在不同的光学调制度水平下测定的对比敏感度函数

前文中,我们根据小尺寸矩形孔的 MTF(T_□)在高频部分的下降比对比敏感度函数(Tcs)下降快得多的事实,揭示了随空间频率(SF)的增加,网膜-大脑系统的 MTF(Trb)表现愈趋明显的补偿作用。     

古植物学在解释中国植物地理上的潜力

中国许多植物分类群的现代分布证明了一些区系重要性中心可能是古老的并且具有进化的意义,或者这些中心可能仅仅是它们在新生代迁移历史中最后的一些场所。本文概述白刺属(Nitraria),麻黄属(Ephedra),山核桃属(Carya),枫香树属(Liquidambar),枫杨属(Pterocarya)以及桦木科(Betu-laceae)这些分类群在中国的新生代和现在的分布。结论是:现在这些属的重要性中心不同于它们在新生代某个地质世代的;每个分类群在新生代中都经历过较大规模的迁移和遭受过具有重要意义的一些消灭作用的影响。桦木科的演变历史虽然在主要方面已经解决但还是不很明朗。     

Sex differences in the electrocommunication signals of the electric fish Apteronotus bonapartii

The South American weakly-electric knifefish (Apteronotidae) produce highly diverse and readily quantifiable electrocommunication signals. The electric organ discharge frequency (EODf), and EOD modulations (chirps and gradual frequency rises (GFRs)), vary dramatically across sexes and species, presenting an ideal opportunity to examine the proximate and ultimate bases of sexually dimorphic behavior. We complemented previous studies on the sexual dimorphism of apteronotid communication signals by investigating electric signal features and their hormonal correlates in Apteronotus bonapartii, a species which exhibits strong sexual dimorphism in snout morphology. Electrocommunication signals were evoked and recorded using a playback paradigm, and were analyzed for signal features including EOD frequency and the structure of EOD modulations. To investigate the androgenic correlates of sexually dimorphic EOD signals, we measured plasma concentrations of testosterone and 11-ketotestosterone. A. bonapartii responded robustly to stimulus playbacks. EODf was sexually monomorphic, and males and females produced chirps with similar durations and amounts of frequency modulation. However, males were more likely than females to produce chirps with multiple frequency peaks. Sexual dimorphism in apteronotid electrocommunication signals appears to be highly evolutionarily labile. Extensive interspecific variation in the magnitude and direction of sex differences in EODf and in different aspects of chirp structure suggest that chirp signals may be an important locus of evolutionary change within the clade. The weakly-electric fish represent a rich source of data for understanding the selective pressures that shape, and the neuroendocrine mechanisms that underlie, diversity in the sexual dimorphism of behavior.     

格网尺度下典型旅游城市生态服务价值估算和时空分异特征——以三亚为例

以典型旅游城市三亚市为案例地，利用2006-2018年4期Landsat遥感影像数据，借助ENVI、ArcGIS平台定量识别土地利用演变特征，在1km×1km格网尺度下估算旅游地生态系统服务价值，并结合空间探索性数据分析揭示生态系统服务价值时空分异特征及其与旅游地发展的时空耦合关系。结果表明：（1）2006-2018年间，三亚市生态系统服务价值总量呈逐年下降趋势，由6.73×10⁹元降至5.76×10⁹元，累计减少9.78×10⁸元；（2）空间格局上，三亚市呈"南低北高"空间分异格局，2006-2018年增值区域连片分布于崖州区、天涯区、吉阳区南部区域，且呈逐年减少趋势，减值区域集聚于天涯区东北部、海棠区；（3）空间集聚上，生态系统服务价值截面各年份均呈显著空间正相关且相关性先降后增。高高集聚区位于天涯区北部区域，低低集聚区分布于沿海、海湾地区；（4）旅游发展与生态系统服务价值时空演化特征关联性较强。三亚市天涯区北部林地生态环境良好，生态系统服务价值略有下降但绝对数值稳定，是生态系统服务价值主要来源；旅游发展较为迅速的三亚湾、崖州湾以及海棠湾，相对增值区域较多，但绝对生态系统服务价值损失显著，严重滞后于其他区域。     

分裂中期的CHO细胞能对其染色体DNA进行切除修补吗?

以中国仓鼠细胞CHO-K_1为材料,用放射自显术检查紫外线照射后细胞内DNA的切除修补,没有得到足以证明分裂中期细胞对其染色体DNA进行切除修补的可靠证据。此结果与1977年Ikushima所报告的不同。对于这种修补能力的缺乏,作者认为应考虑到的原因至少有:(1)分裂中期染色体上的DNA处在高度紧密的状态,难于进行切除修补;(2)分裂中期细胞中担任DNA修补合成的多聚酶β的活性可能下降;(3)分裂中期细胞的核膜解体。