Factors affecting levels of genetic diversity in natural populations.

Genetic variability is the clay of evolution, providing the base material on which adaptation and speciation depend. It is often assumed that most interspecific differences in variability are due primarily to population size effects, with bottlenecked populations carrying less variability than those of stable size. However, we show that population bottlenecks are unlikely to be the only factor, even in classic case studies such as the northern elephant seal and the cheetah, where genetic polymorphism is virtually absent. Instead, we suggest that the low levels of variability observed in endangered populations are more likely to result from a combination of publication biases, which tend to inflate the level of variability which is considered ''normal'', and inbreeding effects, which may hasten loss of variability due to drift. To account for species with large population sizes but low variability we advance three hypotheses. First, it is known that certain metapopulation structures can result in effective population sizes far below the census size. Second, there is increasing evidence that heterozygous sites mutate more frequently than equivalent homozygous sites, plausibly because mismatch repair between homologous chromosomes during meiosis provides extra opportunities to mutate. Such a mechanism would undermine the simple relationship between heterozygosity and effective population size. Third, the fact that related species that differ greatly in variability implies that large amounts of variability can be gained or lost rapidly. We argue that such cases are best explained by rapid loss through a genome-wide selective sweep, and suggest a mechanism by which this could come about, based on forced changes to a control gene inducing coevolution in the genes it controls. Our model, based on meiotic drive in mammals, but easily extended to other systems, would tend to facilitate population isolation by generating molecular incompatabilities. Circumstances can even be envisioned in which the process could provide intrinsic impetus to speciation.     

The frequency distribution of Hymenolepis diminuta cysticercoids in natural,sympatric populations of Tenebrio molitor and T. obscurus

Rau M. E. 1979. The frequency distribution of Hymenolepis diminuta cysticercoids in natural, sympatric populations of Tenebrio molitor and T. obscurus. International Journal for Parasitology9: 85–87. Natural, sympatric populations of Tenebrio molitor and T. obscurus were examined for cysticercoids of Hymemlepis diminuta. The distribution of cysticeroids in both species and both sexes conformed to the negative binomial. Cysticeroids were more prevalent and the mean intensity of infection was higher in T. obscurus than in T. molitor. No differences in the intensity of infection were detected between the sexes. Larvae of both beetle species were always very lightly infected. The significance of these factors in the transmission of the infection to the rat definitive host is discussed.     

Factors affecting tactile spatial acuity

Tactile spatial acuity on the fingerpad was measured using a grating orientation task. In this task, subjects are required to identify the orientation of square-wave gratings placed on the skin. Previous studies have shown that performance varies as a function of the width of the grooves in the gratings. In the present study, both groove width and the overall size and configuration of the contactors were varied. Sensitivity improved with wider grooves and with larger contactors. Additional measurements showed that the improved sensitivity is not the result of the increase in total area contacted, but rather is due to two other factors associated with larger contactors. One is the greater linear extent of the larger contactors. The other appears to be due to the reduction in the interference produced by the outer edge of the contactor. Specifically, as the contactor increases in size, the distance between the outer edge and the center portion of the grooves also increases. It was also shown that subjects are more sensitive to a single, continuous groove as compared with two grooves of the same total length but spatially discontinuous. Similarly, subjects are more sensitive to a contactor with a continuous groove than to a contactor in which just the end points of the groove are presented. The results are generally consistent with the results of peripheral, neurophysiological recordings. The results are discussed in terms of the way in which both spatial and intensive factors may affect sensitivity to grating orientation.     

Lateral asymmetry of eye use in Octopus vulgaris

The lateralization of sensory and motor functions has been recently demonstrated in various groups of vertebrates. We examined lateral asymmetry of eye use in Octopus vulgaris by behavioural methods. Octopus vulgaris uses monocular vision almost exclusively and can move its eyes independently. The amount of binocular vision is small because the eyes are on the sides of the head. We tested eight octopuses in two conditions (one with and one without moving stimuli) where the use of the eye for frontal vision could be determined unequivocally. Data were recorded on videotape. All animals showed a preference for one eye (five left, three right). There was no correlation between eye use and the animal's direction of movement. Pigmentation of the ventral side of the arms tended to be most intense on the side of the preferred eye and the body was most pigmented on the side of the eye currently in use. We found no sex differences for visual lateralization. Pigmentation of the ventral side of the arms was lighter in females than in males. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

The prevalence and spatial distribution of viruses in natural populations of Brassica oleracea

