CONSTANCY OF POPULATION PARAMETERS FOR LIFE-HISTORY AND FLORAL TRAITS IN RAPHANUS SATIVUS L. II. EFFECTS OF PLANTING DENSITY ON PHENOTYPE AND HERITABILITY ESTIMATES

To determine the effect of growing conditions on population parameters in wild radish, (Raphanus sativus L.: Brassicaceae), we replicated maternal and paternal half-sib families of seed across three planting densities in an experimental garden. A nested breeding design performed in the greenhouse produced 1,800 F₁ seeds sown in the garden. We recorded survivorship, measured phenotypic correlations among and estimated narrow-sense and broad-sense heritabilities (h²) of: days to germination, days to flowering, petal area, ovule number/flower, pollen production/flower, and modal pollen grain volume. Survivorship declined with increasing density, but the relative abundances of surviving families did not differ significantly among densities. Seeds in high-density plots germinated significantly faster than seeds sown in medium- or low-density plots, but they flowered significantly later. Plants in high-density plots had fewer ovules per flower than those in the other treatments. Petal area and pollen characters did not differ significantly among densities. Densities differed with respect to the number and sign of significant phenotypic correlations. Analyses of variance were conducted to detect additive genetic variance (V_a) of each trait in each density. At low density, there were significant paternal effects on flowering time and modal pollen grain volume; in medium-density plots, germination time, flowering time and ovule number exhibited significant paternal effects; in high-density plots, only pollen grain volume differed among paternal sibships. The ability to detect maternal effects on progeny phenotype also depended on density. Narrow-sense h² estimates differed markedly among density treatments for germination time, flowering time, ovule number and pollen grain volume. Maternal, paternal and error variance components were estimated for each trait and density to examine the sources of variation in narrow-sense h² across densities. Variance components did not change consistently across densities; each trait behaved differently. To provide qualitative estimates of genetic correlations between characters, correlation coefficients were estimated using paternal family means; these correlations also differed among densities. These results demonstrate that: a) planting density influences the magnitude of maternal and paternal effects on progeny phenotype, and of h² estimates, b) traits differ with respect to the density in which heritability is greatest, c) density affects the variance components that comprise heritability, but each trait behaves differently, and d) the response to selection on any target trait should result in different correlated responses of other traits, depending on density.     

INFLUENCE OF IRRADIANCE ON CELL VOLUME AND CARBON QUOTA FOR TEN SPECIES OF MARINE PHYTOPLANKTON1

Ten species of marine phytoplankton were grown under a range of photosynthetic photon flux densities (PFDs) and examined for variation in cell volume and carbon quota. Results suggest that in response to low PFDs phytoplankton generally reduce their cell volume and frequently reduce their carbon quota. A significant linear relationship between the log of PFD (I) and cell volume (in nine of ten species) and log I and carbon quota (four of ten species) was demonstrated. When exposed, to a transient in light intensity, Thalassiosira pseudonana (Hustedt, clone 3H) Hasle and Heimdal underwent a rapid adaptation in cell volume and carbon quota. Cells going from low light to high light reached maximum mean cell volume within 5 h, and cells going from high light to low light reached a minimum mean cell volume within 12 h. The resulting kinetic constant (k; a measure of the rate of adaptation) was considerably larger than previously reported k values. Ditylum brightwellii (West) Grunow increased in length but did not increase in width during a transient to increased irradiance. Nutrient limitation was shown to override PFD in determining cell volume and carbon quota for Heterosigma akashiwo Hada. Cells grown at equivalent irradiances but N-limited, were smaller than light-limited and nutrient-saturated cells. Therefore, cell volume and carbon quota do not have the same relationship with PFD when factors other than PFD control growth rate. The ecological implications of reduced cell volumes and carbon quotas with decreasing PFD include possible impacts on CO₂ budgets, an influence on sinking rates, potential changes in predation rates, and surface area/cell volume benefits.     

