Development and submicroscopic anatomy of male gametes in Halammovortex nigrifrons (Plathelminthes, Rhabdocoela, ”Dalyellioida”): phylogenetic implications and functional aspects

The steps of spermiogenesis and the submicroscopic anatomy of male gametes in Halammovortex nigrifrons are described. During spermiogenesis the cytophore develops pseudopod-like extensions, and bung-like deposits of dark material become attached to the basal bodies of the cilia. During the phase of cell elongation, cilia stay near the edge of the cytophore. Spermatozoa bear two free cilia or flagella. The axonemata are equipped with glycogen islets appearing at regular spaces. The sperm body is characterized by dot-like dense granules linearly arranged, intense glycogen aggregations in a channel-shaped deposition and giant dense bodies. Events of spermiogenesis and the features of mature male gametes in H. nigrifrons corroborate the hypothesis of the existence of a monophylum within the Rhabdocoela encompassing several, but not all taxa of the ”Typhloplanoida” and ”Dalyellioida”. The Dalyelliidae (including the species of the Temnocephalida) belong to this monophylum.     

Zur Feinstruktur der Protonephridien von Provortex psammophilus (Plathelminthes,Rhabdocoela)

Zusammenfassung Die Cyrtocyten und die proximalen Nephridialkanalabschnitte von Provortex psammophilus wurden elektronenmikroskopisch untersucht. Die Reusen, die nur jeweils einen Kranz von Stäben aufweisen, werden ausschließlich von den multiciliären Cyrtocyten gebildet, deren Kerne wahrscheinlich extrem basal in Zellfortsätzen liegen. Distal gehen die Cyrtocyten ohne Ausprägung einer Zellgrenze in die proximalen, cilienlosen Kanalabschnitte über, die sich nacheinander vereinigen; dabei bilden ihre Wandungen Syncytien. Während die Ultrafiltration auf die Reusen beschränkt ist, finden Resorptionsvorgänge nur in den proximalen Kanalabschnitten statt. Die Reusenbereiche der Protonephridien sind vollständig von einer unterschiedlich dicken Interzellularsubstanz umhüllt.

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On the fine structure of the protonephridia of Provortex psammophilus (Plathelminthes, Rhabdocoela)

Summary The cyrtocytes and the proximal parts of the nephridial canals of Provortex psammophilus have been investigated by electron microscope. The weir-baskets have, respectively, only one rim of rods, and they are formed exclusively by the multiciliary cyrtocytes, whose nuclei are situated probably extremely basally in cell projections. Lacking a distal cell border, the cyrtocytes pass over into the proximal aciliary parts of the canals which join successively, forming syncytia. While the ultrafiltration is restricted to the weir-baskets, resorption only takes place in the proximal parts of the canals. The weir areas of the protonephridia are completely surrounded by an intercellular matrix of variable thickness.

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Abkürzungen b Basalkörper - c Cyrtocyte - ce Centriol - cw Cilienwurzel - i Interzellularsubstanz - k proximaler Nephridialkanal - kl Kanallumen - m Muskulatur des Hautmuskelschlauches - p Parenchymzelle - rl Reusenlumen - rs Reusenstab - v Vakuole     

Characteristics of spermiogenesis and mature spermatozoa in Gyratrix hermaphroditus (Plathelminthes, Rhabdocoela, Kalyptorhynchia)

