Ultrastructure of the vegetative cell ofBrassica napus pollen with particular reference to microbodies

Summary The ultrastructure of the vegetative cell ofBrassica napus tricellular pollen grains, just before anthesis with standard chemical fixation, is reported. The vegetative cell may be regarded as a highly differentiated and metabolically active fat-storage cell. It contains many mitochondria with a well developed internal membrane system, starchless plastids, microbodies, lipid bodies, dictyosomes and numerous vesicles thought to originate from the dictysomes. Rough endoplasmic reticulum organized in stacks of cisternae is also spatially associated with certain organelles, mainly lipid bodies, microbodies and plastids. There are also randomly distributed polyribosome areas. The microbodies are mainly polymorphic in shape and are often observed in contact with lipid bodies. The above spatial relationship implies that the microbodies may have a glyoxysomal function. In the late period of vegetative cell maturation, the microbodies are probably involved in the process of glyconeogenesis in which the conversion of lipid reserves to sugar takes place.Abbreviations VC vegetative cell - VN vegetative nucleus - SC sperm cell - M mitochondria - MB microbodies - L lipid body - P plastid - D dictyosomes     

The Early Neolithic site at Hoge Vaart, Almere, the Netherlands, with particular reference to non-diffusion of crop plants, and the significance of site function and sample location

The construction of a new motorway necessitated the excavation of an Early Neolithic site in the Dutch Flevopolder near the Hoge Vaart canal. In 1994-1996, an area of ca. 1700 m² was excavated in 0.5x0.5-m squares. The excavation revealed 150 surface hearths and more than a hundred deep hearth pits. The finds, mainly flint and pottery, as well as¹⁴C dates from charcoal demonstrated that the area had been used mainly during the Early Neolithic (ca. 4900–4500 cal. B.C.). The investigation of the Early Neolithic remains included detailed botanical analyses of 182 samples and hand-sorting by the excavation team, which included archaeobotanists, of tens of thousands of samples sieved over 2-mm-mesh sieves. No remains of cultivated plants were recognised among the 87 taxa (40 identified to species level) that were preserved, mainly in carbonised form. These results are compared with other Early Neolithic sites studied in the Netherlands, such as Swifterbant S3 and Schokland P14. The possible significance of site function and sample location as important factors influencing presence/absence of cultivated plant remains at a particular site is discussed.     

Biogeography of limno-terrestrial Tardigrada, with particular reference to the Antarctic fauna

The Tardigrada is a cosmopolitan phylum of pre-Pangaean origin, yet tardigrade families and genera show distinct biogeographic components isolated by two major geological events. Separate Laurasian and Gondwanan familial clusters correlate with the Triassic disintegration of Pangaea, while discrete Antarctic, Australian and New Zealand familial/generic clusters relate to the subsequent Jurassic/Cretaceous disintegration of Gondwana.     

Evolutionary ecology of colonial reef-organisms, with particular reference to corals

Sedentary reef-organisms such as sponges, colonial coelenterates, bryozoans and compound ascidians produce repeated modules (aquiferous systems, polyps, zooids) as they grow. Modular construction alleviates constraints on biomass imposed by mechanical and energetic factors that are functions of the surface area to volume ratio. Colonies thus may grow large whilst preserving optimal modular dimensions. Among corals, optimal polyp size is smaller in the more autotrophic than in the more heterotrophic species. Modular construction allows flexibility of growth form, which can adapt to factors such as water currents, silting, light intensity and proximity of competitors. Modular colonies have great regenerative capacities, even separated fragments may survive and grow into new colonies. All fragments from a parental colony are genetically identical and large branching corals frequently undergo clonal propagation through fragmentation during storms. Soft corals can also fragment endogenously. By spreading the risk of mortality among independent units, the generation and dispersal of fragments lessens the likelihood of clonal extinction. In spite of their ability to propagate asexually, most benthic colonial animals also reproduce asexually. The selective advantages of the genetic diversity among sexually produced offspring seem not to be linked with dispersal, but probably lie in the biological interactions with competitors, predators and pathogens in the parental habitat. Age at first sexual maturity and the proportional investment of resources in sexual reproduction are related to colonial survivorship. Small branching corals on reef flats grow quickly, attain sexual maturity within 1–4 years, planulate extensively, but reach only small sizes before dying. Massive corals are longer lived and have the opposite characteristics of growth and reproduction. Most sessile reef organisms compete for space, food or light. Faster growers can potentially outcompete slower growers, but are often prevented from doing so by several forms of aggression from competitors and by the damage inflicted by storms. Competitive interactions among sedentary organisms on coral reefs are unlikely to be linear or deterministic, and so the co-existence of diverse species is possible.     