We report a survey of four viruses (beet western yellows luteovirus (BWYV), cauliflower mosaic caulimovirus (CaMV), turnip mosaic potyvirus (TuMV), turnip yellow mosaic tymovirus (TYMV)) in five natural populations of Brassica oleracea in Dorset (UK). All four viruses were common; 43% of plants were infected with BWYV, 60% with CaMV, 43% with TuMV and 18% with TYMV. For each virus there were significant differences in the proportion of infected plants among populations, which were not completely explained by differences in the age of plants. Multiple virus infections were prevalent, with 54% of plants having two or more virus types. There were statistically significant associations between pairs of viruses. The CaMV was positively associated with the other three viruses, and BWYV was also positively associated with TuMV. There was no detectable association between BWYV and TYMV, whereas TuMV and TYMV were negatively associated. We suggest these associations result from BWYV, CaMV and TuMV having aphid vectors in common, as aphids are attracted to plants that already have a virus infection. Infected plants were distributed randomly or were very weakly aggregated within populations. The implications of widespread multiple virus infections in natural plant populations are discussed with respect to the release of transgenic plants expressing virus-derived genes.     

Eucomis zambesiaca baker: Factors affecting in vitro bulblet induction

Eucomis species having considerable horticultural potential are used in African traditional medicine to treat various ailments. The effects of environmental and physiological parameters on the initiation and growth of bulblets using leaf explants were investigated. These included the effect of temperature (10, 15, 20, 25 and 30 °C), photoperiod (8 h light, 16 h light, continuous light and continuous dark), carbohydrates (sucrose, fructose and glucose) at different concentrations and combinations as well as various plant growth regulators; gibberellic acid (GA₃), indole-3-butyric acid (IBA), napthaleneacetic acid (NAA), N⁶-benzyladenine (BA), zeatin and others. Liquid shake and liquid static cultures versus solid cultures were investigated. Maximum number of bulblets per leaf explant was obtained at 20 °C, with an average of 3 bulbs per leaf explants and a bulblet mass of 57 mg. An 8 h light cycle produced 1.38 bulbs per leaf explant, at a mass of 42 mg. Fructose at 3% produced an average of 1.18 bulbs per leaf explant, 3.39 mm wide and weighing 56.6 mg. Of the plant growth regulators, 4.90 µM IBA was found to be the optimum treatment for bulblet induction, with an average bulb diameter of 4.36 mm and a mean bulblet mass of 79.07 mg. Liquid shake cultures exhibited poor growth while bulblet, leaf and root growth was improved in liquid static cultures. Successful micropropagation from leaf explants established that leaf explants can be used as an alternative explant source to bulbs. This protocol allows for the fast and economic mass propagation of Eucomis plants.     

Factors affecting the occurrence and distribution of entomopathogenic fungi in natural and cultivated soils

Factors affecting the occurrence and distribution of entomopathogenic fungi in 244 soil samples collected from natural and cultivated areas in Spain were studied using an integrated approach based on univariate and multivariate analyses. Entomopathogenic fungi were isolated from 175 of the 244 (71.7 %) soil samples, with only two species found, Beauveria bassiana and Metarhizium anisopliae. Of the 244 soil samples, 104 yielded B. bassiana (42.6 %), 18 yielded M. anisopliae (7.3 %), and 53 soil samples (21.7 %) harboured both fungi. Log-linear models indicated no significant effect of habitat on the occurrence of B. bassiana, but a strong association between M. anisopliae and soils from cultivated habitats, particularly field crops. Also, irrespective of habitat type, B. bassiana predominated over M. anisopliae in soils with a higher clay content, higher pH, and lower organic matter content. Logistic regression analyses showed that pH and clay content were predictive variables for the occurrence of B. bassiana, whereas organic matter content was the predictive variable for M. anisopliae. Also, latitude and longitude predicted the occurrence of these same species, but in opposite directions. Altitude was found to be predictive for the occurrence of B. bassiana. Using principal component analysis, four factors (1 to 4) accounted for 86 % of the total variance; 32.8, 22.9, 19.6 and 10.4 % of the cumulative variance explained, respectively. Factor 1 was associated with high positive weights for soil clay and silt content and high negative weights for soil sand content. Factor 2 was associated with high positive weights for soil organic matter content and high negative weights for soil pH. Factor 3 was associated with high positive weights for latitude and longitude of the sampled localities and factor 4, had high positive weights only for the altitude. Bi-plot displays representing soil samples were developed for different factor combinations and indicated that, irrespective of geographical location, absence of both fungal species was determined by alkaline sandy soils with low organic matter content, whereas heaviness of soil texture, acidity and increasing organic matter content led to progressively higher percentages of samples harbouring entomopathogenic fungi. These results could aid decision-making as to whether or not a particular cultivated or natural soil is suitable for using entomopathogenic fungi as a pest control measure and for selecting the fungal species best suited to a particular soil.     