 Some events of spermiogenesis and the submicroscopic anatomy of male gametes in Gyratrix hermaphroditus are described. Special features occurring during the steps of sperm cell maturation are the development of delicate rootlet-like and spike-shaped structures attached to the basal bodies, an anchor apparatus of cilia staying near the tip of the outgrowing spermatids, the differentiation of a single mitochondrial rod, and the processes of compartmentalizing the chromatin resulting in the formation of extranuclear lamellar stacks. Mature spermatozoa are especially characterized by two incorporated axonemes, a string of large mushroom-shaped dense bodies in the anterior section of the cell, and two lateral rows of nuclear lamellar stacks in the median cell segment. Based on the present findings, the process of spermiogenesis and the organization of spermatozoa in the ground pattern of the monophylum Kalyptorhynchia are reconstructed. Apparently, the sister group of the Kalyptorhynchia can be found within the other free-living and symbiotic Rhabdocoela; the hypothesis of a sister group relationship with the Neodermata is not corroborated. Probably, spermatozoal characteristics can contribute to a discrimination between distinct populations of G. hermaphroditus which is proved to be a complex of several sibling species. Accepted: 25 June 1998     

First evidence of mitochondrial lensing in two species of the “Typhloplanoida” (Plathelminthes,Rhabdocoela): phylogenetic implications

 Based on electron-microscopical observations the light-sensing organs of Proxenetes deltoides and Ptychopera westbladi, representatives of the ”Typhloplanoida” Trigonostominae, are described. The photoreceptors in both species belong to the type of rhabdomeric pigment cup ocelli. P. deltoides has a single pigment cell and three sensory cells. P. westbladi possesses eyes made up of a single pigmented cup cell and a single sensory cell. The dioptric apparatus in the eyes of P. deltoides is formed by three proliferations of the cup cell containing giant mitochondria. In P. westbladi, the elements focalizing incoming light also consist of modified mitochondria which are arranged in the section of the cup cell covering the eye cavity. With regard to the new findings, mitochondrial lensing is hypothesized as an autapomorphy of a monophylum encompassing distinct taxa or all members of the free-living Rhabdocoela; the Neodermata also belong to this monophylum. Accepted: 21 March 1996     

Spermiogenesis and sperm in Mesostoma viaregginum (Plathelminthes, Rhabdocoela): an ultrastructural study to infer sperm orientation

Spermiogenesis in Mesostoma viaregginum begins with the formation of a zone of differentiation containing striated rootlets, two centrioles, and an intercentriolar body in-between. These centrioles generate two parallel free-flagella with the 9+“1” pattern of the Trepaxonemata growing out in opposite directions. Spermatid differentiation is characterised by a 90° latero-ventral rotation of flagella and a subsequent disto-proximal centriolar rotation, with a distal cytoplasmic projection. The former rotation involves the compression of a row of cortical microtubules and allows recognising a flagellar side and an aflagellar side in the late spermatid and in the mature spermatozoon. At the end of the differentiation, centrioles and microtubules lie parallel to the spermatid axis. The disto-proximal centriolar rotation is proposed as a synapomorphy for the Rhabdocoela. The modifications of the intercentriolar body during spermiogenesis and the migration of the nucleus and the centrioles towards the cytoplasmic distal projection are also described. The mature spermatozoon of M. viaregginum is filiform and tapered at both ends and presents many features found in the Rhabdocoela gametes. The nucleus disappears before the flagellar insertion and a density gradient of mitochondria is observed along the sperm axis. The anterior end of the spermatozoon of M. viaregginum is characterised by a tapering capped by a membrane expansion. This study has enabled us to describe precisely the orientation of spermatozoa in the Rhabdocoela in general: the centriolar extremity is proposed as the anterior one for the Rhabdocoela.     

Ultrastructure of spermiogenesis and spermatozoa in Bradynectes sterreri and remarks on sperm cells in Haplopharynx rostratus and Paromalostomum fusculum: phylogenetic implications for the Macrostomorpha (Plathelminthes)

