Using flowering times and leaf numbers to model the phases of photoperiod sensitivity in Antirrhinum majus L

A model has been developed that can be used to determine the phases of sensitivity to photoperiod for seedlings subjected to reciprocal transfers at regular intervals between long (LD) and short day (SD) conditions. The novel feature of this approach is that it enables the simultaneous analysis of the time to flower and number of leaves below the inflorescence. A range of antirrhinum cultivars were grown, all of which were shown to be quantitative long-day plants. Seedlings were effectively insensitive to photoperiod when very young (juvenile). However, after the end of the juvenile phase, SD delayed flowering and increased the number of leaves below the inflorescence. Plants transferred from LD to SD showed a sudden hastening of flowering and a decrease in leaf number once sufficient LD had been received for flower commitment. Photoperiod had little effect on the rate of flower development. The analysis clearly identified major cultivar differences in the length of the juvenile phase and the photoperiod-sensitive inductive phase in both LD and SD.     

Mathematical consequences of the genealogical species concept

A genealogical species is defined as a basal group of organisms whose members are all more closely related to each other than they are to any organisms outside the group ("exclusivity"), and which contains no exclusive group within it. In practice, a pair of species is so defined when phylogenies of alleles from a sample of loci shows them to be reciprocally monophyletic at all or some specified fraction of the loci. We investigate the length of time it takes to attain this status when an ancestral population divides into two descendant populations of equal size with no gene exchange, and when genetic drift and mutation are the only evolutionary forces operating. The number of loci used has a substantial effect on the probability of observing reciprocal monophyly at different times after population separation, with very long times needed to observe complete reciprocal monophyly for a large number of loci. In contrast, the number of alleles sampled per locus has a relatively small effect on the probability of reciprocal monophyly. Because a single mitochondrial or chloroplast locus becomes reciprocally monophyletic much faster than does a single nuclear locus, it is not advisable to use mitochondrial and chloroplast DNA to recognize genealogical species for long periods after population divergence. Using a weaker criterion of assigning genealogical species status when more than 50% of sampled nuclear loci show reciprocal monophyly, genealogical species status depends much less on the number of sampled loci, and is attained at roughly 4-7 N generations after populations are isolated, where N is the historically effective population size of each descendant. If genealogical species status is defined as more than 95% of sampled nuclear loci showing reciprocal monophyly, this status is attained after roughly 9-12 N generations.     

Reciprocal "flipping" underlies substrate recognition and catalytic activation by the human 8-oxo-guanine DNA glycosylase

Both 8oxo-guanine and formamidopyrimidines are major products of oxidative DNA damage that can result in the fixation of transversion mutations following replication if left unrepaired. These lesions are targeted by the N-DNA glycosylase hOgg1, which catalyses excision of the aberrant base followed by cleavage of the phosphate backbone directly 5' to the resultant abasic site in a context, dependent manner. We present the crystal structure of native hOgg1 refined to 2.15 A resolution that reveals a number of highly significant conformational changes on association with DNA that are clearly required for substrate recognition and specificity. Changes of this magnitude appear to be unique to hOgg1 and have not been observed in any of the DNA-glycosylase structures analysed to date where both native and DNA-bound forms are available. It has been possible to identify a mechanism whereby the catalytic residue Lys 249 is "primed" for nucleophilic attack of the N-glycosidic bond.     

Density dependence and population differentiation of genetic architecture in Impatiens capensis in natural environments

We identified environment-dependent constraints on the evolution of plasticity to density under natural conditions in two natural populations of Impatiens capensis. We also examined the expression of population divergence in genetic variance-covariance matrices in these natural environments. Inbred lines, originally collected from a sunny site with high seedling densities and a woodland site with low seedling densities, were planted in both original sites at natural high densities and at low density. Morphological and life-history characters were measured. More genetic variation for plastic responses to density was expressed in the sun site than in the woodland site, so the evolutionary potential of plasticity was greater in the sun site. Strong genetic correlations between the same character expressed at different densities and correlations among different characters could constrain the evolution of plasticity in both sites. Genetically based trade-offs in meristem allocation to vegetative growth and reproduction were apparent only in the high-resource environment with no overhead canopy and no intraspecific competition. Therefore, genetic constraints on the evolution of plasticity depended on the site and density in which plants were grown, and correlated responses to selection on plastic characters are also expected to differ between sites and densities. Population differentiation in genetic variance-covariance matrices was detected, but matrix structural differences, as opposed to proportional differences, were detected between populations only in the sun site at natural high density. Thus, population divergence in genetic architecture can occur rapidly and on a fine spatial scale, but the expression of such divergence may depend on the environment.     

