Inhibition of RNA Synthesis and the Relationship between Nucleolar and Mitotic Cycles in Zea mays Root Meristems

The relationship between nucleolar and mitotic cycles has beendetermined after treatment of root apices of Zea mays with ethidiumbromide. In the meristematic regions of the stele the two cyclesare not much displaced in relation to each other except fora delay in the onset of the disorganization phase. A few nucleolipersist into metaphase and a few nuclei undergo an amitoticdivision. In the cap initials the drug greatly delays the onsetof disorganization of the nucleolus, which normally occurs beforeprophase in this region. It also delays the completion of reorganizationso that fully organized nucleoli are no longer available duringthe last half of telophase. In the quiescent centre the onsetof disorganization and the end of reorganization of the nucleoliare also delayed in relation to mitosis. There is no evidencefor a delay in the onset of reorganization in any region ofthe meristem. Some cells form multiple micronucleoli and this aberrant behaviouroccurs more often in the cap initials than elsewhere as doesamitotic division.     

Persistent nuclear ribosomal DNA sequence polymorphism in the Amelanchier agamic complex (Rosaceae)

Individual plants of several Amelanchier taxa contain many polymorphic nucleotide sites in the internal transcribed spacers (ITS) of nuclear ribosomal DNA (nrDNA). This polymorphism is unusual because it is not recent in origin and thus has resisted homogenization by concerted evolution. Amelanchier ITS sequence polymorphism is hypothesized to be the result of gene flow between two major North American clades resolved by phylogenetic analysis of ITS sequences. Western North American species plus A. humilis and A. sanguinea of eastern North America form one clade (A), and the remaining eastern North American Amelanchier make up clade B. Five eastern North American taxa are polymorphic at many of the nucleotide sites where clades A and B have diverged and are thought to be of hybrid origin, with A. humilis or A. sanguinea as one parent and various members of clade B as the other parent. Morphological evidence suggests that A. humilis is one of the parents of one of the polymorphic taxa, a microspecies that we refer to informally as A. "erecta." Sequences of 21 cloned copies of the ITS1- 5.8S gene-ITS2 region from one A. "erecta" individual are identical to A. humilis sequence or to the clade B consensus sequence, or they are apparent recombinants of A. humilis and clade B ITS repeats. Amelanchier "erecta" and another polymorphic taxon are suspected to be relatively old because both grow several hundred kilometers beyond the range of one of their parents. ITS sequence polymorphisms have apparently persisted in these two taxa perhaps because of polyploidy and/or agamospermy (asexual seed production), which are prevalent in the genus.      

Endodermis-like Sheaths in the Submerged Freshwater MacrophyteRanunculus trichophyllusChaix

Light, fluorescence and electron microscopical analysis of therooted freshwater plantRanunculus trichophyllusrevealed a peculiaranatomical feature. In addition to the true endodermis encirclingthe root stele, endodermis-like sheaths occurred around eachvascular bundle of the leaf segments and of the eustelic stemwith its large central cavity, which assumed an anatomical featureresembling that of some pteridophyte stems. These impermeablesheaths, whose cells differentiate suberized walls, can playa major role in hampering the apoplastic leakage of the pressurizedwater solution which flows throughout the plant in xylem vesselsand contains the mineral nutrients taken up by the roots fromthe sediment. Moreover, these sheaths can function in preventingflooding of the aerenchymatic cavities of the submerged organs.In this way the endodermis-like sheaths preserve the correctcirculation of gas and nutrient solution through the entireorganism and assume great significance as a structural mechanismevolved by this species to survive and grow underwater.Copyright1999 Annals of Botany Company. Ranunculus trichophyllus,freshwater macrophyte, submerged angiosperm, anatomy, endodermis, endodermis-like sheaths, light microscopy, fluorescence microscopy.     

