Dictyosphaerium-like morphotype in terrestrial algae: what is Xerochlorella (Trebouxiophyceae,Chlorophyta)?1

Several strains of terrestrial algae isolated from biological soil crusts in Germany and Ukraine were identified by morphological methods as the widely distributed species Dictyosphaerium minutum (=Dictyosphaerium chlorelloides). Investigation of the phylogeny showed their position unexpectedly outside of Chlorellaceae (Trebouxiophyceae) and distantly from Chlorella chlorelloides, to which this taxon was attributed after revision of the genus Chlorella based on an integrative approach. SSU rRNA phylogeny determined the position of our strains inside a clade recently described as a new genus of the cryptic alga Xerochlorella olmiae isolated from desert biological soil crusts in the United States. Investigation of the morphology of the authentic strain of X. olmiae showed Dictyosphaerium-like morphology, as well as some other characters, common for our strains and morphospecies D. minutum. The latter alga was described as terrestrial and subsequently united with the earlier described aquatic representative D. chlorelloides because of their similar morphology. The revision of Chlorella mentioned above provided only one aquatic strain (D. chlorelloides), which determined its position in the genus. But terrestrial strains of the morphospecies were not investigated phylogenetically. Our study showed that the terrestrial D. minutum is not related to the morphologically similar D. chlorelloides (=Chlorella chlorelloides, Chlorellaceae), and instead represented a separate lineage in the Trebouxiophyceae, recently described as genus Xerochlorella. Therefore, revision of Xerochlorella is proposed, including nomenclatural combinations, epitypifications, and emendations of two species: X. minuta and X. dichotoma. New characters of the genus based on investigation of morphology and ultrastructure were determined.     

Ultrastructural observations on spermiogenesis in the peanut worm,Themiste pyroides (Chamberlin, 1920) (Sipuncula; Sipunculidea)

Summary

The process of spermiogenesis and the ultrastructure of the spermatozoa in the peanut worm, Themiste pyroides, from the Sea of Japan were observed with electron microscopy (SEM and TEM). The testes are composed of groups of spermatogonia and are covered by peritoneal cells. Clusters of spermatocytes are released from the testes into the coelomic fluid. Connected by intercellular bridges, the spermatocytes within a given cluster develop asynchronously. Proacrosomal vesicles and a flagellum appear in spermatocytes. Spermatids in the clusters retain the intercellular connections. During spermiogenesis, the acrosomal vesicle, formed by coalescence of small proacrosomal vesicles in the basal part of the spermatid, migrates to the apical part of the cell to form a conical-shaped acrosome. The basal concavity lying above the nucleus is filled with subacrosomal substance. The midpiece contains four mitochondria, two centrioles, and some residual cytoplasm with dark glycogen-like granules. A peculiar annulus structure develops around the base of the flagellum. The distal centriole has a pericentriolar complex consisting of radially oriented elements. Before the spawning process, the spermatozoa are filtered throughout the ciliary nephrostomal funnel into the excretory sac of paired nephridia where they are stored for a short time. The sperm are released into the sea water via nephridiopores. Spermatozoa remaining in the coelomic fluid after spawning are resorbed by amoebocytes. This species from Vostok Bay is characterized by a prolonged spawning period from June to early October. The reproductive strategy of T. pyroides is discussed in comparison with that of Thysanocardia nigra, the latter having a unique pattern of packaging of the spermatozoa, resulting in the formation of spermatozeugmata, as a reproductive adaptation to the very short spawning period.     

Ultrastructural study of metamorphosis in the freshwater bryozoan Plumatella fungosa (Bryozoa,Phylactolaemata)

Summary

At metamorphosis the attachment of the Plumatella larva to the substrate is effected by secretions from glandular cells in the apical plate, the leading pole during swimming. The larval mantle folds back and slides down towards the substrate. By ciliary activity an adhesive secretion is spread over the metamorphosing larva and the attachment area. Two polypides appear through the larval terminal opening. The mantle fold, together with gland cells, nerve cells, sensory cells, and muscle cells from the larva form a nutritive cell mass. Reduction of this nutritive cell mass is accomplished by autolysis and phagocytosis. An invaginated area of the nutritive cell mass is provided with a dense layer of microvilli, which seem to have an absorbtive function. The nutritive cell mass consisting of transitory larval tissues provides a significant source of nutrient for the developing polypide buds.     

SPERMATOGENESIS IN A TRICHUROID NEMATODE,TRICHURIS MURIS. II. FINE STRUCTURE OF PRIMARY SPERMATOCYTE AND FIRST MEIOTIC DIVISION

Ultrastructural observations indicate that the primary spermatocyte of Trichuris muris is larger than the spermatogonial stage with an increased cytoplasm to nucleus ratio. The cytoplasm contains an extensive reticular system, mitochondria, numerous free ribosomes and prominent Golgi complexes which may contribute to the formation of a sub-surface, vesicular complex. Although only two spermatocytes were seen to be linked by a cytoplasmic bridge it is suggested that the number of conjoined cells is probably greater. The rearrangement of mitochondria in a ring around the nucleus and the indentation and vesiculation of the nuclear envelope preceeded its disappearance and indicated the onset of meiosis. Centrioles were frequently resolved at this stage. They were composed of nine peripheral doublets surrounded by a dense pericentriolar sheath. Three dense chromatin areas indicative of haploid chromosomes were present in later meiotic stages. Each chromosome was surrounded by a number of mitochondria and there was a clear separation of the chromosome-mitochondrial clusters from the remainder of the cytoplasm. This was particularly evident at telophase when two daughter cells were partially separated by membrane infoldings. This reflects incomplete cytokinesis in the dividing spermatocyte of T. muris and is similar to that described in other trichuroid species. A close association with processes of the non-germinal, sustentacular cells was noted throughout the spermatocyte stage.     

