Fitness costs of R-gene-mediated resistance in Arabidopsis thaliana

Resistance genes (R-genes) act as an immune system in plants by recognizing pathogens and inducing defensive pathways. Many R-gene loci are present in plant genomes, presumably reflecting the need to maintain a large repertoire of resistance alleles. These loci also often segregate for resistance and susceptibility alleles that natural selection has maintained as polymorphisms within a species for millions of years. Given the obvious advantage to an individual of being disease resistant, what prevents these resistance alleles from being driven to fixation by natural selection? A cost of resistance is one potential explanation; most models require a lower fitness of resistant individuals in the absence of pathogens for long-term persistence of susceptibility alleles. Here we test for the presence of a cost of resistance at the RPM1 locus of Arabidopsis thaliana. Results of a field experiment comparing the fitness of isogenic strains that differ in the presence or absence of RPM1 and its natural promoter reveal a large cost of RPM1, providing the first evidence that costs contribute to the maintenance of an ancient R-gene polymorphism.     

妊娠期及哺乳期小鼠第四脑室外侧隐窝室管膜超微结构的改变

观察妊娠期和哺乳期小鼠第四脑室外侧隐窝进行超微结构变化,探讨该部位室管膜及室管膜上结构与妊娠和哺乳功能的关系.该处室管膜细胞有旺盛的分泌现象,该现象与妊娠和哺乳周期密切相关.     

广西桂中地区大学生健身意识和健身行为的调查分析

采用随机抽样问卷调查法、文献资料法、数理统计法等对桂中地区在校大学生的健身意识和健身行为进行了调查分析.结果表明:大学生在健身意识上的认识不是很充分,不完全清楚健康和亚健康的界线划定;大学生的终身健身意识差,仅有23%的学生准备终身体育锻炼;在健身行为习惯中有70%左右的学生选择了正确的锻炼频度、锻炼时间和锻炼强度,他们选择的健身项目却有一定的局限性.整体来看大学生在健身运动意识方面有了广泛的认识,但还没有形成良好的健身行为习惯,不能够自觉地进行终身体育锻炼,需要进一步的指导和培养.     

大同市健身俱乐部健身教练员现状调查与分析

运用文献资料法、问卷调查法和访谈法，对大同市健身俱乐部教练员的年龄结构、性别结构、学历层次，所具备的专业素质状况等进行了调查与分析，并对目前存在的问题提出了合理建议．     

Evolution of homeothermy in mammals

We propose that mammalian homeothermy was was acquired in two steps. The first step enabled mammals to invade a nocturnal niche without an increase in resting metabolic rate. The second step enabled them to invade a diurnal niche and involved the acquisition of higher body temperatures and metabolic rates.     

Progress in interspecies cloning of mammals

Interspecies mammalian cloning can be achieved by application of two key techniques, i.e.the technique of interspecies nuclear transfer and the technique of interspecies pregnancy.The general principles, problems and possible solutions, as well as the recent advances of interspecies mammalian cloning have been summarized in this review.     

Coordination of circadian timing in mammals

Time in the biological sense is measured by cycles that range from milliseconds to years. Circadian rhythms, which measure time on a scale of 24 h, are generated by one of the most ubiquitous and well-studied timing systems. At the core of this timing mechanism is an intricate molecular mechanism that ticks away in many different tissues throughout the body. However, these independent rhythms are tamed by a master clock in the brain, which coordinates tissue-specific rhythms according to light input it receives from the outside world.