相同运动负荷下踏车速率与运动耗氧量相关性研究

在四个不同的测试日中,１０名非运动员的健康男性,随机以４０,６０,８０和１００ｒ／ｍｉｎ做极限水平递增负荷运动试验。结果显示：于运动负荷为４５,９０,１３５,１８０和２２５Ｗ时,踏车速率与Ｖ·Ｏ２分别呈抛物线样二项式相关关系。自４５Ｗ起,ｒ分别为０．９３,０．９１,０．８９,０．９２和０．９１（均Ｐ＜０．０１）。不同运动负荷下,最佳代谢踏车速率分别为５１．３,５４．６,５８．３,７２．３和８０．４ｒｐｍ,并与运动负荷呈高度直线相关（ｒ＝０．９３,Ｐ＜０．０１）。不同运动负荷下,１０名受试者的自选踏车速率分别为８４±６．５８,８３±５．８７,９２±５．１７,８０±７．４５和８３±９．７ｒ／ｍｉｎ,与最佳代谢踏车速率和运动负荷间无相关（ｒ为－０．１７和－０．３２,均Ｐ〉０．０５）。结果提示：在相同运动负荷下,踏车速率与Ｖ·Ｏ２间存在抛物线样相关关系。最佳代谢踏车速率随运动负荷的增加而增加。自选踏车率与运动负荷及最佳代谢踏车速率间无明确关系。     

Marine reserves and optimal harvesting

Advocates of no‐take marine reserves emphasize their conservation benefits. Critics counter that reserves would decrease fisheries yield. Analysis of a spatially explicit harvesting model, however, shows that no‐take marine reserves are always part of an optimal harvest designed to maximize yield. The optimal harvest generates a spatial source–sink structure with source populations placed in reserves. The sizes and locations of the optimal reserves depend on a dimensionless length parameter. For small values of this parameter, the maximum yield is obtained by placing a large reserve in the centre of the habitat. For large values of this parameter, the optimal harvesting strategy is a spatial ‘chattering control’ with infinite sequences of reserves alternating with areas of intense fishing. Such a chattering strategy would be impossible to actually implement, but in these cases an approximate yet practicable policy, utilizing a small number of reserves, can be constructed.     

Logistic model for soldier production in aphids

Summary The number of aphid soldiers in a colony is positively correlated with colony size. This positive correlation has been repeatedly confirmed for the bamboo aphid Pseudoregma bambucicola. To explain this, we present a simple model assuming logistic growth of the aphid colony. The model predicts that a first soldier is more readily produced in large colonies than in small colonies. This is because the productivity of each reproductive decreases as the colony size increases, and also because the efficiency of defense increases as the number of reproductives defended by the soldier increases. The latter effect disappears when the number of reproductives exceeds N_p, the critical number of reproductives that a predator could damage. This argument holds for a second, a third, and an ith soldier in general. Although we assume logistic growth of the aphid colony, the model is applicable to any form of colony growth with minor changes of some premises.Received 12 February 2003; revised 28 March 2003; accepted 22 April 2003.     

Effect of strong directional selection on weakly selected mutations at linked sites: implication for synonymous codon usage

The fixation of weakly selected mutations can be greatly influenced by strong directional selection at linked loci. Here, I investigate a two-locus model in which weakly selected, reversible mutations occur at one locus and recurrent strong directional selection occurs at the other locus. This model is analogous to selection on codon usage at synonymous sites linked to nonsynonymous sites under strong directional selection. Two approximations obtained here describe the expected frequency of the weakly selected preferred alleles at equilibrium. These approximations, as well as simulation results, show that the level of codon bias declines with an increasing rate of substitution at the strongly selected locus, as expected from the well-understood theory that selection at one locus reduces the efficacy of selection at linked loci. These solutions are used to examine whether the negative correlation between codon bias and nonsynonymous substitution rates recently observed in Drosophila can be explained by this hitchhiking effect. It is shown that this observation can be reasonably well accounted for if a large fraction of the nonsynonymous substitutions on genes in the data set are driven by strong directional selection.     

Carbamoyl phosphate synthetase 1 deficiency in Italy: clinical and genetic findings in a heterogeneous cohort

Carbamoyl Phosphate Synthetase 1 deficiency (CPS1D) is a rare autosomal recessive urea cycle disorder, potentially leading to lethal hyperammonemia. Based on the age of onset, there are two distinct phenotypes: neonatal and late form. The CPS1 enzyme, located in the mitochondrial matrix of hepatocytes and epithelial cells of intestinal mucosa, is encoded by the CPS1 gene. At present more than 220 clear-cut genetic lesions leading to CPS1D have been reported. As most of them are private mutations diagnosis is complicated.Here we report an overview of the main clinical findings and biochemical and molecular data of 13 CPS1D Italian patients. In two of them, one with the neonatal form and one with the late form, cadaveric auxiliary liver transplant was performed. Mutation analysis in these patients identified 17 genetic lesions, 9 of which were new confirming their “private” nature. Seven of the newly identified mutations were missense/nonsense changes. In order to study their protein level effects, we performed an in silico analysis whose results indicate that the amino acid substitutions occur at evolutionary conserved positions and affect residues necessary for enzyme stability or function.     

