Microsatellite paternity analysis on captive Lake Malawi cichlids supports reproductive isolation by direct mate choice

Three sympatrically occurring members of a cichlid fish species complex from Lake Malawi have previously been reported to show assortative mating in the field. Offspring from three experimental tanks each housing both sexes of all three members of this complex were screened at one to four microsatellite loci to determine paternity. Reproductive isolation was found to persist without exception under laboratory conditions, providing strong evidence for behavioural barriers to mating. Direct mate choice is suggested as the process preventing interbreeding. The results are interpreted with reference to the rapid and explosive evolution of the cichlid group.     

Emergence and dynamics of cyclopoid copepods in an unpredictable environment

SUMMARY. 1. We studied species composition, abundance and population dynamics of cyclopoid copepods emerging from dormancy from the sediments of a temporary pond in South Carolina in 1985, 1988 and 1989. During a drought in 1988–89, the maximum hydroperiod was 19 days; in 1985 and late 1989-early 1990, the hydroperiods were 57 and 118 days. We also report on species present in 1984 and 1987, two years that had longer hydroperiods, and on abundance of cyclopoids in 1984. 2. Within a day after standing water appeared, fourth-instar copepodids of Diacydops haueri and D. crassicaudis brachycercus became active. These two species appeared every time the pond filled in winter, spring, or autumn. Other species, including Acanthocyclops vernalis, were usually not collected until weeks or months after the pond filled. Because the times and durations of fillings were extremely variable, species composition differed among years, with the most species (eleven) appearing in 1984, the year with the longest hydroperiod. 3. The abundances of emerging Diacydops were much lower in 1988 and 1989 (range of means from seven fillings in spring and autumn: 675–7382 animals m^?2) than 1985 (range of means from three fillings in winter: 26,037–107,418 animals m^?2). Low abundances of emerging animals could have been caused by poor survival of dormant animals, poor production during preceding seasons, or incomplete emergence of the dormant populations. 4. Substrate samples from the dry pond were collected in spring, summer, and autumn 1988 and winter 1989 to measure emergence of the cyclopoids in laboratory incubations. Population densities of emerging animals were much lower in 1988 and 1989 (range of means from seven experiments: 0–120 animals m^?2 over the first 3 days) than in similar experiments in 1984 (Taylor & Mahoney, 1990, means from two experiments: 3630 and 6890 animals m^?2). 5. Despite the low abundance of animals emerging from dormancy in late 1989, the cyclopoid populations in 1990 reached similar densities of copepodids (10^4?-10⁵ animals m^?2) to those reached in 1984 and 1985. These results suggest that short generation time and high reproductive capacity permit rapid recovery from population reductions.     

Entamoeba Motility: Dynamics of Cytoplasmic Streaming, Locomotion and Translocation of Surface-Bound Particles, and Organization of the Actin Cytoskeleton in Entamoeba invadens

ABSTRACT. The dynamics of cytoplasmic streaming, retrograde translocation of externally bound particles and locomotion by Entamoeba invadens were compared. Locomoting amoebae were monopodial, exhibited fountain flow cytoplasmic streaming and translocated externally bound erythrocytes to the rear of cells. The rates of rearward flow of peripheral cytoplasmic vacuoles and of the externally bound particles were equal to the rate of cell forward locomotion. Rhodamine-phalloidin staining revealed a distinct cortical polymerized actin cytoskeleton. This was least evident about the periphery of the advancing pseudopod, increased in density toward the rear of the cell and was most concentrated in the uroid. A monoclonal anti-eucaryotic actin antibody, which recognized monomeric Entamoeba actin on immunoblots, stained trophozoites by indirect immunofluorescence throughout the cytoplasm, but not in the cortical regions stained by rhodamine-phalloidin. This and other evidence implied that the antibody recognized only unpolymerized actin in Entamoeba . We propose that locomotion, cytoplasmic streaming and translocation of externally bound particles are driven by a common actin-based mechanism in Entamoeba , possibly involving retrograde cortical actin flow and recycling.     

