Paranucleospora theridion n. gen., n. sp. (Microsporidia,Enterocytozoonidae) with a Life Cycle in the Salmon Louse (Lepeophtheirus salmonis,Copepoda) and Atlantic Salmon (Salmo salar)

ABSTRACT. Paranucleospora theridion n. gen, n. sp., infecting both Atlantic salmon (Salmo salar) and its copepod parasite Lepeophtheirus salmonis is described. The microsporidian exhibits nuclei in diplokaryotic arrangement during all known life‐cycle stages in salmon, but only in the merogonal stages and early sporogonal stage in salmon lice. All developmental stages of P. theridion are in direct contact with the host cell cytoplasm or nucleoplasm. In salmon, two developmental cycles were observed, producing spores in the cytoplasm of phagocytes or epidermal cells (Cycle‐I) and in the nuclei of epidermal cells (Cycle‐II), respectively. Cycle‐I spores are small and thin walled with a short polar tube, and are believed to be autoinfective. The larger oval intranuclear Cycle‐II spores have a thick endospore and a longer polar tube, and are probably responsible for transmission from salmon to L. salmonis. Parasite development in the salmon louse occurs in several different cell types that may be extremely hypertrophied due to P. theridion proliferation. Diplokaryotic merogony precedes monokaryotic sporogony. The rounded spores produced are comparable to the intranuclear spores in the salmon in most aspects, and likely transmit the infection to salmon. Phylogenetic analysis of P. theridion partial rDNA sequences place the parasite in a position between Nucleospora salmonis and Enterocytozoon bieneusi. Based on characteristics of the morphology, unique development involving a vertebrate fish as well as a crustacean ectoparasite host, and the results of the phylogenetic analyses it is suggested that P. theridion should be given status as a new species in a new genus.     

Evolutionary aspects of the arthropod heart

Evolution has led to changes in the gross anatomy of the arthropod hearts. Changes are also seen in the ultrastructural organization of the cardiomyofiber. Thus the myofilament organization and the membrane systems (T-system and SR) vary within both Chelicerata, Crustacea and Uniramia. Yet, the variation is not haphazard, but constitutes a pattern which cannot be deduced from the gross anatomy. In the three taxa the evolutionary tendency seems to be towards a more strict sarcomeral organization of the myofilaments. This is due to parallelism. The organization of the membrane systems and the spatial relation of the interior couplings are not identical for all arthropods. However, no variations has so far been detected within one and the same order, despite differences in adaptation and size. These systems are conservative and it is suggested that they could be useful in studies of arthropod phylogeny.     

Effects of artificial frost hardening and winter stress on net photosynthesis, photosynthetic electron transport and RuBP carboxylase activity in seedlings of Pinus silvestris

Net photosynthesis of seedlings of Pinus silvestris has been measured and compared with the activities of photosynthetic electron transport and extracted RuBP carboxylase. The effects of prolonged frost hardening (photoperiod 8 h, + 3°C) followed by winter stress at subzero temperatures were analysed. There was a parallel effect of frost hardening and winter stress on the photosynthetic properties of both intact seedlings and isolated chloroplast thylakoids. The activity of extracted RuBP carboxylase was less affected by the treatments. In relation to earlier works we conclude that the decay of net photosynthesis in winter climate is determined by the electron transport properties of the chloroplast thylakoids, i.e. by the pool sizes of photosynthetically active plastoquinone. The results of this work justify the definition of two phases in the response of conifers towards autumn and winter climates: I. Frost hardening occurs at temperatures slightly above zero and it does not affect the efficiency of photosynthesis as defined by the quantum yield at rate limiting light absorption. II. Winter stress occurs at subzero temperatures and it is characterized by a suppression of the photosynthetic efficiency as a result of damage within the photosynthetic apparatus.     

Effects of SO2 on Photosynthesis and Nitrogen Fixation

Spikelets of Themeda triandra are dormant when mature and require an after-ripening period in dry storage of approximately 12 months before full germination potential is realized. Successful germination of spikelets entails the splitting of the tough upper glumes by radicles. Dormany appears to result from a combination of embryo dormancy and mechanically resistant glumes. Glume removal from dormant spikelets increases germination while glume removal plus gibberellic acid increases germination even more. During the after-ripening period, the growth potential of spikelets and caryopses increases as measured by their ability to germinate in the presence of the osmoticum polyethylene glycol 6000. The inhibition of germination by decreasing osmotic potential of the germination medium significantly interacts with the promotion caused by gibberellic acid indicating that both factors affect germination by altering the growth potential of the embryos. The increase in growth potential during after-ripening is probably related to the synthesis of gibberellin-like substances. It is hypothesized that dormancy breaking during after-ripening occurs because the growth potential of embryos increases and this consequently increases the ability of radicles to split the upper glumes during germination.     

The Myocardial Ultrastructure of Hemilamprops rosea and Leucon nasica (Crustacea, Cumacea) and its Phylogenetic Implications

The heart wall of Hemilamprops rosea and Leucon nasica is innervated and consists of a single-layered epicardium and a single-layered myocardium. Their heart ultrastructure does not differ. Large lipid-containing cells are found in the heart lumen. The membrane systems conform to the typical eumalacostracan condition, with the isopods as the only exception. The transverse tubular system at the Z- and H-levels is connected by longitudinal tubules. The sarcoplasmic reticulum, which forms a fenestrated sheath around the myofibrils, is continuous across the Z-band and modified at the H-level only. The interior and peripheral couplings are located at the H-level. The posterior and anterior aorta lack contractile material. The aorta valves are bicuspid.