Simulation and Parameter Estimation Study of a Simple Neuronal Model of Rhythm Generation: Role of NMDA and Non-NMDA Receptors

Simple neural network models of the Xenopus embryo swimming CPG, based on the one originally developed by Roberts and Tunstall (1990), were used to investigate the role of the voltage-dependent N-methyl-D-aspartate (NMDA) receptor channels, in conjunction with faster non-NMDA components of synaptic excitation, in rhythm generation. The voltage-dependent NMDA current "follows" the membrane potential, leading to a postinhibitory rebound that is more efficient than one without voltage dependency and allows neurons to fire more than one action potential per cycle. Furthermore, the model demonstrated limited rhythmic activity in the absence of synaptic inhibition, supporting the hypothesis that the NMDA channels provide a basic mechanism for rhythmicity. However, the rhythmic properties induced by the NMDA current were observed only when there was moderate activation of the non-NMDA synaptic channels, suggesting a modulatory role for this component. The simulations also show that the voltage dependency of the NMDA conductance, as well as the fast non-NMDA current, stabilizes the alternation pattern versus synchrony. To verify that these effects and their implications on the mechanism of swimming and transition to other types of activity take place in the real preparation, constraints on parameter values have to be specified. A method to estimate synaptic parameters was tested with generated data. It is shown that a global analysis, based on multiple iterations of the optimization process (Foster et al., 1993), gives a better understanding of the parameter subspace describing network activity than a standard fit with a sensitivity analysis for an individual solution.     

游离股前外侧皮瓣Ⅰ期修复大面积深度眼镜蛇伤溃疡

目的 探讨大面积深度眼镜蛇伤溃疡的修复方法。方法 对8例大面积深度眼镜蛇伤溃疡,采用游离股前外侧皮瓣,Ⅰ期修复创面。结果 8例皮瓣均全部成活,患肢外观及活动功能均满意。结论 游离股前外侧皮瓣是一种Ⅰ期修复大面积深度眼镜蛇伤溃疡的理想方法。     

Is local adaptation in Mimulus guttatus caused by trade‐offs at individual loci?

Local adaptation is considered to be the result of fitness trade‐offs for particular phenotypes across different habitats. However, it is unclear whether such phenotypic trade‐offs exist at the level of individual genetic loci. Local adaptation could arise from trade‐offs of alternative alleles at individual loci or by complementary sets of loci with different fitness effects of alleles in one habitat but selective neutrality in the alternative habitat. To evaluate the genome‐wide basis of local adaptation, we performed a field‐based quantitative trait locus (QTL) mapping experiment on recombinant inbred lines (RILs) created from coastal perennial and inland annual races of the yellow monkeyflower (Mimulus guttatus) grown reciprocally in native parental habitats. Overall, we detected 19 QTLs affecting one or more of 16 traits measured in two environments, most of small effect. We identified 15 additional QTL effects at two previously identified candidate QTLs [DIV ERGENCE (DIV)]. Significant QTL by environment interactions were detected at the DIV loci, which was largely attributable to genotypic differences at a single field site. We found no detectable evidence for trade‐offs for any one component of fitness, although DIV2 showed a trade‐off involving different fitness traits between sites, suggesting that local adaptation is largely controlled by non‐overlapping loci. This is surprising for an outcrosser, implying that reduced gene flow prevents the evolution of individuals adapted to multiple environments. We also determined that native genotypes were not uniformly adaptive, possibly reflecting fixed mutational load in one of the populations.     