Utility of Trigonelline as a Biochemical Market for Interspecific Competition between Soybean and the Weed Common Waterhemp

Interspecific competition between four soybean cultivars (PI471938, Stressland, Essex and Forrest) and the weed, common waterhemp was investigated under increasing weed densities (i.e. 0, 1, 4 and 16 plants per pot). Soybean height and leaflet number were measured over a 45-d period and used to calculate relative growth rates (RGR). Trigonelline (TRG) concentration was determined within the V1 leaf of 45-d-old soybean plants. Soybean leaflet number (P[lt ]0.05), soybean height (P[lt ]0.05) and soybean RGR_h (expressed in terms of height) differed significantly (P[lt ]0.05) according to waterhemp density. At each waterhemp density Stressland matured at a significantly faster rate whereas the maturation rate of Essex decreased in the presence of waterhemp. Final TRG concentrations were affected by the interaction between soybean cultivar and waterhemp density. Under no competition, TRG concentration was significantly lower in Forrest relative to PI471938, Stressland and Essex. TRG concentrations in Essex declined in higher waterhemp densities.     

An Analysis of Ultrastructural Changes during Oosphere-initial Development in Oogonia from Ca2+-sufficient and Ca2+-deficient cultures of Saprolegnia terrestris Cookson ex Seymour

The central vacuole system of oogonia of Saprolegnia terrestrisfrom Ca²⁺-sufficient cultures was fully enlarged prior to theformation of oosphere initials, which did not involve cleavagevesicles. In oogonia with fully-enlarged central vacuole systems,subsidiary vacuoles at the periphery of the system sometimescontained dense bodies, and dense-body profiles were sometimespresent within sections of the central vacuole system itself.As the central vacuole system enlarged, volume densities ofdense-body vesicles, peripheral vacuoles, lipid bodies and thecytoplasmic matrix decreased relative to total oogonial volume(peripheral protoplasm volume plus central vacuole volume),while the volume density of nuclei increased and that of mitochondriaremained constant. Relative to the peripheral protoplasm only,volume densities of dense-body vesicles, lipid bodies and mitochondriaincreased and volume densities of peripheral vacuoles and ofthe cytoplasmic matrix decreased, while the volume density ofnuclei increased during central vacuole enlargement but subsequentlydecreased during formation of oosphere initials. Under conditionsof Ca²⁺ deficiency, the volume densities of mitochondria andof the cytoplasmic matrix were significantly increased, whilethat of lipid bodies was significantly decreased, at early stagesof oogonial development; the volume densities of other organelleswere not significantly altered at any stage. Saprolegnia terrestris, oogonia, development, calcium, ultrastructure, stereology     

LIMNOLOGICAL BIBLIOGRAPHY FOR AFRICA SOUTH OF THE SAHARA

SUMMARY

Before inundation in September 1976 Barbus holubi and Labeo capensis were the dominant species each making up 46% of the total catch. Immediately after impoundment the relative density of B. holubi increased to 75% while that of L. capensis dropped to 18%. With minor changes the relative densities remained roughly constant until April 1978 when the turbidity of the dam increased. After this date the relative density of B. holubi rapidly declined, probably due to environmental changes, while that of L. capensis increased due to post-impoundment recruitment. The catches of other species remained low. Changes in catch per unit effort, which were determined from April 1978 onwards substantiate these findings.     

DENSITY-DEPENDENT EVOLUTION OF LIFE-HISTORY TRAITS IN DROSOPHILA MELANOGASTER

Populations of Drosophila melanogaster were maintained for 36 generations in r- and K-selected environments in order to test the life-history predictions of theories on density-dependent selection. In the r-selection environment, populations were reduced to low densities by density-independent adult mortality, whereas populations in the K-selection environment were maintained at their carrying capacity. Some of the experimental results support the predictions or r- and K-selection theory; relative to the r-selected populations, the K-selected populations evolved an increased larval-to-adult viability, larger body size, and longer development time at high larval densities. Mueller and Ayala (1981) found that K-selected populations also have a higher rate of population growth at high densities. Other predictions of the thoery are contradicted by the lack of differences between the r and K populations in adult longevity and fecundity and a slower rate of development for r-selected individuals at low densities. The differences between selected populations in larval survivorship, larval-to-adult development time, and adult body size are strongly dependent on larval density, and there is a significant interaction between populations and larval density for each trait. This manifests an inadequacy of the theory on r- and K-selection, which does not take into account such interactions between genotypes and environments. We describe mechanisms that may explain the evolution of preadult life-history traits in our experiment and discuss the need for changes in theories of density-dependent selection.     

