterior end and remaining bound together after sporogony by a persistent accessory membrane or sporocyst. The two spores in the sporocyst are produced by an unusual morphogenetic sequence in which, after one mitosis, the binucleate sporont elongates, forming two lobes that fold toward one another and cleave along a central plane, forming two parallel sporoblasts. The general ultrastructural features of this process are described, and diagnostic characters of this new species of Telomyxa are presented.     

Ultrastructure of a microsporidian hyperparasite, Unikaryon legeri (Microsporida), of trematode larvae

Unikaryon legeri (Dollfus, 1912) Canning and Nicholas, 1974, has been reexamined by electron microscopy from material collected in Portugal. It parasitises metacercariae of Meigymnophallus sp. in Cardium edule. A sporophorous vesicle forms around the sporonts, arising as a blister that separates from the electron-dense surface coat of the sporont. Sporogony is disporoblastic, giving rise to 2 spores that are retained in pairs within the sporophorous vesicle. Unikaryon piriformis, which is the type species of the genus and is also hyperparasitic in platyhelminth larvae, has not been examined by electron microscopy, and it is not known whether this species also produces sporophorous vesicles. If it does, then all that will be required is a simple addition of this character to the definition; if not, U. legeri will have to be transferred to a new genus and reclassified with other disporoblastic genera that sporulate in sporophorous vesicles.     

Morphology,Molecular Phylogeny,and Ecology of Binucleata daphniae n. g., n. sp. (Fungi: Microsporidia), a Parasite of Daphnia magna Straus, 1820 (Crustacea: Branchiopoda)

ABSTRACT. We describe a new microsporidian species Binucleata daphniae, n. g., n. sp., that infects the integument cells lining the hemocoele cavity of the carapace and the postabdomen of the cladoceran Daphnia magna Straus. Infected cells filled with spores accumulate as large clusters in the carapace cavity and heavily infected hosts are detected by their opaque appearance. Despite the parasite's presence, infected Daphnia grow and molt, but have a reduced fecundity. During the parasite's life cycle, chain‐like meronts with isolated nuclei are formed, giving rise to binucleate presporonts, the most frequently observed, characteristic developmental stage. In sporogony, the nuclei of the presporont separate, divide, and eight spores enclosed in a thin‐walled sporophorous vesicle are formed. Spores are 4.9 × 2.5 μm in size (fresh) and have an anisofilar polar filament with eight coils. DNA sequence analysis places B. daphniae in a clade of microsporidians that parasitize crustaceans and mosquitoes and have assumed complex life cycles. Binucleata daphniae, however, has a simple and direct life cycle and can be transferred to naïve hosts and maintained as persistent infections in populations of its host D. magna. We propose that B. daphniae has simplified its life cycle by losing its secondary host, rendering it unique in this clade.     

Ultrastructural Study and Description of Ovavesicula popilliae N. G., N. Sp. (Microsporida: Pleistophoridae) from the Japanese Beetle,Popillia japonica (Coleoptera: Scarabaeidae)1

A new genus and species of microsporidia, Ovavesicula popilliae n. g., n. sp., is described from the Japanese beetle, Popillia japonica, on the basis of studies by light and electron microscopy. Parasite development primarily occurs within the Malpighian tubules of larvae, and spores are formed in a sporophorous vesicle. Meronts have diplokaryotic nuclei, develop in direct contact with the host cell cytoplasm, and divide by binary fission. Sporonts have unpaired nuclei, develop within a thick sporophorous vesicle, and undergo synchronous nuclear divisions producing plasmodia with 2, 4, 8, 16, and 32 nuclei. Cytokinesis of sporogonial plasmodia does not occur until karyokinesis is complete with 32 nuclei. Intact sporophorous vesicles are ovoid, containing numerous secretory products, and are surrounded by a persistent two-layered wall. The uninucleate spores are regularly formed in groups of 32, and the polar tube in each has six coils.     

Ultrastructure ofNosema marucae sp. n. (Microspora,Nosematidae), a pathogen ofMaruca testulalis (Lepidoptera,Pyralidae)

The development ofNosema marucae was followed in its host, the legume pod borerMaruca testulalis. The meronts had an irregular cell body and were binucleated, with the large diplokaryon occupying most of the cell. The sporonts contained many rough endoplastic reticula. The sporoblasts had a granular cytoplasm with a large number of ribosomes, and nuclear division and cellular cleavage were prevalent, particularly from 84 to 120 h after inoculation. Sporoblasts and young spores were evident from 108 to 144 h after inoculation.In the mature spores, the exocuticle had a rough and sculptured surface and the endocuticle thinned considerably at the apical end. The anchoring disc was thick at the point of attachment. The polar tube had 12–15 loops, and the polaroplast was multilayered with two distinct regions and occupied almost half of the spore. The diplokaryon was centrally located in the cell; the nuclear membranes of the diplokarya were separated by an electron-dense region about 0.6 nm thick.     

Description of Chytridiopsis Trichopterae N. Sp. (Microspora, Chytridiopsidae), A Microsporidian Parasite of the Caddis Fly Polycentropus Flavomaculatus (Trichoptera, Polycentropodidae), With Comments On Relationships Between the Families Chytridiopsidae and Metchnikovellidae

ABSTRACT. The microsporidium Chytridiopsis trichopterae n. sp., a parasite of the midgut epithelium of larvae of the caddis fly Polycentropus flavomaculatus found in southern Sweden, is described based on light microscopic and ultrastructural characteristics. All life cycle stages have isolated nuclei. Merogonial reproduction was not observed. the sporogony comprises two sequences: one with free spores in parasitophorous vacuoles, the other in spherical, 5.6-6.8 μm wide, sporophorous vesicles which lie in the cytoplasm. the free sporogony yields more than 20 spores per sporont. the vesicle-bound sporogony produces 8, 12 or 16 spores. the envelope of the sporophorous vesicle is about 82 nm thick and layered. the internal layer is the plasma membrane of the sporont; the surface layer is electron dense with regularly arranged translucent components. Both spore types are spherical. They have an ～ 35-nm thick spore wall, with a plasma membrane, an electron-lucent endospore, and an ～ 14-nm thick electron-dense exospore. the polar sac is cup-like and lacks a layered anchoring disc. the polar filament is arranged in two to three isofilar coils in the half of the spore opposite the nucleus. the coupling between the polar sac and the polar filament is characteristic. the surface of the polar filament is covered with regularly arranged membraneous chambers resembling a honeycomb. There is no polaroplast of traditional type. the cytoplasm lacks polyribosomes. the nucleus has a prominent, wide nucleolus. the two spore types have identical construction, but differ in dimensions and electron density. Free living spores are about 3.2 μm wide, the diameter of the polar filament proper is 102-187 nm, the chambers of the honeycomb are 70-85 nm high, and the polar sac is up to 425 nm wide. Living spores in the vesicle-bound sporogony are about 2.1 μm wide, the polar filament measures 69-102 nm, the chambers of the honeycomb are about 45 nm high, and these spores are more electron dense. Comparisons of cytology (especially the construction of the spore wall and the polar filament and associated structures) and life cycles reveal prominent differences among the Chytridiopsis-like microsporidia, and close relationships between the families Chytridiopsidae and Metchnikovellidae.