元江干热河谷山地五百年来植被变迁探讨

元江河谷是云南省最干热地区之一,在海拔800—900米以下的山地上广泛分布着稀树灌草丛。根据《元江府志》(1714年编纂)、《元江州志》(1826年编纂)、《元江志稿》(1922年编篡)及对现存植被的考察,本文探讨了元江干热河谷山地五百年来植被的变迁。 元江县森林复盖率的减少与人口的增加有密切关系,十七世纪中期以前,森林复盖率在75％以上,十八、十九世纪时为70％左右,1958年为61.5％,1975年为27.3％,至1982年则为19.3％。研究表明,在十九世纪以前,这个地区分布的主要植被是热带季雨林,甚而热带季节雨林,以后热带稀树灌草丛则迅速发展。植被的历史变化与土壤流失密切相关。植物群落的演变是由以乔木树种为优势演变为以灌木种类为优势,再演变为以多年生草木植物为优势,而最后则成为裸地。本文也讨论了这个地区植被恢复的方法。     

颈动脉窦内压的变化速率对反射性降压效应的影响

在成年狗身上制备右侧孤离颈动脉窦,借助灌流装置用饱和氧的任氏液对其进行灌流。灌流压为搏动性的。通过改变每搏泵出量、灌流管道的阻力和弹性来调节灌流压及其变化速率。用多导生理记录仪同步记录股动脉血压、窦内灌流压及其变化速率。本文主要观察窦内压的变化速率对降压反射的影响。在8只狗身上共进行了93次实验。结果表明,在窦内压相同的情况下,灌流压的上升速率愈快,降压效应愈明显,而其下降速率则无显著作用。已有资料证明搏动性窦内压所引起的降压效应较非搏动性压力更为明显。由此可见,狗的颈动脉窦压力感受器不仅对搏动性压力而且对其上升的变化速率也很敏感。     

Phytoplankton community structure and distribution in the nearshore zone of Lake Ontario

Spatial and temporal distributions of phytoplankton were studied during 1979 in the nearshore zone of Lake Ontario. Genera such as Dinobryon, Oscillatoria, and Gymnodinium tended to be found at discrete depths whereas other genera such as Cryptomonas and Rhodomonas tended to be more uniformly distributed throughout the water column. The discrete depth distribution of some of these genera compared with the weak thermal stabilization of the area suggests depth selection and habitat preference within the algal community.Physical processes probably played a major role in regulating the timing and duration of vertical diatom pulses, and resuspension from the sediment played a major role as to the timing of the autumn growth. Vertical mixing in the nearshore zone was sufficient for some diatom genera such as Tabellaria to maintain populations throughout the summer. These mixing processes, however, affect the stability of community structures and associations that develop within the water column. The role of physical processes and their potential effects on algal production in the Great Lakes is discussed.     

Nonrandomness of point mutation as reflected in nucleotide substitutions in pseudogenes and its evolutionary implications

Summary We have obtained a revised estimate of the pattern of point mutation by considering more pseudogene sequences. Compared with our previous estimate, it agrees better with expectations based on the double-strand structure of DNA. The revised pattern, like the previous one, indicates that mutation occurs nonrandomly among the four nucleotides. In particular, the proportion of transitional mutations (59%) is almost twice as high as the value (33%) expected under random mutation. The same high proportion of transitions is observed in synonymous substitutions in genes. The proportion of transitional changes observed among electrophoretic variants of human hemoglobin is about the same as that predicted by the revised pattern of mutation. We also show that nonrandom mutation increases, by about 15%, the proportion of synonymous mutations due to single-nucleotide changes in the codon table, and increases, from 10% to 50%, the rate of synonymous mutation in the seven genes studied. However, nonrandom mutation reduces (by about 10%) the proportion of polar changes among nonsynonymous mutations in a gene. As far as single-nucleotide changes (in the codon table) are concerned, nonrandom mutation only slightly favors relatively conservative amino acid interchanges, and has virtually no effect on the proportions of radical changes and nonsense mutations.     

Ordination of vegetation change in Guadalupe Mountains,New Mexico,USA

Vegetation change over a nine-year period was studied in Guadalupe Mountains, New Mexico. Permanent transects in desert shrub vegetation were sampled in 1972 and 1980. Emphasis was given to shrubs because of their importance to big game diets. Univariate paired t-tests and reciprocal averaging ordination were used to detect and display coordinated changes in species composition over time. Despite apparently less browsing pressure in desert shrub vegetation in 1980 there were few significant changes in species composition. In addition, preferred forage species showed reduced reproduction while species of intermediate and poor forage value dis-played increased reproduction during this time. These data do not support traditional rangeland succession theory which states that enhanced reproduction of preferred species should follow grazing or browsing pressure reduction.     

Organization and function of cytochromeb and ubiquinone in the cristae membrane of beef heart mitochondria