 The fine structure of spermiogenesis and spermatozoa in three species of the Macrostomorpha was studied, with emphasis on Bradynectes sterreri. Two centrioles appear during the development of sperm cells, at least in B. sterreri and Paromalostomum fusculum. Initially these organelles have a perpendicular position, but later they come to lie in line with each other. In P. fusculum, the differentiation of rootlet structures inserting on both centrioles was found. However, ciliary axonemes do not grow out, either in B. sterreri or in P. fusculum. These two species, and also Haplopharynx rostratus, have aciliated spermatozoa. The mature male gametes of B. sterreri are characterized by a filiform nucleus, numerous mitochondria, dense bodies irregular in shape, membranous lacunae, a pair of electron-dense lateral ledges and two sets of cortical microtubules in addition to a closed ring of microtubules in the posterior segment of the cell. Both lateral ledges do not originate from the centrioles. ’Lateral ledges’ or ’lateral bristles’ were not observed in spermatozoa of H. rostratus and P. fusculum. Such structures cannot be considered autapomorphic for the Macrostomorpha. The known spermatological characteristics contribute to elucidating the interrelationships of the Macrostomorpha. Haplopharynx and Macrostomida are sister groups. Spermatozoa with cortical microtubules separated into two sets are hypothesized as an autapomorphy of the Macrostomida. The two lateral ledges found in spermatozoa of B. sterreri are discussed to correspond to the pair of ’lateral bristles’ known from Macrostomum species, indicating a sister-group relationship of these two taxa. Apparently, the aciliated spermatozoa of Macrostomorpha species originated from biciliated male gametes. Hence, biciliated spermatozoa are not an evolutionary novelty of the Trepaxonemata, but of the Rhabditophora. Accepted: 22 February 1999     

Sperm morphology in the Pterastericolidae (Platyhelminthes, Rhabdocoela): phylogenetic implications

The sperm morphology of five species of the Pterastericolidae was studied with transmission electron microscopy. The spermatozoa of all species have two axoncmes, which are incorporated in the sperm cell body for most of their length. The axonemes arc of the'9 + 1'pattern characteristic of the flatworm taxon Trepaxonemata. Outer dynein arms are absent from the microtubule doublets in the axonemes. Dense bodies are few, and occur only in the distal part of the spermatozoa, but small electron dense granules are numerous. A sister group of the Neodermata consisting of the Pterastericolidae and the Fecampidae is proposed. The monophyly of the taxon Dalyellioida is discussed     

Adhesive glands in the Pterastericolidae (Plathelminthes,Rhabdocoela)

Summary The ultrastructure of anteroventral gland cells with processes penetrating the epidermis inPterastericola bergensis, P. fedotovi, P. pellucida and the undescribedP. (sp. Rottnest) was studied with transmission and scanning electron microscopy. Specimens ofP. pellucida were shock frozen in situ in the epithelium of their asteroid host to study the function of the glands. Secretory products released from the gland cell processes fan out towards the host epithelium. The glands are concluded to have an adhesive function. They are compared with similar structures in Neodermata and other rhabdocoel taxa. The phylogenetic significance of the glands is discussed.Abbreviations b basal lamina - c cilium - cr ciliary rootlets - d septate desmosome - g gland cell process - gc gland cell - h host epithelium - m mitochondria - mc muscle cell - mv microvilli - mt microtubules - n nucleus - o ootype - pm plasma membrane - s secretory granule - sm secretory material released from dissolving secretory granules     

Epidermal ultrastructure of Anoplodium stichopi (Plathelminthes,Dalyellioida), a flatworm endosymbiotic in Stichopus tremulus (Holothurioida)

Summary The epidermal ultrastructure of Anoplodium stichopi Bock 1925 (Platyhelminthes, Dalyellioida, Umagillidae) was studied using transmission and scanning electron microscopy. The species lives in the perivisceral coelom of the aspidochirote holothurian Stichopus tremulus Gunnerus 1767. Two types of cells were observed in the epidermis of A. stichopi: ciliated cuboidal epithelial cells and nonciliated pear-shaped cells. The surface of the ciliated epidermal cells is folded into anastomosing ridges. Numerous coated vesicles are subjacent to the surface folds and mitochondria are abundant just below them. Observations indicate that A. stichopi takes up nutrients pinocytically from the coelomic fluid of the host. The ciliation of A. stichopi is sparse.     