Determination of Nitrate Reductase Activity in Barley Leaves and Roots

The inactivation of nitrate reductase in the leaves and rootsof barley (Hordeum vulgare L. cv. Mazurka) during and afterextracting was investigated. At 0 °C in the absence of casein,25 per cent of ‘total’. i.e. maximal in vitro, nitratereductase activity was lost during the 2 min extraction process,followed by a slower loss of activity while the extract wasstored in ice. Activity was maintained by adding a minimum of1 per cent casein to the extraction medium containing 0·1M phosphate (pH 7·5), 1 mM EDTA and 1 mM dithiothreitol.Nitrate reductase was stable for several hours in these extracts,but declined in a first order manner in the absence of dithiothreitol.Casein also prevented the initial loss while making root extracts,but had less effect during storage. Using casein and thiols, nitrate reductase activity in light,(as product of maximal in vitro rates and wt g^–1) in leaveswas 98 per cent of the total activity in 31-day-old plants grownwith full nutrient in water culture and 60-day-old field-grownplants receiving no fertilizer. Field-grown plants, however,exhibited only 17 per cent of the activity of culture-grownplants. Nitrate reductase in leaves of barley plants grown in waterculture had a diurnal rhythm. During the first 3 h of the lightperiod, activity increased to 1·3 x the ‘dark’value. This was followed by a temporary decrease and then byanother increase to a maximum of 1·7 x the ‘dark’value, occurring about 8 h after illumination. Activity thendecreased during the rest of the light period and in darkness. Hordeum vulgare L., barley, nitrate reductase     

The effects of rates of supply of nitrogen, phosphorus, potassium and magnesium on leaf and stem growth, flowering and 'topple' of Golden Harvest and Elmus tulip

In factorial nutrition experiments with three rates of nitrogen, phosphorus and potassium—with some additional treatments with magnesium—on two bulb sizes of Golden Harvest and Elmus tulip, the effect of nitrogen was outstanding. Flowering was retarded by low and high rates of nitrogen supply. Increased nitrogen decreased the length of the first internode of Elmus. Mineral deficiency symptoms in leaf and stem showed minor varietal differences.
Elmus tulips receiving amounts of calcium normally considered adequate developed 'topple' when given no nitrogen especially when combined with high potassium: these treatments also induced stem contortion in Golden Harvest, which appeared to be an analogous disorder to 'topple'.     

The Inducible Formation and Stability of Nitrate Reductase in Higher Plants: I. EFFECTS OF NITRATE AND MOLYBDENUM ON ENZYME ACTIVITY IN CAULIFLOWER (BRASSICA OLERACEA VAR. BOTRYTIS)

The enzyme nitrate reductase could not be detected in leaf tissuesof cauliflower plants grown in sterile cultures with glutamicacid or ammonium sulphate if nitrate was absent. Excised leaftissues from these plants formed the enzyme for several hoursat a steady rate when infiltrated with nitrate. Plants starvedof nitrate for short periods lost enzyme activity which wasrestored in excised tissues upon infiltration with nitrate butnot with ammonium sulphate or nitrite. Molybdenum-deficientplants grown with nitrate also lacked enzyme activity whichwas restored in excised tissues after infiltration with molybdenum.Both nitrate and molybdenum were required to produce maximalrates of enzyme formation in excised tissues of plants grownwith ammonium sulphate and no molybdenum. Apparent Michaelisconstants for nitrate and molybdenum were found to be about10^-5 and 10^-7 respectively. The capacity of excised tissuesto respond to the inducer varied with their age and leaf positionon the plant and was exercised under conditions where growthwas unlikely. Increases in specific activities were similar.There was no evidence of a lag in response to nitrate or molybdenumwith tissues of plants grown with ammonium sulphate or glutamicacid in sterile cultures but lag periods were observed withtissues from plants deprived of nitrate. Cell-free preparationswere unable to respond to either factor. The results are interpretedas evidence for induced enzyme formation in vivo in responseto the substrate or the constituent metal.     

The Inducible Formation and Stability of Nitrate Reductase in Higher Plants: II. EFFECTS OF ENVIRONMENTAL FACTORS, ANTIMETABOLITES, AND AMINO-ACIDS ON INDUCTION

The induction of nitrate reductase by molybdenum or nitratein excised tissues of cauliflower leaf was dependent on temperature;for the range 2^? to 12^? C, Q₁₀ was about 2; for the range 12^?to 22^? C, Q₁₀ was greater than 3. Enzyme formation was initiallymost rapid at 32^? C but did not continue for as long as it didat 22^? or 24^? C. Decreased oxygen supply lessened the rate ofenzyme formation. The effects on enzyme formation of a widerange of natural and synthetic antimetabolites were tested withrespect to induction by either nitrate or molybdenum, when introducedat the same time by infiltration. Actidione (cycloheximide),patulin, cycloserine, polymyxin B, L-2-thiolhistidine D-methionine,L-dihydroxyphenylalanine, D,L--methylglutamic acid, sarcosineand 1 ,2-dichloro-4-(p-nitrobenzenesulphonylamido)-5-nitrobenzene(DCDNS) were the most inhibitory compounds tested. Serine stimulatedproduction of enzyme activity; kinetin, benzimidazole, and p-fluorophenylalanine,3--methyltryptophane and the 4- isomer, chloramphenicol, gramicidin,and several thio- and^aza- derivatives of purines or pyrimidineswere practically without effect. Differential effects of inhibitorson enzyme formation in response to nitrate or molybdenum wererarely observed, and no deductions regarding the possible sequencein which the substrate and prosthetic metal induce activitycould be inferred from the results.     