Heme-heme and heme-ligand interactions in the di-heme oxygen-reducing site of cytochrome bd from Escherichia coli revealed by nanosecond absorption spectroscopy

Cytochrome bd is a terminal quinol:O₂ oxidoreductase of respiratory chains of many bacteria. It contains three hemes, b₅₅₈, b₅₉₅, and d. The role of heme b₅₉₅ remains obscure. A CO photolysis/recombination study of the membranes of Escherichia coli containing either wild type cytochrome bd or inactive E445A mutant was performed using nanosecond absorption spectroscopy. We compared photoinduced changes of heme d-CO complex in one-electron-reduced, two-electron-reduced, and fully reduced states of cytochromes bd. The line shape of spectra of photodissociation of one-electron-reduced and two-electron-reduced enzymes is strikingly different from that of the fully reduced enzyme. The difference demonstrates that in the fully reduced enzyme photolysis of CO from heme d perturbs ferrous heme b₅₉₅ causing loss of an absorption band centered at 435 nm, thus supporting interactions between heme b₅₉₅ and heme d in the di-heme oxygen-reducing site, in agreement with previous works. Photolyzed CO recombines with the fully reduced enzyme monoexponentially with τ ∼ 12 μs, whereas recombination of CO with one-electron-reduced cytochrome bd shows three kinetic phases, with τ ∼ 14 ns, 14 μs, and 280 μs. The spectra of the absorption changes associated with these components are different in line shape. The 14 ns phase, absent in the fully reduced enzyme, reflects geminate recombination of CO with part of heme d. The 14-μs component reflects bimolecular recombination of CO with heme d and electron backflow from heme d to hemes b in ∼ 4% of the enzyme population. The final, 280-μs component, reflects return of the electron from hemes b to heme d and bimolecular recombination of CO in that population. The fact that even in the two-electron-reduced enzyme, a nanosecond geminate recombination is observed, suggests that namely the redox state of heme b₅₉₅, and not that of heme b₅₅₈, controls the pathway(s) by which CO migrates between heme d and the medium.     

THE BIOLOGY OF SPECIATION

Since Darwin published the “Origin,” great progress has been made in our understanding of speciation mechanisms. The early investigations by Mayr and Dobzhansky linked Darwin's view of speciation by adaptive divergence to the evolution of reproductive isolation, and thus provided a framework for studying the origin of species. However, major controversies and questions remain, including: When is speciation nonecological? Under what conditions does geographic isolation constitute a reproductive isolating barrier? and How do we estimate the “importance” of different isolating barriers? Here, we address these questions, providing historical background and offering some new perspectives. A topic of great recent interest is the role of ecology in speciation. “Ecological speciation” is defined as the case in which divergent selection leads to reproductive isolation, with speciation under uniform selection, polyploid speciation, and speciation by genetic drift defined as “nonecological.” We review these proposed cases of nonecological speciation and conclude that speciation by uniform selection and polyploidy normally involve ecological processes. Furthermore, because selection can impart reproductive isolation both directly through traits under selection and indirectly through pleiotropy and linkage, it is much more effective in producing isolation than genetic drift. We thus argue that natural selection is a ubiquitous part of speciation, and given the many ways in which stochastic and deterministic factors may interact during divergence, we question whether the ecological speciation concept is useful. We also suggest that geographic isolation caused by adaptation to different habitats plays a major, and largely neglected, role in speciation. We thus provide a framework for incorporating geographic isolation into the biological species concept (BSC) by separating ecological from historical processes that govern species distributions, allowing for an estimate of geographic isolation based upon genetic differences between taxa. Finally, we suggest that the individual and relative contributions of all potential barriers be estimated for species pairs that have recently achieved species status under the criteria of the BSC. Only in this way will it be possible to distinguish those barriers that have actually contributed to speciation from those that have accumulated after speciation is complete. We conclude that ecological adaptation is the major driver of reproductive isolation, and that the term “biology of speciation,” as proposed by Mayr, remains an accurate and useful characterization of the diversity of speciation mechanisms.     

Medaka fish stem cells and their applications

Stem cells are present in developing embryos and adult tissues of multicellular organisms. Owing to their unique features, stem cells provide excellent opportunities for experimental analyses of basic developmental processes such as pluripotency control and cell fate decision and for regenerative medicine by stem cell-based therapy. Stem cell cultures have been best studied in 3 vertebrate organisms. These are the mouse, human and a small laboratory fish called medaka. Specifically, medaka has given rise to...