Peripheral variability and central constancy in mammalian visual system evolution

Neural systems are necessarily the adaptive products of natural selection, but a neural system, dedicated to any particular function in a complex brain, may be composed of components that covary with functionally unrelated systems, owing to constraints beyond immediate functional requirements. Some studies support a modular or mosaic organization of the brain, whereas others emphasize coordination and covariation. To contrast these views, we have analysed the retina, striate cortex (V1) and extrastriate cortex (V2, V3, MT, etc.) in 30 mammals, examining the area of the neocortex and individual neocortical areas and the relative numbers of rods and cones. Controlling for brain size and species relatedness, the sizes of visual cortical areas (striate, extrastriate) within the brains of nocturnal and diurnal mammals are not statistically different from one another. The relative sizes of all cortical areas, visual, somatosensory and auditory, are best predicted by the total size of the neocortex. In the sensory periphery, the retina is clearly specialized for niche. New data on rod and cone numbers in various New World primates confirm that rod and cone complements of the retina vary substantially between nocturnal and diurnal species. Although peripheral specializations or receptor surfaces may be highly susceptible to niche-specific selection pressures, the areal divisions of the cerebral cortex are considerably more conservative.     

On the effect of scene motion on color constancy

A series of experiments with human subjects have shown that color constancy improves when an object moves. It has been hypothesized that this effect is due to some kind of influence of high-level motion processing. We have built a computational model for color perception which replicates the results qualitatively which have been obtained with human subjects. We show that input from high-level motion processing is not required. In our model, the dependence is an effect of eye movement in combination with neural processing. Depending on the type of stimulus used, the eye either tracks the object or the background. When the object moves but is tracked by the observer, the background appears to move when considering the stimulus with respect to eye coordinates. Hence, the retinal input is different for the two conditions leading to a difference in color constancy performance.     

Interhemisphere asymmetry and the constancy of visual perception of large and small sizes

The constancy of perception of linear 3–40-cm sizes at a distance of 0.7 to 5.6 m was studied. The results of investigations carried out on 66 subjects are presented. Sizes less than 10 cm and more than 10 cm are perceived according to different laws. Large sizes, as they are moved away, are assessed according to two modes of perception: aconstantly (with a diminution of the size value) and constantly (with the preservation of the apparent size value). The sizes not exceeding 10 cm are reassessed with an increase in the distance to the extent that reevaluation of the small size value attains 40%. It is evident that only one modus of perception— hyperconstancy—exists for sizes less than 10 cm.     

Sexual selection and its effect on the fixation of an asexual clone

Sexual selection is a powerful and ubiquitous force in sexual populations. It has recently been argued that sexual selection can eliminate the twofold cost of sex even with low genomic mutation rates. By means of differential male mating success, deleterious mutations in males become more deleterious than in females, and it has been shown that sexual selection can drastically reduce the mutational load in a sexual population, with or without any form of epistasis. However, any mechanism that claims to maintain sexual reproduction must be able to prevent the fixation of an asexual mutant clone with a twofold fitness advantage. Here, I show that despite very strong sexual selection, the fixation of an asexual mutant cannot be prevented under reasonable genomic mutation rates. Sexual selection can have a strong effect on the average mutational load in a sexual population, but as it cannot prevent the fixation of an asexual mutant, it is unlikely to play a key role on the maintenance of sexual reproduction.     

The effect of natural selection on enzymic catalysis.

The karyotype of Drosophila nasutoides reveals a very large autosome pair at the metaphase plate. The application of the C-banding technique shows that this chromosome is almost entirely heterochromatic and an isochromosome (Cordeiro et al., 1975). Examination of the DNA isolated from purified nuclei of D. nasutoides in neutral CsCl gradients reveals four major satellites. As much as 60% of the total DNA appears as satellites in the DNA from larval brains. The buoyant densities of the four satellites, designated as I through IV in the order of descending density, are 1.687, 1.682, 1.669 and 1.665 g/cm³, respectively. All four satellites show strand separations in alkaline CsCl gradients with the least separation in satellite III. Thermal denaturation studies with purified native satellites show that satellites I and IV consist of repeats of identical sequences, whereas satellites II and III show a large sequence variation between repeating units. As much as 10 to 24% base-pair mis-matching is observed in the reassociated satellite II. The sequence complexities obtained from DNA reassociation kinetics data are 5, 103, 2.3 × 10⁶ and 46 nucleotide pairs for the satellites I, II, III and IV, respectively. The complexity of satellite III is almost as large as that of Escherichia coli, when the reassociation rate is corrected according to the amount of mis-matching in this satellite. All four satellite sequences are localized in one chromosome (dot chromosome) according to in situ hybridizations to polytene chromosomes. The large heterochromatic chromosome seen at the metaphase plate appears as the dot chromosome after polytenization. Therefore, the large heterochromatic chromosome contains all four satellite DNA components.     