Ultrastructure of possible sites of ultrafiltration in some gastropoda,with particular reference to the auricle of the freshwater prosobranch Viviparus viviparus L.

Summary In the Gastropoda pressure-ultrafiltration of the blood is assumed to be the first step in urine formation. The most probable site of ultrafiltration is the wall of the heart. Since in other animal groups ultrafilters are characterized by a special cell type, the podocyte, the hearts of two pulmonates (Lymnaea stagnalis, Biomphalaria glabrata) and of four prosobranchs (Viviparus viviparus, Bithynia tentaculata, Ampullaria gigas, Littorina littorea) were ultrastructurally investigated, in order to establish whether or not podocytes occur in these structures. It appeared that only in the wall of the auricle of Viviparus podocytes are present. They form a layer underneath the epicardium, the epithelium covering the auricle. It is assumed that in Viviparus ultrafiltration proceeds in the auricle. The possible route of the pro-urine is discussed. The location of the ultrafilters in the other species studied remains still unknown.     

The biology of terns nesting at Aldabra Atoll, Indian Ocean, with particular reference to breeding seasonality

Five species of tern breed on Aldabra Atoll (09° 24' S; 46° 20' E). The Caspian tern Sterna caspia and Crested tern S. bergii feed exclusively in very shallow reef/lagoon water, the Fairy tern Gygis alba and Brown noddy Anous stolidus feed out at sea, and the Black-naped tern Sterna sumatrana is intermediate in its foraging. Both of the shallow-water species lay during the south-east monsoon season, the Caspian tern from April to August and the Crested tern from June to August, but the Crested tern also lays in December and January. The remaining three species have extended laying periods largely circumscribed by the north-west monsoon season from November to March. Breeding population size of the Caspian tern is in the low tens and of the Brown noddy in the low thousands, with the other species each numbering in the hundreds. The distribution and abundance of the nine species of tern breeding within the Seychelles ( sensu lato ) vary on the different island groups in a manner interpretable in terms of depth of the surrounding waters. Systematic differences between the central Seychelles and Aldabra groups in timing of breeding by terns which feed out at sea may be associated with seasonal latitudinal movement of the divergence zone between the South Equatorial Current and the Equatorial Counter-current, acting via correlated latitudinal shifts of prey species and game-fish abundance.     

Sampling strategy related to fish distribution, with particular reference to the Norfolk Broads

Large-scale movements of fish within a complex freshwater system such as the Norfolk Broads can give rise to erroneous sampling data if the extent of these movements is not fully appreciated. The Norfolk Broads habitat is briefly described, and data are presented showing that mean estimates of standing crop in the broads vary from <1gm^-2 in the winter to 9.4 gm^-2 in the summer. The widespread nature of the phenomenon is demonstrated by winter and summer data from five broads, and a more detailed picture of the seasonal changes is shown by an intensive 2-year study on one of these broads. Winter aggregations of fish have been found in sites adjacent to rivers connected with the broads, with densities of up to 36.7 fish m^-2 and a biomass of 1787 gm^-2. Accurate overall estimates of fish standing crop can only be obtained when fish are maximally dispersed during the summer months.     

Response of the spinal trigeminal nucleus neurons to electric stimulation of the rat dura mater

Aseptic inflammation of tissues surrounding large meningeal blood vessels, e.g. the superior sagittal sinus, underlies pathogenesis of migraine. This inflammation develops due to antidromic activation of sensory trigeminal nerve endings and is followed by changes in responses of the spinal nucleus of the trigeminal nerve neurons to electrical stimulation of the superior sagittal sinus. However, characteristics of these reactions are still unclear. In experiments ou urethane-anesthetized rats, responses of 387 neurons of the spinal nucleus of the trigeminal nerve to electrical stimulation of the superior sagittal sinus, were recorded. It was tial discharge with the latency 7 to 19 ms (11.4 +/- 0.17 ms) and a subsequent long-lasting discharge with the latency 20 to 50 ms (34.2 +/- 0.8 ms). It is presumed that the first phase reflects orthodromic activation of prevascular A delta and C-fibers of the trigeminal nerve while the second phase is connected with activation of meningeal C-fibers which have low conduction velocity, and/or with a secondary activation of perivascular sensory endings of trigeminal nerve by releasing algogenic and vasoactive substances. These changes could be used as an indicator of efficacy of some antimigraine substances in animal experiments.     