Induction and development of exogastrulae in the starfish,Pisaster ochraceus

The process of mouth and coelom formation in exogastrulae of the starfish, Pisaster ochraceus, induced by LiCl, has been studied with the light microscope, scanning and transmission electron microscopes. Bending and segmentation of the exogastrulated archenteron with the formation of either single or double coelomic pouches follows the same schedule as the control. In addition, a region of the exogastrular ectoderm, which corresponds to the area of the mouth in controls, undergoes invagination. Early morphogenesis of the archenteron and invagination of the ectoderm during mouth formation appear to be intrinsic properties of these structures.

At the time of mouth formation in the controls, a discrete region adjacent to the distal end of the exogastrulated archenteron becomes sticky. Examination of this region shows that the surfaces of the archenteron cells are relatively smooth and that processes of the mesenchyme cells extend between them. The evidence suggests that the mesenchyme cells are responsible for the stickiness, and that they may guide the archenteron and ectoderm into contact and maintain the contact during normal mouth formation.     

Spermatogenesis and sperm—spermatheca relations in Spirorbis spirorbis (L.)

The differentiation from early spermatid to spermatozoon is described with special emphasis on the formation of the helix of chromatin and mitochondrial junctions. The role of microtubules in morphogenesis is discussed.

New observations on the role of the recently described spermatheca are presented; phagocytosis and digestion of spermatozoa are proven, and the various origins of the sperm found in the spermatheca are specified.     

Scanning electron microscopy of glochidia and juveniles of the freshwater mussel,Hyriopsis myersiana

Summary

The ultrastructure of early stages of the mussel, Hyriopsis (Limnoscapha) myersiana (Lea, 1856), was observed by scanning electron microscopy from the glochidial period until the onset of the juvenile stage 10 days later. Further observations were performed for an additional 13 days to assess juvenile development. Glochidia extracted from the brood chambers have a hookless, semi-oval and equivalve calcareous shell with numerous pores in the internal surface, pits in the external surface and cuticular spines in the ventral region. Keratin fibers with a random arrangement in the cuticle of the glochidial shell were also detected. The appearance of the foot within 10 days of in vitro glochidial culture was considered the main feature of metamorphosis to the juvenile stage. Another change during the following 13 days was the formation of a new periostracum exhibiting growth lines under the old glochidial shell. This development occurs mainly in the anterior region and is followed by hardening of the periostracum matrix by calcium deposition. Periostracum growth gradually became apparent in the lateral and posterior regions at the end of this period. The retraction of spines and the alteration of the external surface of the old shell are also described. It is speculated that transcuticular filaments identified in the juvenile stage may have sensory or metabolic exchange functions. The prominent foot, gradually covered by long dense cilia, shows rhythmical movements which suggest a role in feeding. Similarly, cilia present in the mantle may also be involved in the capture of food, while microvilli may facilitate absorption of dissolved materials. Longer cilia, sparsely distributed in the mantle, may function as chemo- or tactile sensors.     

The fine structure of the sperm bundles of the coccid,Aspidiotus perniciosus Cumst., and sperm movement

The mature sperm of A. perniciosus are organized into bundles, about 350 μm long by 9–10 μm wide. Each bundle contains 32 sperm enclosed by a common sheath. The sperm contains an elongated ‘central core’, representing nuclear material, surrounded by a spiral microtubular sheath and cytoplasm. The electron-dense nuclear material is localized in the more pointed half of the sperm. The spiral microtubular sheath is composed of 30— 100 microtubules (depending on the cross-sectional level), situated parallel to the longitudinal axis of the sperm. On the basis of this ultrastructural organization, the motility of the sperm and sperm bundle as a whole is discussed. The sperm of A. perniciosus provide strong evidence that the microtubules arranged asymmetrically represent the elements directly involved in sperm motility.     

Development of secondary sexual characters in the seabob shrimp Xiphopenaeus kroyeri (Heller 1862) (Crustacea,Decapoda, Penaeidae): a scanning electron microscope study

The development of secondary sexual characters, the petasma, and thelycum growth were studied in Xiphopenaeus kroyeri. In adult females, the thelycum is a single plate and its anterolateral portion is characterized by a reduced hood. The aperture resembles a transverse ridge. In immature stages, the ridge has a space between the plates, which becomes narrower as it reaches the end of development. The female gonopore is ‘comma’ shaped. In adult males, the endopods of the petasma are linked at the dorsomedial margin by a large quantity of cincinnuli. In juveniles, cincinnuli gradually increase in number until they join both endopods. At the end of development the petasma is T-shaped. The male gonopore is C-shaped. The relative growth of the petasma total length versus juvenile body length showed a highly positive allometry, whereas in adults the growth was isometric. For the relationship carapace length versus thelycum width, the juvenile phase of females is characterized by an isometry and the adult phase by a negative allometry.