The seasonality of phytoplankton in African lakes

Although some study of the subject began in 1899, wide-ranging information from African water-bodies has only become available since 1950. Important developments included the establishment of long-term centres of research, the adoption of improved methods for quantitative algal sampling, the more intensive study of environmental conditions, the beginnings of experimental testing, and the improvement of taxonomic knowledge.At higher latitudes (> 20 °) examples of pronounced algal seasonality are long-established; they are accompanied and influenced by marked changes in radiant energy income and so water temperature, and often by effects of seasonal water input. Illustrations are given from lakes in Morocco and South Africa.More generally in Africa, including the tropical belt, annual patterns of phytoplankton seasonality are usually either dominated by hydrological features (water input-output) or by hydrographic ones (water-column structure and circulation). Examples of both types are discussed, together with instances (e.g. L. Volta) of combined hydrological and hydrographic regulation. In both the seasonal abundance of diatoms is often distinct and complementary to that of blue-green algae, with differing relationships to vertical mixing and water retention.Horizontal variability in the seasonal cycle is especially pronounced in the larger or morphometrically subdivided lakes. Some inshore-offshore differentiation is also known to affect phytoplankton quantity (e.g. L. George) and species composition (e.g. L. Victoria). Longitudinal differentiation is common in elongate basins especially when with a massive or seasonal inflow at one end (e.g. L. Turkana, L. Nubia, L. Volta); occasional terminal upwelling can also be influential (e.g. southern L. Tanganyika). Such examples grade into the longitudinally differentiated seasonality of flowing river-reservoir systems, as studied on the Blue and White Niles.The annual amplitude of population density, expressed in orders of magnitude (=log₁₀ units), is one measure of seasonal variability. It can exceed 3 orders both in systems subject to hydrological wash-out (e.g. Nile reservoirs) and in the more variable species components of lakes of long retention (e.g. L. Victoria). Low amplitudes can be characteristic of some components (e.g. green algae in L. Victoria) or of total algal biomass (e.g. L. George, L. Sibaya).Seasonal changes may be subordinated to inter-annual ones, especially in shallow and hydrologically unstable lakes (e.g. L. Nakuru).     

Trypanosoma brucei: analysis of relapsing populations in sensitive and resistant breeds of cattle

The clone DiTat 1.1 of Trypanosoma brucei brucei was injected into four bovids, and clones obtained from successive waves of parasitemia were used to study the expressed variant-specific surface glycoprotein repertoire. Twenty-four clones were obtained which could be classified into 12 different variable antigen types, in addition to the clone injected, using agglutination or immunofluorescence with monospecific antisera. The variable surface glycoproteins of the 25 clones were extracted using the detergent octyl-beta-D-glucopyranoside in the presence of the protease inhibitor, N-cbz-L-phenylalaninechloromethylketone. The molecular weights varied from 52,000 to 69,000 and the pI from 5.0 to 8.8. The virulence of 14 clones representing 13 variable antigen types was ascertained in mice. The mean survival time ranged from 20.5 to 43.0 days. Clones isolated from early peaks of parasitemia in the bovid were the most virulent while clones derived from later peaks were less virulent. It seems that organisms of diminishing virulence appear in bovids, leading to self-cure of the disease. All clones were sensitive to human serum in a blood infectivity inhibition test. Antibody against all virulent clones appeared in 20 cattle (10 Zebus, 10 Baoulés) which had been injected with T. brucei DiTat 1.1. There was no evidence for parasites of high or low virulence being preferentially expressed in resistant or sensitive hosts.     