Spermatozoa and spermatogenesis in Genostoma kozloffi (Plathelminthes, Rhabdocoela)

Genostoma kozloffi Hyra, 1993, is a symbiotic plathelminth living beneath the carapace of Nebalia pugettensis (Clark, 1932) (Leptostraca Crustacea). Because of similarities in the structure of its epidermis with that of the major group of parasitic flatworms, the Neodermata, and because of similarity in body form to a member of the Revertospermata, which includes the Neodermata and taxa that may be the sister group to the Neodermata, Genostoma was recently classified in the Revertospermata. By electron microscopy we found, however, that spermiogenesis in G. kozloffi does not occur in the manner characteristic of the Revertospermata. Instead, positioning of the axoneme and nucleus takes place as in free-living turbellarians in which the axoneme is fully incorporated, that is, in a distal-proximal fashion. Mature spermatozoa of G. kozloffi are filiform, possess an elongate rod-like nucleus, and one short single, fully incorporated axoneme. A rod of multiple, fused mitochondria accompanies the nucleus and axoneme, and an array of cortical microtubules with thickened walls runs the length of the sperm. Neither dense bodies nor acrosomal vesicles could be found. These features of its spermatozoa as well as the presence of a tunica surrounding its testes are reminiscent of the free-living group Kalyptorhynchia, specifically the Schizorhynchia. It is unlike the Kalyptorhynchia in other respects, however, and its systematic position remains uncertain, albeit removed from the Revertospermata.     

Frontal glandular and sensory structures inNemertoderma (Nemertodermatida) andParatomella (Acoela): ultrastructure and phylogenetic implications for the monophyly of the Euplathelminthes (Plathelminthes)

Summary The ultrastructural organization of the frontal glandular and sensory structures in aNemertoderma species and in the acoelParatomella rubra is described. In both species, a frontal gland complex with an accumulation of different body wall glands and of ciliated sensory receptors exists at the anterior tip of a specimen. Such an organization can be hypothesized as a ground pattern characteristic of the Acoelomorpha. A frontal organ with a common apical pore of many necks of mucoid glands is known for most species of the Acoela; however, such an organ is not a basic feature either of this taxon or of the Acoelomorpha. This substantiates the hypotheses of a homology of the frontal glandular structures in the Acoelomorpha and the Rhabditophora and of the monophyly of the taxon Euplathelminthes. Within the Euplathelminthes, positional, morphological and histochemical differences of the frontal glands and the secretions exist between distinct taxa. These different organizations contribute to the phylogenetic systematization of the Plathelminthes, as shown in a diagram of the phylogenetic relationships among the basic closed descendent communities of this taxon.Abbreviations e,e ₁,e ₂ ellipsoid granules, types 1 and 2 - ep epidermis cell - f mucoid granules - gf mucoid gland neck - is intensively staining secretions - mit mitochondria - r rod-shaped rhabdoids - sr ₁ monociliated sensory receptors with vertical rootlet - sr ₂ monociliated sensory receptor with rhizomelike structure - sr ₃ monociliated sensory receptor with tubularlike structure - t target granules - tw terminal web     

Photoreceptors in species of the Macrostomida (Plathelminthes): ultrastructural findings and phylogenetic implications