TOXICITY OF COPPER TO THALASSIOSIRA PSEUDONANA IN UNENRICHED INSHORE SEAWATER1

Toxicity of copper to T. pseudonana (formerly Cyclotella nana, clone 13-1) wax examined in inshore seawater using a 96-hr bioassay method. Raw unenriched seawater wax filtered through a 0.22-μ membrane filter and then pasteurized, for 30 min at 60 C. Following this treatment, samples contained 0.68–1.14 μg Cu/liter. Copper was added as the chloride in 5-μg increments over the range of 5 to 30 μg/liter (about 0.1–0.5 μM). Population densities, mean cell, volume, and ¹⁴C bicarbonate uptake were measured. Population growth, and ¹⁴C uptake by T. pseudonana displayed inhibition over the entire range of added copper. Growth rate constant (k) of T. pseudonana decreased with increasing copper concentration and during the course of growth at each concentration. Correspondingly, mean cell volumes increased with copper concentration and time. Copper toxicity varied in different water samples. The presence of decomposed natural plankton and detritus decreased toxicity. In the absence of enrichment, bacteria had little effect on copper toxicity. H exults were influenced by glassware treatment, collection and storage of seawater, and absence of enrichments.     

ASYMMETRIC COMPETITION IN MIXED POPULATIONS OF TADPOLES OF THE HYBRIDOGENETIC: RANA ESCULENTA COMPLEX

Hybridogenetic Rana esculenta tadpoles display tolerance to extreme environmental conditions and fit criteria of the “general-purpose” genotype. A trade-off between generality and competitive ability is assumed to occur in asexual species, but the evidence remains unclear. The purpose of my experiment was to test the competitive ability of hemiclonal hybrid Rana esculenta tadpoles relative to the parental species Rana lessonae. Mixed and single genotype populations of R. esculenta and R. lessonae tadpoles were reared at three densities in artificial ponds. Survival of R. esculenta was higher than for R. lessonae tadpoles, but did not differ among densities. Body size at metamorphosis was the same between genotypes, but decreased with increasing density. Larval period was not affected by density, but R. esculenta tended to metamorphose earlier than R. lessonae. Percentage of individuals metamorphosing was higher for R. esculenta at both medium and high densities, but the same as R. lessonae at the low density. The difference in survival, body size, and larval period between tadpoles reared in single and mixed genotype populations was unaffected by genotype or density. The difference in the percentage of metamorphs, however, was strongly affected. The percentage of hybrids metamorphosing was 9% above the responses of single genotype populations at the highest density. Conversely, the percentage of R. lessonae metamorphosing was 12% below the responses of single genotype populations at the same density. Hybrid success in this experiment further supports the criterion of a “general-purpose” genotype without assumptions of reduced competitive ability.     

A centrifugation method for measuring the relative density (specific gravity) of planktonic rotifers (Rotifera), with values for the relative density of Polyarthra major Burckhardt and Keratella cochlearis (Gosse)

The relative density of Polyarthra major and Keratella cochlearis was measured by centrifugation in solutions of different densities. Ficoll was used as the solute. The mean values for the relative density of living specimens of P. major was found to be 1.0277 ± 0.0062. The value for preserved animals was lower at 1.0160 ± 0.0029. The relative density of fixed specimens of K. cochlearis was measured and found to be approximately 1.0450 ± 0.0175.     

Increasing Himalayan tahr and decreasing chamois densities in the eastern Southern Alps, New Zealand: evidence for interspecific competition

There is anecdotal evidence that increasing densities of Himalayan tahr (Hemitragus jemlahicus) are associated with declining densities of chamois (Rupicapra rupicapra) in the Southern Alps, New Zealand. To examine this phenomenon, densities of tahr and chamois were measured at 53 sites within their sympatric range in the eastern Southern Alps during 1978–1979. In sites where only one species was present, tahr density was significantly higher than chamois density (P=0.032), probably reflecting species differences in sociality. Chamois density was higher in catchments without tahr than in those with tahr (P=0.012). Similarly, tahr density was significantly higher at sites without chamois than at those with chamois (P=0.033). Sites with both species present (termed sympatric) were significantly larger than those with only chamois or tahr (P<0.001). Following the prohibition of aerial hunting of tahr in 1983, 16 of the 17 sites where tahr and chamois were sympatric during 1978–1979 were recounted during 1991–1996. There was a 6-fold increase in the mean density of tahr between the two counts (P=0.001), whereas chamois density had declined significantly (P=0.006). Chamois persisted at only three sites, two of which had the highest chamois densities in 1978–1979. This is evidence that increasing densities of tahr exclude chamois from all but the `best' habitats. We conclude that intensive aerial hunting of tahr during 1967–1983 reduced tahr densities such that chamois could co-exist with tahr. Received: 20 March 1997 / Accepted: 8 September 1997     