Limnology and plankton periodicity of Jos Plateau water reservoir,Nigeria, West Africa

Limnological data (Dec. 1980–Jan. 1982) on the plankton and water chemistry of Lamingo Dam, located within the Jos biotite granite area of the Plateau State (Nigeria) are presented. The water-body falls in Beadle's (1981) category I of African lakes (conductivity < 40 µS cm^–1). Alkalinity (% MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGabmiEayaara% aaaa!3914!\[\bar x\] = 0.3 meq l^–1), principally composed of bicarbonates, dominated the anions % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0Jf9crFfpeea0xh9v8qiW7rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaK4waiaajI% eacaqIdbGaaK4tamaaxababaGaeyOeI0caleaacaaIZaaabeaakiaa% b6dacaqIdbGaaKiBaiaaj2cacaqG+aGaaK4uaiaaj+eadaqhaaWcba% GaaKinaaqaaiaajkdacaqITaaaaOGaaKyxaaaa!4657!\[\user1{[HCO}\mathop -\limits_3 {\text{ > }}\user1{Cl - }{\text{ > }}\user1{SO}_\user1{4}^{\user1{2 - }} \user1{]}\]The plankton were characterized by a moderate standing crop of phytoplankton, and zooplankton were, generally, very limited in species and abundance. The order of dominance for the categories of phyto and zooplankton was: [Bacillariophyceae > Chlorophyceae > Dinophyceae] and [Rotifera > Crustacea] respectively. A diel cycle was characterized by nocturnal upward migration of the zooplankton, and the reverse behaviour in the phytoplankton.Interrelations between the biotic assemblages of plankters and various physical and chemical variables are discussed.     

Spontaneous malignant lymphoma in an African green monkey naturally infected with simian T-lymphotropic virus (STLV)

An African green monkey naturally infected with simian T-lymphotropic virus (STLV) developed spontaneous malignant lymphoma of diffuse pleomorphic type. The clinical, hematological and histopathological characteristics were very similar to those of human adult T-cell leukemia.     

The effects of aromatizable androgens,non-aromatizable androgens,and estrogens on gonadotropin release in castrated African catfish,Clarias gariepinus (Burchell)

Summary To study the feedback mechanism of gonadal hormones on GTH secretion in male African catfish, the effects of castration and steroid replacement on GTH release, pituitary GTH content, and ultrastructural appearance of gonadotropes were investigated.Castration resulted in an increase in plasma GTH levels, a decrease in pituitary GTH content, and a degranulation of many gonadotropes. The aromatizable androgens testosterone and androstenedione were able to abolish the castration-induced increase in plasma GTH. This was accompanied with a restoration of pituitary GTH content and a regranulation of gonadotropes. The non-aromatizable androgens 5-dihydrotestosterone and 11-hydroxyandros tenedione did not have these effects. Replacement with estrone or estradiol resulted in an increase in pituitary GTH, however, without abolishing the elevated plasma GTH levels; ultrastructurally, many gonadotropes showed a welldeveloped granular endoplasmic reticulum together with a regranulation.The results of the present study indicate the significance of androgen aromatization in the feedback mechanism of gonadal steroids on the brain-pituitary axis.     

Isolation of low-copy-number sequences that neighbor satellite DNA in mammals

To investigate the role of satellite DNA in eukaryotic genomes, we isolated from an African green monkey (Cercopithecus aethiops) genomic library cloned segments containing the previously described deca-satellite linked to low-copy-number genomic sequences. Three such clones were obtained. The low-copy-number sequences in the three clones do not cross-hybridize suggesting that they derive from different genomic loci. The structure of one of the clones, λAMkA, is described in detail. Subcloned segments containing the low-copy-number sequences from λAMkA anneal to monkey, human and mouse genomic DNA. The subcloned probes were used to select clones containing homologous sequences from a second, independent monkey library as well as from human and mouse genomic libraries. Several of the newly isolated monkey clones hybridized to probes containing the species-specific deca- and -satellites, confirming the genomic association of the low-copy-number sequence in λAMkA with satellite DNA. Moreover, several of the human and mouse clones hybridized to species-specific human and mouse satellite DNAs, respectively. These experiments indicate that the low-copy-number sequence in λMkA and its association with satellite DNA is conserved in primates and rodents.     