The arrangement and function of the redox centers of the mammalianbc ₁ complex is described on the basis of structural data derived from amino acid sequence studies and secondary structure predictions and on the basis of functional studies (i.e., EPR data, inhibitor studies, and kinetic experiments). Two ubiquinone reaction centers do exist—a QH₂ oxidation center situated at the outer, cytosolic surface of the cristae membrane (Q₀ center), and a Q reduction center (Q _i center) situated more to the inner surface of the cristae membrane. The Q₀ center is formed by theb-566 domain of cytochromeb, the FeS protein, and maybe an additional small subunit, whereas the Q _i center is formed by theb-562 domain of cytochromeb and presumably the 13.4kDa protein (QP-C). The Q binding proteins are proposed to be protein subunits of the Q reaction centers of various multiprotein complexes. The path of electron flow branches at the Q₀ center, half of the electrons flowing via the high-potential cytochrome chain to oxygen and half of the electrons cycling back into the Q pool via the cytochromeb path connecting the two Q reaction centers. During oxidation of QH₂, 2H⁺ are released to the cytosolic space and during reduction of Q, 2H⁺ are taken up from the matrix side, resulting in a net transport across the membrane of 2H⁺ per e^– flown from QH₂ to cytochromec, the H⁺ being transported across the membrane as H (H⁺ + e^–) by the mobile carrier Q. The authors correct their earlier view of cytochromeb functioning as a H⁺ pump, proposing that the redox-linkedpK changes of the acidic groups of cytochromeb are involved in the protonation/deprotonation processes taking place during the reduction and oxidation of Q. The reviewers stress that cytochromeb is in equilibrium with the Q pool via the Q _i center, but not via the Q₀ center. Their view of the mechanisms taking place at the reductase is a Q cycle linked to a Q-pool where cytochromeb is acting as an electron pump.     

Genetic diversity and environmental associations of wild barley,Hordeum spontaneum (Poaceae), in Iran

Genetic diversity and structure of populations of the wild progenitor of barleyHordeum spontaneum in Iran was studied by electrophoretically discernible allozymic variation in proteins encoded by 30 gene loci in 509 individuals representing 13 populations of wild barley. The results indicate that: a)Hordeum spontaneum in Iran is extremely rich genetically but, because of predominant self-pollination, the variation is carried primarily by different homozygotes in the population. Thus, genetic indices of polymorphismP-1% = 0.375, range = 0.267–0.500, and of genetic diversity,He = 0.134, range = 0.069–0.198, are very high. b) Genetic differentiation of populations includes clinal, regional and local patterns, sometimes displaying sharp geographic differentiation over short distances. The average relative differentiation among populations isGst = 0.28, range = 0.02–0.61. c) A substantial portion of the patterns of allozyme variation in the wild gene pool is significanctly correlated with the environment and is predictable ecologically, chiefly by combinations of temperature and humidity variables. d) The natural populations studied, on the average, are more variable than two composite crosses, and more variable than indigenous land races of cultivated barely,Hordeum vulgare, in Iran. — The spatial patterns and environmental correlates and predictors of genetic variation ofH. spontaneum in Iran indicate that genetic variation in wild barley populations is not only rich but also at least partly adaptive. Therefore, a much fuller exploitation of these genetic resources by breeding for disease resistance and economically important agronomic traits is warranted.     

A jump in an Arrhenius plot can be the consequence of a phase transition. The binding of ATP to myosin subfragment 1

The temperature dependence of the kinetics of the binding of ATP to myosin subfragment-1 was studied by an ATP chase technique in a rapid-flow-quench apparatus: (formula; see text) A temperature range of 30 degrees C to -15 degrees C was obtained with ethylene glycol as antifreeze. The Arrhenius plot of k2 is discontinuous with a jump at 12 degrees C. Above the jump delta H+ = 9.5 kcal/mol, below delta H+ = 28.5 kcal/mol. Few such Arrhenius plots are recorded in the literature but they are predicted from theory. Thus, we explain our results as a phase change of the subfragment 1-ATP system at 12 degrees C. This is in agreement with certain structural studies.     

Relationships of the chromosomal species in the eurasian mole rats of theSpalax ehrenbergi group as determined by DNA-DNA hybridization,and an estimate of the spalacid-murid divergence time

Summary DNA-DNA hybridization was used to measure the average genomic divergence among the four chromosomal species of the Eurasian mole rats belonging to theSpalax ehrenbergi complex (Rodentia: Spalacidae). The percent nucleotide substitutions in the single-copy nuclear DNA among the species ranged from 0 to 5%, suggesting that speciation has occurred with minor genomic changes in these animals. The youngest chromosomal species appear to differ by 0.2–0.6% base pair mismatch, which is only between one and three base differences in a 500-bp fragment. The interspecific values of percent nucleotide differences permit the recognition of two well-separated speciation events in theS. ehrenbergi complex, the older (of Lower Pleistocene age) having isolated the chromosomal species 2n=54 before the divergence of the three other species.DNA-DNA hybridization was also used to compare the Spalacinae (Eurasian mole rats), Murinae (Old World rats and mice), and Arvicolinae (voles and lemmings). These data enabled us to estimate the time of divergence of the spalacids at ca. 19 million years ago. The dates of divergence among the other rodent lineages, as predicted by DNA hybridization results, agree well with paleontological data. These dates of divergence are obtained by the relation between geological time and single-copy nuclear DNA change, a relation that was calibrated by Catzeflis et al. (1987) through the use of fossil Arvicolinae and Murinae data.     

Unsuspected cryptic polymorphism in the polychromatic Midas cichlid.

The Midas cichlid ( Cichlasoma citrinellum ) of Nicaragua shows stable polychromatism in that a consistent, small proportion of the fish in nature lose their melanophores and thereby become gold-coloured. Before metamorphosis, all fish look alike. We asked whether the pre-transformed offspring of gold and normal morphs might differ in their morphological and physiological responses to white and to black backgrounds despite looking the same. More chromatophores developed in all juveniles exposed for two weeks to a black than to a white background. When moved from one background to the other, the fish adjusted to white faster than to black. Most importantly, genetically normal fish (N) colour-matched quicker than genetically gold fish (G) irrespective of the direction of change. Adjustment in G was also more variable, indicating poorer control. A parallel pattern was recorded photometrically in melanophores of excised scales: aggregation of pigment was faster than dispersion. And pigment in the melanophores of N aggregated and dispersed swifter than did those of G. Therefore, pre-metamorphosed juvenile G could be more vulnerable to predation than are juvenile N. This reveals a possible source of selection against the offspring of gold morphs before they become gold-coloured.