Increase in cell mass during the division cycle of Escherichia coli B/rA.

Increase in the mean cell mass of undivided cells was determined during the division cycle of Escherichia coli B/rA. Cell buoyant densities during the division cycle were determined after cells from an exponentially growing culture were separated by size. The buoyant densities of these cells were essentially independent of cell age, with a mean value of 1.094 g ml-1. Mean cell volume and buoyant density were also determined during synchronous growth in two different media, which provided doubling times of 40 and 25 min. Cell volume and mass increased linearly at both growth rates, as buoyant density did not vary significantly. The results are consistent with only one of the three major models of cell growth, linear growth, which specifies that the rate of increase in cell mass is constant throughout the division cycle.     

Density-dependent germination and the role of seed leachate

Abstract While density dependence is a central issue in much of plant ecology, it is often overlooked during the crucial seed germination period of the plant life-cycle. Here, patterns of germination in relation to initial seed density for 12 phylogenetically-diverse perennial plant species are described from laboratory experiments. When each of the 12 species was analysed individually, seeds of Alysicarpus rugosus, Callistemon citrinus, Eragrostis curvula and Panicum miliaceum showed a significant decrease in the proportion of seeds germinating at high densities of conspecifics. A meta-analysis carried out by grouping 11 of the 12 species together revealed an overall significant effect for a decrease in the proportion of seeds germinating at high conspecific densities compared with low con-specific densities. Significant decreases in the proportion of seeds germinating are interpreted as risk reappraisal by seeds through dormancy in response to potentially hazardous conditions imposed by high density clusters of seeds all germinating at once. The four species that responded significantly to high densities individually were each treated at low densities with a leachate solution obtained from high density conspecifics. For Alysicarpus rugosus and Panicum miliaceum, this resulted in a significant decrease in the proportion of seeds germinating at simulated high densities implicating the leachate as a causative agent. Heterospecific effects were investigated similarly for A. rugosus and E. curvula by the addition of leachate from high density clusters of seeds of one species upon the other. Only A. rugosus decreased germination significantly through the addition of leachate. These results demonstrate the ability of seeds to predict environmental conditions of the habitat into which they will emerge in terms of potential competitive interactions from neighbouring seedlings.     

The density of apical cells of dark-grown protonemata of the mossCeratodon purpureus

Summary Determinations of plant or algal cell density (cell mass divided by volume) have rarely accounted for the extracellular matrix or shrinkage during isolation. Three techniques were used to indirectly estimate the density of intact apical cells from protonemata of the mossCeratodon purpureus. First, the volume fraction of each cell component was determined by stereology, and published values for component density were used to extrapolate to the entire cell. Second, protonemal tips were immersed in bovine serum albumin solutions of different densities, and then the equilibrium density was corrected for the mass of the cell wall. Third, apical cell protoplasts were centrifuged in low-osmolarity gradients, and values were corrected for shrinkage during protoplast isolation. Values from centrifugation (1.004 to 1.015 g/cm³) were considerably lower than from other methods (1.046 to 1.085 g/cm³). This work appears to provide the first corrected estimates of the density of any plant cell. It also documents a method for the isolation of protoplasts specifically from apical cells of protonemal filaments.Abbreviations BSA bovine serum albumin - ER endoplasmic reticulum - V_v volume fraction     