Factors affecting the distribution of dorcas gazelle

The dorcas gazelle Gazella dorcas was very common and widespread in Tunisia. Nowadays, only some small isolated populations still survive in the desert areas of the southern part of the country. Factors affecting the distribution of this species in Tunisia have never been investigated despite the importance of such investigations for elaborating long-term conservation plans for the remaining wild populations. Using data on gazelle occurrence and on a set of habitat and human variables collected in south-eastern Tunisia, we aimed to identify the factors affecting gazelle distribution in this area. In particular, we investigated the relevance of habitat versus human factors for gazelle occurrence probability. As predicted, we found that human variables were the most relevant factors shaping the distribution patterns of gazelles in the studied area. Gazelles tended to avoid areas where agricultural development has occurred but did not seem to be disturbed by livestock. Overall, our results suggest that the occurrence probability of dorcas gazelle in southern Tunisia was mainly dependent on human presence and land use, rather than habitat characteristics. The recent intensification of agriculture in the more remote areas of southern Tunisia may thus constitute a serious threat to the conservation of this endangered species.     

Assessing the validity of stylets as ageing tools in Octopus pallidus

A method to age octopus was determined through the quantification of growth increments within the stylet structure (highly reduced internal shells). To validate the periodicity of the increments, stylets were analysed from known-age laboratory-raised Octopus pallidus. The animals, which ranged from 3 to 8 months old, were exposed either to a simulated natural or to constant temperature regime. Transverse stylet sections were embedded in thermo-plastic cement and ground until a thin section was achieved. A pre-hatch region and first post-hatch increment was identified in the stylet microstructure. The number of increments, across all ages and temperature treatments, was in remarkably close agreement to age (number of days), clearly demonstrating that stylet increments are deposited daily throughout the lifecycle of O. pallidus. Morphometric analyses of the stylet indicated that increments were laid down regularly during stylet growth and that stylets grew in concert with body size, further supporting daily periodicity. This study successfully validates daily increments in stylets and thus demonstrate a method to age octopus, therefore, potentially having critical implications for future octopus research and the effective management of stocks worldwide.     

Factors affecting tissue distribution of heavy metals

Metal distribution in an organism at any one time is a resultant of many extrinsic and intrinsic factors. Improved analytic methodology has made it feasible to obtain highly sensitive determinations for many metals in a single sample. Thus, it is now feasible to examine patterns of metal distribution. We report on a study of the effect of age on metal patterns in the common tern,Sterna hirundo, a fisheating seabird. We contrast the levels of nine metals in the liver of adult and young terns and compared these with levels in tern eggs. Unlike many previous studies, adults did not have significantly higher levels of metals, although for most metals, levels in eggs were significantly lower than liver levels of young and adults. The intermetal correlations showed more significant positive correlations for adults and eggs than for chicks, the latter showing instead a correlational chaos, probably reflecting the immaturity of their physiologic defense mechanisms and the absence of dynamic equilibrium.     

Factors affecting horizontal transmission of the microsporidium Amblyospora albifasciati to its intermediate copepod host Mesocyclops annulatus

Factors that directly impact horizontal transmission of the microsporidium Amblyospora albifasciati to its intermediate copepod host, Mesocyclops annulatus were examined in laboratory bioassays. Results were evaluated in relation to life history strategies that facilitate persistence of the parasite in natural populations of its definitive mosquito host, Ochlerotatusalbifasciatus. A moderately high quantity of meiospores from mosquito larvae was required to infect adult female copepods; the IC50 was estimated at 3.6 × 10⁴ meiospores/ml. Meiospore infectivity following storage at 25 °C was detected up to 30 days, while meiospores stored at 4 °C remained infectious to copepods for 17 months with virtually no decline in infectivity. Uninfected female M. annulatus are long-lived; no appreciable mortality was observed in field-collected individuals for 26 days, with a few individuals surviving up to 70 days. The pathological impact of A. albifasciati infection on M. annulatus resulted in a 30% reduction in survivorship after 7 days followed by gradual progressive mortality with no infected individuals surviving more than 40 days. This moderate level of pathogenicity allows for a steady continual release of spores into the environment where they may be ingested by mosquito larvae. Infected female copepods survived in sediment under conditions of desiccation up to 30 days, thus demonstrating their capacity to function as a link for maintaining A. albifasciati between mosquito generations following periods of desiccation. The susceptibility of late stage copepodid M. annulatus to meiospores of A. albifasciati and subsequent transstadial transmission of infection to adult females was established.     