The submicroscopic anatomy of intracerebral and pericerebral photoreceptors in six species of the Macrostomida is described. Cylindromacrostomum notan-dum, Paramyozonaria simplex and Macrostomum hystricinum marinum possess two rhabdomeric intracerebral photoreceptors each consisting of two pigmented cup cells and three (C. notandum and P. simplex) or two sensory cells (M. hystricinum marinum). In C. notandum and P. simplex two of the sensory cells are equal in size, while the third one is much smaller. This organisation is hypothesised as an autapomorphy of the Dolichomacrostomidae. Photoreceptors with two mantle cells are also known for Microstomum spiculifer. Since only one cup cell exists in representatives of nearly all other high-ranked taxa of the Rhabditophora, it is concluded that the characteristic ”two cup cells in rhabdomeric photoreceptors” has evolved in the stem lineage of the taxon Macrostomida or Macrostomorpha, respectively. In Myozona purpurea and Psammomacrostomum turbanelloides rhabdomeric intracerebral photoreceptors of a special type were encountered. These light-sensing organs consist of numerous cells forming an ellipsoid. The surface membranes of these cells are elongated to form filiform extensions which are tightly intertwined with each other. Pericerebral ciliary aggregations consisting of cells with an internal cavity into which axonemata of modified cilia project were observed in all species mentioned above and in Bradynectes sterreri as well. Such putative light-perceiving organs are widespread within taxa of the Plathelminthes Rhabditophora and have been hypothesised either as homologous characteristics or as analogous ones. With increasing examples being described it becomes likely that pericerebral ciliary aggregations are an apomorphic ground pattern characteristic of the Rhabditophora. Accepted: 22 January 2001     

The sperm ultrastructure of Caurinus dectes Russell (Mecoptera: Boreidae) and its phylogenetic implications

The sperm structure of the enigmatic mecopteran species Caurinus dectes (Boreidae) is described for the first time. Diagnostic features are the bi-layered acrosome, a cylindric nucleus provided with two longitudinal opposite grooves, and a simple 9 + 2 axoneme which degenerates in the posterior tail end. The results are conform with the monophyly of Mecoptera including Boreidae. A possible autapomorphy of the order is the presence of the two longitudinal opposite grooves along the nucleus, and the presence of two electron-dense fibres beneath the axoneme. Some apparently plesiomorphic features are preserved in the sperm of Caurinus. Features characterizing the distal part of the flagellum, including the presence of an axial cylindrical structure and the distinctive type of axoneme degeneration, are potential synapomorphies of Caurinus and Boreus, i.e. autapomorphic traits of Boreidae.     

On sperm ultrastructure,spermiogenesis and the spermatophore of Heterolaophonte minuta (Copepoda,Harpacticoida)

Summary The spermatophore, mature spermatozoon and spermiogenesis of Heterolaophonte minuta have been investigated by light and electron microscopy. The spermatophore contains three different secretions which are responsible for the discharge of the contents of the spermatophore, the formation of the fertilization tube and the storage of the spermatozoa. The spermatozoon represents a type new for the Copepoda. It is a filiform cell about 25 m in length, ellipsoid in transverse section and tapered at the posterior end. The elongated nucleus contains chromatin fibrils and does not possess a nuclear envelope. Posterior to the nucleus, six mitochondria are placed one after the other. The posterior part of the spermatozoon contains parallel pseudomembranes. The gamete is not helically twisted and is without a flagellum and centrioles. The most remarkable feature of the spermatozoon is an osmiophilic cap in front of the nucleus. This cap corresponds to the acrosome of the spermatozoon. Early stages of spermiogenesis take place in the testis, where the spermatids are incorporated into accessory cells. The origin of the chromatin fibrils and the glycocalyx, as well as the breakdown of the nuclear envelope and centrioles, represent the final steps of spermiogenesis which occur in the vas deferens.     

On the spermiogenesis of Invenusta aestus (Plathelminthes,Proseriata) an ultrastructural study with implications for plathelminth phylogeny

Summary The spermiogenesis of Invenusta aestus (Coelogynoporidae) is described based on electron microscopical observations. Special attention is paid to the intercentriolar body and the anchoring apparatus of the cilia. These organelles are differentiated at the beginning of spermatid maturation and undergo striking changes during gametogenesis. The importance of the organelles mentioned for discussion of phylogenetic relationships within the Plathelminthes is considered.Abbreviations ci cilium - cy cytophore - db dense bodies - go Golgi complex - icb intercentriolar body - mi mitochondrion - mt microtubules - n nucleus - r rootlet - spo spermatogonia - spt spermatids - spy spermatocytes - tb tubular body - zd zone of differentiation     