Stimulation of aquatic bacterial activity by cyanobacteria

The time-course response of natural bacterial populations and isolates from lake water to various densities of the filamentous cyanobacteriaAphanizomenon flos-aquae andLyngbya birgei collected from the same lake is reported. The cyanobacteria were separated from the bacteria by dialysis membranes that allowed only dissolved cyanobacterial products to pass. Bacterial³H-thymidine incorporation and cell number were significantly (p<0.05) correlated with cyanobacterial density for both species. Estimated dissolved organic carbon (DOC) utilization, based on bacterial biomass changes over time, were usually significantly (p<0.01) correlated with cyanobacterial density and the decrease in bulk pool DOC for both species. Bacterial volume per cell increased significantly (p<0.05) in response to cyanobacterial density on day 5 of the experiments; cell volume remained unchanged on day 1. Bacterial cell numbers on outer surfaces of the tubular membrane containing the cyanobacteria (on the side exposed to the test bacteria) were significantly (p<0.01) correlated with cyanobacterial density. Statistical analysis inferred that bacteria closely associated with cyanobacteria (i.e. attached) responded more strongly to cyanobacterial products than free-living bacteria. Overall, our results indicate that cyanobacterial products have a potentially important role in regulating bacterioplankton productivity in aquatic systems.     

PHYSIOLOGICAL CHARACTERISTICS OF SPIRULINA PLATENSIS (CYANOBACTERIA) CULTURED AT ULTRAHIGH CELL DENSITIES1

Photosynthetic activity and growth physiology of Spirulina platensis (Nordstedt) Geitler cultures maintained at ultrahigh cell densities (i.e. above 100 mg chlorophyll-L^?1) in a newly designed photobioreactor were investigated. Nitrogen (NaNO₃) in standard Zarouk medium was characterized as a major nutrient-limiting factor in such cultures. The effect of ultrahigh cell density on photoinhibition of photosynthesis, as reflected by chlorophyll fluorescence and photosynthetic oxygen evolution, was studied: elevating the population density may arrest photoinhibition induced by high photon flux density, as well as low temperature. The relationship between incident irradiance and oxygen production rate was linear in situ for cultures at the optimal cell density, indicating that light limitation rather than light saturation or photoinhibition is the dominant condition outdoors in cultures of ultrahigh cell densities. In contrast with other reports, the extent of biomass loss at night due mainly to dark respiration was found to be relatively small when cell density was optimal, exerting only a minor effect on overall net productivity. Measurements of oxygen consumption at night revealed low rates of respiration, which may be explained by the low value of the volumetric mass transfer coefficient (K_La) of oxygen. Hence, reduced oxygen tension may play a role in preventing full expression of the respiratory potential in ultrahigh cell density cultures in which photoadaptive strategy may explain cell composition. Ultrahigh cell densities optimized with respect to the intensity of the light source, the length of the light path, and the extent of stirring represent the key for obtaining high output rates of cell mass and some natural products.     

IMPORTANCE OF PHYSICAL VARIABLES ON THE SEASONAL DYNAMICS OF EPILITHIC ALGAE IN A HIGHLY SHADED CANYON STREAM1

The seasonal abundance of epilithic algae was correlated with major physico-chemical parameters in a first-order, heavily shaded stream in northern Arizona. Diatoms made up over 85%, by numerical abundance, of the epilithon community Light energy, water temperature, and stream discharge were most highly correlated with seasonal abundance of epilithic diatom taxa when analyzed with stepwise multiple regression. None of the chemical variables measured in the study (NO₃-N, O-PO₄, SiO₂, including PH) was found to be significantly correlated with the seasonal community structure of epilithic diatoms. Total diatom cell densities showed a significant negative correlation to stream bed light energy. Likewise, total diatom cell densities along a transect in the stream bed showed a negative correlation to current velocity during those months when base flow was low and stable, and current velocity was ≤25 cm·sec-¹. Most diatom taxa had highest cell densities at temperatures < 16°C and at daily mean stream bed light levels < 400 μE·m^?2·s^?1. Highest cell densities of green algae occurred at temperatures between 6–16°C and at daily mean stream bed light levels of > 400 μE·m^?2·s^?1. Blue-green algae (cyanobacteria) grew best at the highest recorded water temperatures and daily mean stream bed light energy (16–20°C and 900–1200 μE·m^?2·s^?1). Abrupt increases in NO₃-N coincided with a brief pulse of Nostoc pruniforme colonies during June, and leaf drop from Alnus oblongifolia during October.     