拉萨河谷上段江孜沙棘Hippophea gyantsensis(Rousi)Lian种群的海拔梯度性变异

江孜沙棘[Hippophea gyantsensis(Rousi)Lian]是青藏高原特有的一种广生态幅的小乔木,在拉萨河谷地区的海拔3500～4200 m范围内均有分布.前人工作多集中在江孜沙棘果实的开发利用方面,对其基础生态学研究较少.本研究旨在探讨江孜沙棘沿海拔梯度的群落组成和表型变异的规律.为此,在拉萨河谷上段沿海拔梯度由东向西设置了4个样带:3850 m、3950 m、4050 m和4200 m,每个样带设置2至3个10 m×10 m的样方进行研究.首先,详细记录了每个样方内林下维管植物的物种组成、样方内的沙棘盖度、海拔、样方与河岸的实际距离,并用DCA [detrended correspondence analysis (去势对应分析)] 排序方法对群落及其组成物种进行排序分析.随机抽取了每个样方内的20个江孜沙棘植株个体,测定其胸径、基径、株高和叶片长度,用回归分析法分析这些变量和海拔之间的关系.研究结果表明,江孜沙棘在拉萨河谷内的主要生境分为4种类型,即:河边砾石滩地、河阶草滩、河边草甸和河边林缘, 样方排序结果主要受海拔的影响;同时,江孜沙棘植株的基径、胸径和高度都随着海拔的升高而显著减小,而叶片长度与海拔之间无显著相关.本文研究结果表明,对江孜沙棘而言,海拔所代表的综合环境因子对其分布和表型有显著的影响,而局部光照可能也是影响其表型特征的重要生态因子.     

Sex differences in circulating antibodies in nestling Pied Flycatchers Ficedula hypoleuca

Sex differences in immune function are relatively well studied in vertebrate animals, although the patterns are not always clear in birds. The study of immune responses in nestlings of wild bird populations may constitute an appropriate way to investigate inherent intersexual differences while controlling for environmental conditions such as parasitism that affect male and female individuals growing in the same nest. We studied whether the cell‐mediated immune response, as measured by phytohaemagglutinin (PHA) injection, and the levels of circulating antibodies differ between sexes of Pied Flycatcher nestlings Ficedula hypoleuca. No sex differences in nestling cell‐mediated immune response were found, but females showed significantly higher levels of plasma immunoglobulins than males did. Although nestling birds may not have a fully functional humoral immune defence, our study indicates that sex differences in the humoral component exist at this early stage of life. Given the importance of antibodies in the fight against parasite, bacterial and viral infections, the intrinsic sex disparity in circulating antibodies may have important implications for the life history of each sex.     

Talent in the taxi: a model system for exploring expertise

While there is widespread interest in and admiration of individuals with exceptional talents, surprisingly little is known about the cognitive and neural mechanisms underpinning talent, and indeed how talent relates to expertise. Because many talents are first identified and nurtured in childhood, it can be difficult to determine whether talent is innate, can be acquired through extensive practice or can only be acquired in the presence of the developing brain. We sought to address some of these issues by studying healthy adults who acquired expertise in adulthood. We focused on the domain of memory and used licensed London taxi drivers as a model system. Taxi drivers have to learn the layout of 25 000 streets in London and the locations of thousands of places of interest, and pass stringent examinations in order to obtain an operating licence. Using neuropsychological assessment and structural and functional magnetic resonance imaging, we addressed a range of key questions: in the context of a fully developed brain and an average IQ, can people acquire expertise to an exceptional level; what are the neural signatures, both structural and functional, associated with the use of expertise; does expertise change the brain compared with unskilled control participants; does it confer any cognitive advantages, and similarly, does it come at a cost to other functions? By studying retired taxi drivers, we also consider what happens to their brains and behaviour when experts stop using their skill. Finally, we discuss how the expertise of taxi drivers might relate to the issue of talent and innate abilities. We suggest that exploring talent and expertise in this manner could have implications for education, rehabilitation of patients with cognitive impairments, understanding individual differences and possibly conditions such as autism where exceptional abilities can be a feature.