The retina of the phalangid,Opilio ravennae,with particular reference to arhabdomeric cells

Summary The retina of the phalangid, Opilio ravennae, consists of retinula cells with distal rhabdomeres, arhabdomeric cells, and sheath cells. The receptive segment of retinula cells shows a clear separation into a Proximal rhabdom, organized into distinct rhabdom units formed by three or four retinula cells, and a Distal rhabdom, consisting of an uniterrupted layer of contiguous rhabdomeres. One of the cells comprising a retinula unit, the so-called distal retinula cell (DRC), has two or three branches that pass laterally alongside the rhabdom, thereby separating the two or three principal retinula cells of a unit. The two morphologically distinct layers of the receptive segment differ with respect to the cellular origin of rhabdomeral microvilli: DRC-branches contribute very few microvilli to the proximal rhabdom and develop extremely large rhabdomeres in the distal rhabdom only, causing the rhabdom units to fuse. Principal retinula cells, on the other hand, comprise the majority of microvilli of the proximal rhabdom, but their rhabdomeres diminish in the distal rhabdom. It is argued that proximal and distal rhabdoms serve different functions in relation to the intensity of incident light.In animals fixed 4 h after sunset, pigment granules retreat from the distal two thirds of the receptive segment. A comparison of retinae of day- and night-adapted animals shows that there is a slight (approximately 15%) increase in the cross-sectional area of rhabdomeral microvilli in dark-adapted animals, which in volume corresponds to the loss of pigment granules from the receptive segment. The length of the receptive segment as well as the pattern and shape of rhabdom units, however, remain unchanged.Each retinula unit is associated with one arhabdomeric cell. Their cell bodies are located close to those of retinula cells, but are much smaller and do not contain pigment granules. The most remarkable feature is a long, slender distal dendrite that extends up to the base of the fused rhabdom where it increases in diameter and develops a number of lateral processes interdigitating with microvilli of the rhabdom. The most distal dendrite portion extends through the center of the fused rhabdom and has again a smooth outline. All dendrites end in the distal third of the proximal rhabdom and are never present in the layer of the contiguous distal rhabdom. Arhabdomeric cells are of essentially the same morphology in day- and night-adapted animals. They are interpreted as photoinsensitive secondary neurons involved in visual information-processing that channel current collected from retinula cells of the proximal rhabdom along the optic nerve. A comparison is made with morphological equivalents of these cells in other chelicerate species.     

Oviduct function in pigs, with particular reference to the pathological condition of polyspermy

Because the exceptionally high incidence of polyspermic fertilisation has been emphasised as a major defect in systems of in vitro fertilisation in pigs, the aetiology of the condition has been analysed in a series of experiments in vivo in the search for a common underlying cause and possible means of mitigation. Whereas the defense mechanism against polyspermy in pig oocytes is classically viewed as zona reaction, more recent evidence suggests a secondary block at the vitelline surface. Both blocks may be compromised in situations leading to polyspermy, although deleterious influences seem to be expressed principally in an inadequate zona block, as judged by the presence of perivitelline spermatozoa. Postovulatory aging of mammalian oocytes prior to sperm penetration leads to polyspermy, as can be demonstrated in pig eggs. The primary lesion may concern the cortical reaction, owing to a delayed and incomplete exocytosis of the vesicular contents. Eggs ovulated after gonadotrophin treatment during the luteal phase of the cycle show a high incidence of polyspermic penetration (60.6%), as do those shed at estrus in animals treated with progesterone systemically (40%) or by local microinjections in the oviduct wall (32.3%). Whereas progesterone may be modifying interactions of the gametes and responses of the egg organelles in all four above experimental situations, enhanced numbers of spermatozoa ascending a more patent isthmus appear to be the principal cause of polyspermy.(ABSTRACT TRUNCATED AT 250 WORDS)