Coated Vesicles from the Protozoan Parasite Trypanosoma brucei: Purification and Characterization

ABSTRACT. A procedure was developed to purify a coated vesicle fraction from the protozoan parasite Trypanosoma brucei. Electron microscopy revealed a difference between T. brucei coated vesicles and clathrin-coated vesicles from other eukaryotes: trypanosome vesicles were larger (100 to ISO nm in diameter) and contained an inner coat of electron-dense material in addition to the external coat. Evidence suggests that the internal coat is the parasite's variant surface glycoprotein (VSG) coat. The SDS-PAGE analysis shows the major protein of T. brucei coated vesicles has a molecular mass of 61 kD, similar to VSG; this protein was recognized in an immunoblot by anti-VSG serum. Trypanosome coated vesicles also contain a protein which comigrates with the major protein (clathrin) of coated vesicles purified from rat brains. However, this protein is a minor component and it is not serologically cross-reactive with mammalian clathrin. Immunoblot analysis demonstrated that the parasite vesicles contained host IgG, IgM, and serum albumin.     

Genetic variation of recent Alu insertions in human populations

The Alu family of intersperesed repeats is comprised of ovr 500,000 members which may be divided into discrete subfamilies based upon mutations held in common between members. Distinct subfamilies of Alu sequences have amplified within the human genome in recent evolutionary history. Several individual Alu family members have amplified so recently in human evolution that they are variable as to presence and absence at specific loci within different human populations. Here, we report on the distribution of six polymorphic Alu insetions in a survey of 563 individuals from 14 human population groups across several continents. Our results indicate that these polymorphic Alu insertions probably have an African origin and that there is a much smaller amount of genetic variation between European populations than that found between other populations groups. Present address: Department of Pathology, Stanley S. Scott Cancer Center, Louisiana State University Medical Center, 1901 Perdido St., New Orleans, LA 70112 Correspondence to: M.A. Batzer     

Survival of six species of African ticks in relation to saturation deficits

The survival of unfed males and females of six species of African ticks was monitored at five different saturation deficits at constant temperature (25°C). The survivorship curves for each species comprised a pre-mortality period, prior to when ticks started to die and a mortality period corresponding to a rapid increase in the mortality rate. Longevity was defined as pre-mortality plus mortality. A negative correlation between the longevity of the ticks and the saturation deficits was found with ticks surviving longer at lower deficits. The survival of males and females was similar. At low saturation deficits (2–4 mmHg) Amblyomma hebraeum survived the longest periods (74 weeks). Some correlation was found between the tick survival under dehydrating conditions and habitat associations. Rhipicephalus appendiculatus and Haemaphysalis leachii, the most mesic in distribution, had the shortest longevity (21 and 13 weeks, respectively) at high saturation deficits (7–21 mmHg). Hyalomma marginatum rufipes, the most xerophilic in distribution, had the longest survival (39.3±10.5 weeks) at high saturation deficits. Other factors apart from the adult survival should be taken into account when accounting for the tick distribution, in particular the tolerance of earlier developmental stages to desiccation.     

Effects of an African grass invasion on Hawaiian shrubland nitrogen biogeochemistry

African perennial C₄ grasses are highly successful invaders in Hawaiian ecosystems. We examined the effects of African molasses grass (Melinis minutiflora Beauv.) on Hawaiian shrubland nitrogen (N) dynamics without the influence of fire disturbance. Vegetation tissue carbon and nitrogen chemistry, soil inorganic N pools, net N mineralization rates, and total soil N were studied in three adjacent areas: a monospecificMelinis grassland, a mixed grass/shrubland mosaic, and an un-invaded shrubland.Melinis plots within the mosaic area exhibited the largest inorganic N pools and fastest net N mineralization rates, but were temporally variable with grass phenology. Un-invaded shrubland plots contained the smallest inorganic N pools and lowest net N mineralization rates. Grass foliar C:N and litter C:N were lower than those of common shrubland species, providing one possible link between species and ecosystem N dynamics at this site. The combined effects of N cycle modification, successful light competition, and fire-cycle enhancement make the invasion ofMelinis a significant perturbation to Hawaiian shrubland ecosystem function and successional dynamics. ei]Section editor H Lambers