An Improved Codon Modeling Approach for Accurate Estimation of the Mutation Bias

Phylogenetic codon models are routinely used to characterize selective regimes in coding sequences. Their parametric design, however, is still a matter of debate, in particular concerning the question of how to account for differing nucleotide frequencies and substitution rates. This problem relates to the fact that nucleotide composition in protein-coding sequences is the result of the interactions between mutation and selection. In particular, because of the structure of the genetic code, the nucleotide composition differs between the three coding positions, with the third position showing a more extreme composition. Yet, phylogenetic codon models do not correctly capture this phenomenon and instead predict that the nucleotide composition should be the same for all three positions. Alternatively, some models allow for different nucleotide rates at the three positions, an approach conflating the effects of mutation and selection on nucleotide composition. In practice, it results in inaccurate estimation of the strength of selection. Conceptually, the problem comes from the fact that phylogenetic codon models do not correctly capture the fixation bias acting against the mutational pressure at the mutation–selection equilibrium. To address this problem and to more accurately identify mutation rates and selection strength, we present an improved codon modeling approach where the fixation rate is not seen as a scalar, but as a tensor. This approach gives an accurate representation of how mutation and selection oppose each other at equilibrium and yields a reliable estimate of the mutational process, while disentangling the mean fixation probabilities prevailing in different mutational directions.     

CpG + CpNpG analysis of protein-coding sequences from tomato

We develop codon-based models for simultaneously inferring the mutational effects of CpG and CpNpG methylation in coding regions. In a data set of 369 tomato genes, we show that there is very little effect of CpNpG methylation but a strong effect of CpG methylation affecting almost all genes. We further show that the CpNpG and CpG effects are largely uncorrelated. Our results suggest different roles of CpG and CpNpG methylation, with CpNpG methylation possibly playing a specialized role in defense against transposons and RNA viruses.     

Evaluating optimal diet models for an African browsing ruminant,the kudu: How constraining are the assumed constraints?

Summary The linear programming model (LPM) of Belovsky (1978, 1986) and modifications of the classical or contingency model incorporating a digestive constraint (CM) were tested using foraging data recorded for kudus (Tragelaphus strepsiceros) browsing savanna vegetation over the late wet season. Food choice was between the herbaceous and woody plant components for LPM and among plant species or categories for CM. The constraints considered were consumption (cropping) rate, foraging time and digestive capacity. Woody communities dominated byBurkea andAcacia represented alternative habitat types. Following a minor adjustment, LPM represented the overall average diet and predicted the dietary differences between habitat types. However, the kudus failed to respond dietarily to variations among days and foraging sessions (meals) in the parameters constraining intake. The kudus accepted a wider dietary range than predicted to be optimal by CM. Evidence suggested that neither foraging time, nor digestive capacity, formed an effective constraint under the study conditions. Thermal tolerance and gut space may become limiting only towards the extremes of environmental variability that animals experience. LPM is vulnerable to circularity if average parameter values are used to estimate constraint settings. The energy maximizer—time minimizer dichotomy fails to take into account the fitness consequences of alternative foraging responses. CM is less cryptic in its application than LPM and so has greater heuristic value, despite its predictive failures. However, there may be no consistent ranking of food types where multiple constraints that are variable in their effectiveness apply. Dynamic programming models offer a solution, but pose a formidable challenge in complex natural environments.     

Niche width collapse in a resilient top predator following ecosystem fragmentation

Much research has focused on identifying species that are susceptible to extinction following ecosystem fragmentation, yet even those species that persist in fragmented habitats may have fundamentally different ecological roles than conspecifics in unimpacted areas. Shifts in trophic role induced by fragmentation, especially of abundant top predators, could have transcendent impacts on food web architecture and stability, as well as ecosystem function. Here we use a novel measure of trophic niche width, based on stable isotope ratios, to assess effects of aquatic ecosystem fragmentation on trophic ecology of a resilient, dominant, top predator. We demonstrate collapse in trophic niche width of the predator in fragmented systems, a phenomenon related to significant reductions in diversity of potential prey taxa. Collapsed niche width reflects a homogenization of energy flow pathways to top predators, likely serving to destabilize remnant food webs and render apparently resilient top predators more susceptible to extinction through time.     

Nucleotide sequences of two corn histone H3 genes. Genomic organization of the corn histone H3 and H4 genes

Summary Two histone H3 genes have been cloned from a gtWES.B corn genomic library. The nucleotide sequences show 96% homology and both encode the same protein, which differs from its counterpart in wheat and pea by one amino acid substitution. The 5-flanking regions of the two corn H3 genes contain the classical histone-gene-specific consensus sequences and possess several regions of extensive nucleotide homology. A conserved octanucleotide 5-CGCGGATC-3 occurs at approximately 200 nucleotides upstream from the initiation ATG codon. This octanucleotide was found to exist in all of the 7 plant histone genes sequenced so far. Codon usage is characterized by a very high frequency of C (67%) and G (28%) at the third position of the codons, those ending by A (1%) and T (4%) being practically excluded.Comparison of Southern blots of EcoRI, EcoRV and BamHI digested genomic DNA suggests that the corn H3 and H4 genes are not closely associated. The H3 genes exist as 60 to 80 copies and the H4 genes as 100 to 120 copies per diploid genome. re]19851002 rv]19851212 ac]19851216