Graves’ disease: Hereditary peculiarities and effect of selection on its prevalence population

The features of genetic determination and natural selection of diffuse toxic goiter (DTG) are studied on the basis of the clinical and genealogical data obtained from 242 patients with thyrotoxicosis and from obstetric anamneses of 2105 healthy women and 369 women with thyrotoxicosis. On the basis of the results of genetic analysis, the conformity of thyrotoxicosis inheritance to the alternative model parameters is revealed. In this model, the penetrance of homozygotes was 78.8%, and the penetrance of heterozygotes was 17.3%. It is shown that, in the population of Kharkiv oblast, the frequency of the gene of predisposition to thyrotoxicosis has grown 0.8% per generation.     

Temporal change in mite abundance and its effect on barn swallow reproduction and sexual selection

The tropical fowl mite Ornithonyssus bursa parasitizing barn swallows Hirundo rustica in a Danish population demonstrated a dramatic change in abundance during 1982–2000. Prevalence of mites in nests showed a decrease from 66% in 1987 to a minimum of 1% in 1999. Two parasite manipulation experiments of barn swallow nests in 1988 and 1999 revealed a strong effect of parasites on host reproductive success in the first year (with an average reduction in seasonal reproductive success of 30% when 50 mites were added to nests as compared with controls), but only a weak effect the last year. This pattern was paralleled by a positive relationship between reduction in host reproductive success between egg laying and fledging and mite prevalence during different years of the study period. Mite abundance on adult hosts was negatively related to tail length of males (a secondary sexual character) at the peak of mite abundance in 1988, while that relationship became weaker in the beginning of the 1990s and disappeared at the end of the 1990s. Assortative mating with respect to mite loads in the 1980s also disappeared in the 1990s. Mean tail length of male hosts increased by 1.3 standard deviations during the study period because of changes in phenotype‐dependent patterns of mortality outside the breeding season. This microevolutionary change in mean male phenotype of the host was accompanied by a correlated response in mite abundance. These results are consistent with the hypothesis that the host has become more resistant to the mite during the study period, and that the mite from earlier playing an important role in natural and sexual selection of the host now is of little importance.     

Localizing cortical computations during visual selection

Local field potentials (LFPs) and spikes are two signals that can be recorded from the brain using extracellular microelectrodes. A study by Monosov et al. in this issue of Neuron using timing relations between these signals suggests that selection of a target from an array of distractors is a computational operation performed specifically and locally in the frontal eye field (FEF).     

阿维菌素对棉铃虫的抗性选育及其对解毒酶活性的影响

为了了解棉铃虫对阿维菌素的抗药性机理,用阿维菌素对棉铃虫(Heliothis armigera)进行了抗性选育,并对选育的27代棉铃虫解毒酶的活性进行了检测.选育从F0至F21代,抗性缓慢波动上升,达到选育前的121.19倍;F0至F27代抗性迅速增长,达到选育前的821.73倍,抗性发展趋势呈现S型曲线.随着选育代数的增加,对乙酰胆碱酯酶(AChE)没有明显的影响;羧酸酯酶(CarE)活性,F27是F0的2.52倍(α-NA)和2.83倍(β- NA),从20开始,活性在较高水平上波动;谷胱甘肽转移酶(GST)活性,F27是F0的2.12倍,且从F16开始,活性在较高水平上波动.选育的抗性品系,增效醚对阿维菌素增效6.77倍.上述结果说明棉铃虫对abamectin的抗性机制也涉及MFO、GSTs、CarE等多个酶系统,但与乙酰胆碱酯酶(AChE)无关.     

Beneficial mutation selection balance and the effect of linkage on positive selection

When beneficial mutations are rare, they accumulate by a series of selective sweeps. But when they are common, many beneficial mutations will occur before any can fix, so there will be many different mutant lineages in the population concurrently. In an asexual population, these different mutant lineages interfere and not all can fix simultaneously. In addition, further beneficial mutations can accumulate in mutant lineages while these are still a minority of the population. In this article, we analyze the dynamics of such multiple mutations and the interplay between multiple mutations and interference between clones. These result in substantial variation in fitness accumulating within a single asexual population. The amount of variation is determined by a balance between selection, which destroys variation, and beneficial mutations, which create more. The behavior depends in a subtle way on the population parameters: the population size, the beneficial mutation rate, and the distribution of the fitness increments of the potential beneficial mutations. The mutation-selection balance leads to a continually evolving population with a steady-state fitness variation. This variation increases logarithmically with both population size and mutation rate and sets the rate at which the population accumulates beneficial mutations, which thus also grows only logarithmically with population size and mutation rate. These results imply that mutator phenotypes are less effective in larger asexual populations. They also have consequences for the advantages (or disadvantages) of sex via the Fisher-Muller effect; these are discussed briefly.     