Effects of competition,herbivory and substrate disturbance on growth and size structure in pin cherry (Prunus pensylvanica L.) seedlings

We examined separate and interactive effects of intraspecific competition, vertebrate browsing and substrate disturbance on the growth and size structure of pin cherry (Prunus pensylvanica L.) in the first two seasons of growth after clearcutting, in a hardwoods forest in New Hampshire, United States. Over the 15-month study period, 97.5% of 1801 individuals survived, and mean plant height increased from 4-fold at high density to 5-fold at low density. Relative height growth was significantly lower at higher plant densities in two of the three growth periods examined. Vertebrate browsers (moose and deer) significantly preferred taller plants. Browsed plants had higher relative height growth following browsing than unbrowsed plants (compensatory growth) at low and intermediate densities. The degree of compensation declined with density and compensation was not significant at the highest density level. At low and intermediate densities, plants browsed early in life regained height dominance through compensatory growth; they failed to regain dominance at high density. Because compensatory growth tended to offset the effects of size-selective browsing, there was no difference in the degree of size inequality between browsed and unbrowsed plots. However, size inequality increased with plant density. Substrate disturbance caused by logging had no significant effects on either relative height growth or size inequality. The slope of the relationship between relative height growth and initial height increased significantly with density and time, and was higher in unbrowsed than in browsed plots, suggesting that competition among plants was size-asymmetric. Despite the preference of browsers for large plants, there was a clear net growth advantage for plants of large initial size, when the effects of competition, browsing and compensatory growth were combined. The interactive effects of density and browsing demonstrate the importance of a multifactorial approach to the analysis of individual plant performance and population structure.     

Differences in Turnover Rates of Vinculin and Talin Caused by Viral Transformation and Cell Density

Vinculin and talin are two major components of focal contacts which interact with each other. In order to understand how the relative levels of these proteins are maintained under various conditions, the synthesis rates and half-lives of vinculin and talin in chick embryonic fibroblasts were determined by autoradiography combined with immunoblotting. High cell density and transformation by Rous sarcoma virus decreased the vinculin synthesis rate by 40%. Upon viral transformation, the synthesis rate of talin decreased by 30%. In contrast to vinculin, the synthesis rate of talin was not affected by cell density. The effect of cell density on the synthesis rate of vinculin was retained after viral transformation, suggesting that cell density and viral transformation affect vinculin synthesis by two independent mechanisms. The synthesis rate of vinculin was approximately two to three times greater than that of talin under all conditions tested. The half-lives of vinculin and talin remained constant at different cell densities in untransformed cells (t_1/2= 18–21 h), but transformation slightly decreased half-lives of both proteins (t_1/2= 16–18 h). These results suggest that the decreased expression of vinculin and talin in transformed chick fibroblasts can be attributed mainly to changes in their biosynthesis rates rather than degradation. This may contribute to a decrease in the number of focal contacts in transformed cells.     

Intraspecific competition of Glyptotendipes paripes (Diptera: Chironomidae) larvae under laboratory conditions

Glyptotendipes paripes larvae were reared in wells of tissue culture plates, in groups of 2, 4, 8, 16, and 32 (representing densities of about 1,300, 2,600, 5,200, 10,400, and 20,800 larvae per m², respectively). Larval groups were supplied with one of two concentrations (low or high) of food and larvae were individually observed to evaluate the effects of density on mortality, growth, development, behavior, and adult body size. Increased larval densities resulted in higher mortality, as well as slower larval growth and development. The distribution of developmental time became flatter at higher density, with a wider range of values, or even became bimodal. This was a consequence of the most rapidly developing individuals at higher densities emerging as adults sooner than the fastest developing individuals at lower densities, although overall mean developmental time was longer at higher densities. At higher densities, growth and development of smaller larvae were slowed, based on the relative difference in body length between competitors. When larger competitors emerged as adults or died, the growth of smaller larvae may have accelerated, resulting in increased variability of developmental times. The effect of larval density on adult body size was complex, with the largest body size found at the lowest density and a second peak of adult size at high-middle densities, with smaller adult body sizes found at low-middle, and high densities. Similarly, as with developmental time, the range of body size increased with increasing density. Examined food concentrations had no effect on larval mortality, but significantly affected developmental time, growth rate, and adult body size. At higher densities, larvae spent more time gathering food and were engaged in aggressive or antagonistic behaviors.