The role of mammals in maintaining plant species richness in a floating Typha marsh in southern Ontario

Animals are known to influence and sometimes help maintain plant species richness in terrestrial systems. This study investigated the effects of mammals in influencing plant species richness of a floating Typha marsh at Point Pelee National Park. The extent of use of the marsh by terrestrial mammals was documented by mapping mammal trails from air photographs. Trail densities ranging from 70 to 1550 m ha–1 in various areas of the marsh, provided evidence for widespread, and in some places frequent, mammal activity. Plant surveys indicated a pattern of increased species richness and frequency associated with these trails, particularly in late summer when Typha biomass increased off trails. Three mechanisms by which mammals might influence species richness: disturbance of the dominant Typha, nutrient enrichment, and seed dispersal, were investigated using field and lab experiments. Two types of disturbance, trampling and herbivory, at two levels of intensity were simulated within 2×2m plots with and without fertilizer added. Disturbance significantly affected species richness; high levels of trampling decreased while high levels of clipping increased species richness. Nutrients had no effect; there were no significant disturbance×nutrients interactions. The importance of seed dispersal by raccoon was studied by collecting raccoon scat from within the marsh, incubating scat in a growth chamber and comparing the species that emerged to those growing in the marsh. Although substantial numbers of viable seeds of terrestrial species were found in scat, only one of these species was actually part of the Typha mat community. We concluded that mammals do influence plant species richness in this marsh primarily through disturbance of the dominant Typha.     

Spring photosynthesis in a cool temperate bog

In northern ecosystems, the onset and growth of spring photosynthesis may have an important influence on the annual carbon (C) budget, yet the controls have not been clearly identified, especially for peatlands. We used a 5‐year set of daily carbon dioxide (CO₂) exchange measurements derived from an eddy covariance tower located at Mer Bleue, an ombrotrophic bog near Ottawa, Canada, from March to May [day‐of‐year (DOY) 60–150], 1999–2003. We used half‐hourly measured net ecosystem exchange minus modelled ecosystem respiration to estimate daily photosynthesis, as gross ecosystem production (GEP). The onset of GEP in each year was closely related to the thinning and disappearance of the snow cover, occurring between DOY 86 and 101. GEP increased during the spring, reaching 10‐day average values of between 5 and 9 g CO₂ m^?2 day^?1 by the end of May. This increase was initially associated with moss activity (Sphagnum and Polytrichum), followed by the evergreen shrubs. Peat temperatures in the rooting zone (10–20 cm depth) and increases in shrub leaf nitrogen and chlorophyll a concentrations contributed to this rapid increase in GEP. Examination of moderate‐resolution imaging spectroradiometer (MODIS) images over several years revealed that the temporal resolution (16‐day composites) was inadequate to capture the onset of GEP but estimates of gross primary productivity and photosynthesis from MODIS 8‐day composites for the most part followed the pattern and magnitude of CO₂ exchange observed at the tower.     

Mixotrophy among Dinoflagellates1

Mixotrophy, used herein for the combination of phototrophy and phagotrophy, is widespread among dinoflagellates. It occurs among most, perhaps all, of the extant orders, including the Prorocentrales, Dinophysiales. Gymnodiniales, Noctilucales, Gonyaulacales, Peridiniales, Blastodiniales. Phytodiniales, and Dinamoebales. Many cases of mixotrophy among dinoflagellates are probably undocumented. Primarily photosynthetic dinoflagellates with their “own” plastids can often supplement their nutrition by preying on other cells. Some primarily phagotrophic species are photosynthetic due to the presence of kleptochloroplasts or algal endosymbionts. Some parasitic dinoflagellates have plastids and are probably mixotrophic. For most mixotrophic dinoflagellates, the relative importance of photosynthesis, uptake of dissolved inorganic nutrients, and feeding are unknown. However, it is apparent that mixotrophy has different functions in different physiological types of dinoflagellates. Data on the simultaneous regulation of photosynthesis, assimilation of dissolved inorganic and organic nutrients, and phagotophy by environmental parameters (irradiance. availablity of dissolved nutrients, availability of prey) and by life history events are needed in order to understand the diverse roles of mixotrophy in dinoflagellates.