The sperm ultrastructure of Merluccius merluccius (Teleostei,Gadiformes): phylogenetic considerations

The anacrosomal aquasperm of the gadiform Merluccius merluccius is ultrastructurally similar to the advanced type II spermatozoa (perciform‐type sperm) typically found in most Perciformes. The perciform‐type spermatozoon is characterized by the lateral insertion of the flagellum and the location of the centrioles outside the nuclear fossa. Apart from these characteristics, the spermatozoon of M. merluccius is remarkable because of the mutually parallel arrangement of the centrioles, a rare feature among fishes, which is considered an apomorphic condition for animal sperm cells. Within the superorder Paracanthopterygii, which contains a large diversity of sperm patterns resulting from a high number of apomorphies, a perciform‐type sperm is present only in the order Gadiformes. The significance of the presence of perciform‐type spermatozoa in the three investigated gadiform families is discussed in a phylogenetic context.     

The ultrastructure of the eyes of<Emphasis Type="Italic"> Paravortex karlingi</Emphasis> (Plathelminthes,Rhabdocoela)

Both eyes of Paravortex karlingi belong to the rhabdomeric type. Each eye consists of a single pigment cup cell and three sensory cells. Modified mitochondria lie in three protrusions of the cup cell, and several of these mitochondria form giant mitochondrial derivates. The centres of the derivates include electron-dense substances. These modified structures may have lenticular functions. Furthermore, it is shown that giant mitochondrial derivates develop by fusion of smaller ones. Embryos have many small mitochondria in the developing eyes whereas the adult possesses less numerous, but giant forms. This circumstance leads to the assumption that all such lenticular structures within the rhabdocoels result from identical processes.     

Epidermis and sensory receptors of Temnocephala minor (Plathelminthes, Rhabdocoela, Temnocephalida): an electron microscopic study

 The epidermis, rhabditic glands and receptors of the commensal flatworm Temnocephala minor are described using electron microscopic techniques. The epidermis is syncytial and non-ciliated at the anterior body end; it bears folds, microvilli and other structures which differ according to the body side. The nuclei are located intraepithelially and distally from the basal membrane. Long cilia occur at the posterior end anteriorly from the sucker. All receptor structures described belong to a single morphological type and stand in groups arising from epidermal pits. On the tentacles these groups are regularly distributed. Each receptor has a single cilium and a long rootlet. More than 15 000 receptors of this type have been estimated to occur on the surface of a single medium-sized specimen of T. minor. Although the total number of receptor structures appears very high, the number of different receptor types is extremely low in comparison to other taxa of flatworms. Accepted: 8 July 1997     

Mesocestoides lineatus (Goeze, 1782) (Mesocestoididae): new data on sperm ultrastructure

Spermiogenesis and the ultrastructural characters of the spermatozoon of Mesocestoides lineatus are described by means of transmission electron microscopy, including cytochemical analysis for glycogen. Materials were obtained from a golden hamster (Mesocricetus auratus) after experimental infection with tetrathyridia metacestodes obtained from naturally infected lizards (Anolis carolinensis) from Louisiana. Spermiogenesis in M. lineatus is characterized by the orthogonal growth of a free flagellum, a flagellar rotation, and a proximodistal fusion. The zone of differentiation contains 2 centrioles associated with striated rootlets and a reduced intercentriolar body. The mature spermatozoon of M. lineatus lacks a mitochondrion, and it is characterized by the presence of (1) a single, spiraled, crested body 150 nm thick; (2) a single axoneme of the 9+'1' pattern of trepaxonematan Platyhelminthes; (3) a parallel and reduced row of submembranous cortical microtubules; (4) a spiraled cordon of glycogen granules; and (5) a spiraled nucleus encircling the axoneme.