Factors affecting the molecular weight distribution of liver glycogen

The molecular weight distribution of mouse liver glycogen under different conditions has been investigated. Factors which diminish glycogen content, such as fasting, epinephrine, glucagon, insulin, and ischemia, were found to produce only a slight decrease in molecular weight.After glucose administration, the molecular weight distribution remained nearly unchanged although glycogen content increased considerably.Glycogen synthesized by different metabolic pathways, as after administration of different monosaccharides or of hydrocortisone, could not be distinguished as judged by their molecular weight. No change was observed after injection of growth hormone.The possible mechanisms by which the molecular weight distribution is maintained nearly constant are discussed.     

Home occupancy by male Octopus vulgaris in a large seawater tank

Pairs of individually recognizable male Octopus vulgaris were observed in a large seawater tank containing two suitable homes (brick pots or plastic buckets). None of the animals established exclusive occupancy of one home and for much of the time both animals were associated together at the same site. Usually one of the two homes was preferred and its occupant was most likely to be the larger animal, or the earlier resident if they were of equal size. Large animals were observed to take food forcefully from smaller octopus. An arm alignment interaction is described which, it is suggested, may be a means by which two octopuses establish their relative sizes.     

Incidence of Entomophthora sp. and other natural control agents in populations of Pseudoplusia includens and Trichoplusia ni

A species of Entomophthora was observed as a mortality factor in populations of Pseudoplusia includens and Trichoplusia ni in cotton and soybean fields throughout Alabama. Resting spores appear identical to those of Tarichium gammae. The incidence of this fungus in host populations, as well as other disease agents and parasites, was determined from larval samples collected throughout the State. Entomophthora infected 8% of all loopers collected on cotton as compared to 35% on soybeans. T. ni nuclear polyhedrosis virus infected 41% of all loopers collected from cotton but no mortality attributable to virus was found in loopers collected on soybeans, which were predominantly P. includens.Studies in 1969 and 1970 on the incidence of Entomophthora sp. in a population of P. includens infesting soybeans in central Alabama have shown it to infect between 50 and 95% of all individuals present from early August through mid-September, the period of densest abundance of the caterpillars. Spicaria sp. was also present during August and September but did not contribute significantly to reduction in population levels until late in the season. Parasites accounted for less than 1% of all mortality in each year and predator populations were very small during the periods of observation.     

Factors affecting the thermal behavior of the sand lizard Liolaemus wiegmannii in natural and modified grasslands of temperate coastal dunes from Argentina

The present study describes and compares the thermal behavior of individuals of Liolaemus wiegmannii, inhabiting patches of natural grassland and modified patches with exotic trees of Acacia longifolia, by analyzing the factors affecting its behavior. Thermal behavior of L. wiegmannii was assessed by radiotracking 22 adult individuals in a coastal dune area from Argentina. In order to account for individual responses we analyzed the factors affecting thermal behavior using Generalized Linear Mixed Models. Thermal behavior of L. wiegmannii was mainly affected by the time of day and the substrate temperature. The individuals basking on bare sand in the morning shifted to filtered sunlight during midday and to the shade of clump and erect stems and herbs during the afternoon. The individuals buried in sand when the substrate temperature was low and the wind speed was high. The use of sub-shrubs and shrubs could not be explained solely by thermal factors. In natural L. wiegmannii regulated its body temperature by shuttling between microhabitats that provided them with different levels of exposure to sunlight during the day. However, in modified patches individuals varied in the choice of plants used as shaded places to mitigate high microenvironmental temperatures and they were also more exposed to full shaded sites and less to warm sand to bask, which could have negative consequences for its thermal biology.     