Implicit attentional selection of bound visual features

Traditionally, research on visual attention has been focused on the processes involved in conscious, explicit selection of task-relevant sensory input. Recently, however, it has been shown that attending to a specific feature of an object automatically increases neural sensitivity to this feature throughout the visual field. Here we show that directing attention to a specific color of an object results in attentional modulation of the processing of task-irrelevant and not consciously perceived motion signals that are spatiotemporally associated with this color throughout the visual field. Such implicit cross-feature spreading of attention takes place according to the veridical physical associations between the color and motion signals, even under special circumstances when they are perceptually misbound. These results imply that the units of implicit attentional selection are spatiotemporally colocalized feature clusters that are automatically bound throughout the visual field.     

MEDUSA: large scale automatic selection and visual assessment of PCR primer pairs.

MEDUSA is a tool for automatic selection and visual assessment of PCR primer pairs, developed to assist large scale gene expression analysis projects. The system allows specification of constraints of the location and distances between the primers in a pair. For instance, primers in coding, non-coding, exon/intron-spanning regions might be selected. Medusa applies these constraints as a filter to primers predicted by three external programs, and displays the resulting primer pairs graphically in the Blixem (Sonnhammer and Durbin, COMPUT: Appl. Biosci. 10, 301-307, 1994; http://www.cgr.ki.se/cgr/groups/sonnhammer/Blixem.html) viewer. AVAILABILITY: The MEDUSA web server is available at http://www.cgr.ki.se/cgr/MEDUSA. The source code and user information are available at ftp://ftp.cgr.ki.se/pub/prog/medusa.     

二斑叶螨对螺螨酯的抗性选育及其解毒酶活性测定

用螺螨酯对二斑叶螨Tetranychus urticae Koch进行35代抗性选育,抗性指数达到8.34,并在选育过程中,对二斑叶螨解毒酶的活性进行测定。结果表明,随着选育代数的增加,抗性增强,细胞色素P450活性亦显著提高,而羧酸酯酶、谷胱甘肽转移酶活性较敏感品系差异不显著。说明二斑叶螨对螺螨酯抗药性的增强与其体内细胞色素P450活性的增强有关。     

日本柄瘤蚜茧蜂与其寄主豆蚜的相互作用：寄主龄期选择及其对发育的影响

在25℃下研究了日本柄瘤蚜茧蜂Lysiphlebus japonicus Ashmead对寄主豆蚜Aphis craccivora Koch龄期的选择和被寄生豆蚜的龄期对蚜茧蜂发育的影响。结果表明：在混合虫态寄主中,日本柄瘤蚜茧蜂通常选择较小龄期的若蚜寄生,其中2龄若蚜的相对被寄生率最高,为26.4%;其次是1龄若蚜,为20.6%;无翅成蚜与3、4龄有翅若蚜和成蚜的相对被寄生率较低。日本柄瘤蚜茧蜂的寄生延长了豆蚜若蚜的发育,其中1龄若蚜被寄生后,１～３龄历期显著延长;有翅3龄若蚜被寄生后,３、４龄历期明显延长;但无翅和有翅4龄若蚜被寄生后的发育历期均不受影响。各龄若蚜被寄生后羽化的成蚜寿命明显缩短,其中,被寄生的1龄若蚜不能发育至成蚜,其它较早龄期被寄生的若蚜羽化的成蚜繁殖力均显著降低。寄生时寄主的发育期也影响寄生蜂的发育,2龄豆蚜被寄生时的日本柄瘤蚜茧蜂个体发育最快,为194.10 h;1龄寄主被寄生时蚜茧蜂的发育最慢,需215.80 h。并对不同发育期蚜虫总蛋白质和总糖原含量进行了测定。     

The effect of selection on genealogies

The coalescent process can describe the effects of selection at linked loci only if selection is so strong that genotype frequencies evolve deterministically. Here, we develop methods proposed by Kaplan, Darden, and Hudson to find the effects of weak selection. We show that the overall effect is given by an extension to Price's equation: the change in properties such as moments of coalescence times is equal to the covariance between those properties and the fitness of the sample of genes. The distribution of coalescence times differs substantially between allelic classes, even in the absence of selection. However, the average coalescence time between randomly chosen genes is insensitive to the current allele frequency and is affected significantly by purifying selection only if deleterious mutations are common and selection is strong (i.e., the product of population size and selection coefficient, Ns>3). Balancing selection increases mean coalescence times, but the effect becomes large only when mutation rates between allelic classes are low and when selection is extremely strong. Our analysis supports previous simulations that show that selection has surprisingly little effect on genealogies. Moreover, small fluctuations in allele frequency due to random drift can greatly reduce any such effects. This will make it difficult to detect the action of selection from neutral variation alone.