Factors affecting the amount and the activity of the glutamate dehydrogenases of coprinus cinereus

Kinetic analyses done with cell-free extracts of this basidiomycete fungus showed that the NADP-linked glutamate dehydrogenase exhibited positively co-operative interactions with the substrates 2-oxoglutarate and NADPH, negatively co-operative kinetics with NADP⁺ and was extremely sensitive to inhibition of deamination activity by ammonium and/or ammonia. The NAD-linked enzyme showed positive co-operativity with NADH, Michaelis-Menten kinetics with all other substrates and was subject only to mild inhibitions by the reaction products. Considered together with the values of the Michaelis constants, these results indicate that the former enzyme is primarily concerned with the amination of 2-oxoglutarate when the concentration of this substrate exceeds about 4 mM, while the NAD-linked enzyme is able to aminate or deaminate as metabolic conditions require. Synthesis of both enzymes was repressed by addition of carbamyl phosphate or N-acetyl-glutamate to mycelial cultures growing in media containing glucose and ammonium as carbon and nitrogen sources. Growth in media containing urea results in repression of the NADP-linked glutamate dehydrogenase and depression of the NAD-linked enzyme. Such results indicate a connexion between the glutamate dehydrogenases and the urea cycle. It is suggested that under normal conditions of growth on complex media nitrogen is assimilated in the form of amino acids and that the glutamate dehydrogenases act in support of transminases to allow this process to continue, and in support of the urea cycle to allow the disposal of excess nitrogen.     

Factors affecting the natural occurrence of entomopathogenic nematodes in turfgrass

We conducted a study to determine the relationship between turfgrass management intensity and natural occurrence of entomopathogenic nematodes (EPNs). We surveyed for EPNs on putting greens, fairways, and rough areas—three distinct surface types on golf courses that are managed with different intensities. We collected 159 soil samples from putting greens, contiguous fairways, and rough areas from 19 golf courses in Ohio, USA. Nematodes were recovered from soil samples using the insect baiting technique. We also analyzed the soil samples for texture, organic matter, pH, phosphorus, calcium, magnesium and potassium. We used principal components analysis and Pearson correlation to determine the relationship between nematode occurrence and the measured parameters. Surface type was the most important factor in predicting EPNs occurrence. Putting greens differed significantly from fairways and rough areas in the number of EPN-positive sites. No EPNs were recovered from putting greens but were recovered from 43% of the fairways and 57% of the rough areas. Putting greens also differed significantly from fairways and rough areas in organic matter, pH, calcium, and phosphorus. The fairways and rough areas did not however differ in number of EPN-positive sites and measured soil parameters. Presence of EPNs correlated significantly, although weakly, with sand, silt, phosphorus, organic matter, and magnesium content, but not with clay, pH, calcium, and potassium. Nematode isolates were identified as Heterorhabditis bacteriophora, Steinernema carpocapsae, and S. glaseri. This suggests that EPNs are more likely to occur in less intensively managed sites that receive fewer inputs and have relatively high sand, and moderate silt, organic matter, phosphorus, and magnesium content.     

Factors affecting chilled storage of zebrafish (Danio rerio) embryos

Chilled storage of zebrafish embryos was investigated at a temperature that arrests embryonic development as this technique might offer interesting practical applications. Five parameters played an important role for chilled storage: (a) storage temperature, (b) development stage of embryos, (c) storage solution (extender), (d) postchilling treatment, and (e) inhibition of growth of microorganisms by antibiotics. The optimal chilling temperature was 8 °C. Prim-5 stage (24 h postfertilization [hpf]) and prim-25 stage (36 hpf) embryos had similar high chilling resistance and could be chilled for 33 h without a loss in viability. Five-somite stage (12 hpf) embryos had a lower chilling resistance and could be chilled only for 14 h without a loss in viability. After longer incubation periods, the viability started to decrease. Under these conditions, chilling in physiologic saline solutions was superior to that in water. Fifty percent of the prim-5 stage and prim-25 stage embryos survived for 41 h at 8 °C in water but for 46 h in physiologic saline solution. A similar effect was observed for 5-somite stage embryos (50% survival rate in water, 28 h; 50% survival rate in physiologic saline solution, 35 h). When embryos were incubated in physiologic saline solution instead of water in the postchilling phase, the embryo viability was positively affected, too. Also, supplementation of the storage solution with antibiotics (penicillin and streptomycin) increased the viability of chilled embryos. In summary, the current study shows that chilled storage of zebrafish embryos is possible for sufficiently long periods to synchronize the development of embryos deriving from different spawning dates or to delay the development for experimental purposes. To prolong the storage periods, further development and standardization of the methodology is necessary.