THE SELECTIVE ADVANTAGE OF CRYPSIS IN MICE

The light color of mice that inhabit the sandy dunes of Florida's coast have served as a textbook example of adaptation for nearly a century, despite the fact that the selective advantage of crypsis has never been directly tested or quantified in nature. Using plasticine mouse models of light and dark color, we demonstrate a strong selective advantage for mice that match their local background substrate. Further our data suggest that stabilizing selection maintains color matching within a single habitat, as models that are both lighter and darker than their local environment are selected against. These results provide empirical evidence in support of the hypothesis that visual hunting predators shape color patterning in Peromyscus mice and suggest a mechanism by which selection drives the pronounced color variation among populations.     

ON THE EVOLUTION OF PHENOTYPIC PLASTICITY IN A SPATIALLY HETEROGENEOUS ENVIRONMENT

A genetic model for the dynamics of a quantitative trait is analyzed in terms of gene frequencies, linkage disequilibria, and environmental effects on the trait. In a randomly mating population, at each generation progeny move to niches where they are subject to weak Gaussian selection on the trait, with different fitness levels in the different niches. Initially, the variability of the trait is due to additive loci with heterozygous homeostasis. The evolution of plasticity is then described in terms of the invasion of the population by genetic modifiers that may epistatically affect the trait, its optimum in each niche, the strengths of selection, and other parameters characteristic of the niches. We show that the evolution of trait means within niches depends on the overall evolution in the whole system, and in general, optimum phenotypic values are not attained. The reaction norm and genotype-environment interaction may evolve even if the only effects of the modifier are on individual rates of dispersal, or on fitness effects resulting from the different environments in the different niches; this evolution does not require that the modifier affect parameters that influence the values of the trait. It is conjectured that in the least frequently reached niches with low fitness levels, the deviations from the trait optima should be larger than those in more commonly experienced and less stringent niches. Our analysis makes explicit the different contribution of between- and within-niche effects on the evolutionary dynamics of phenotypic plasticity in heterogeneous environments.     

ADAPTIVE DYNAMICS IN ALLELE SPACE: EVOLUTION OF GENETIC POLYMORPHISM BY SMALL MUTATIONS IN A HETEROGENEOUS ENVIRONMENT

We demonstrate how a genetic polymorphism of distinctly different alleles can develop during long-term frequency-dependent evolution in an initially monomorphic diploid population, if mutations have only small phenotypic effect. As a specific example, we use a version of Levene's (1953) soft selection model, where stabilizing selection acts on a continuous trait within each of two habitats. If the optimal phenotypes within the habitats are sufficiently different, then two distinctly different alleles evolve gradually from a single ancestral allele. In a wide range of parameter values, the two locally optimal phenotypes will be realized by one of the homozygotes and the heterozygote, rather than by the two homozygotes. Unlike in the haploid analogue of the model, there can be multiple polymorphic evolutionary attractors with different probabilities of convergence. Our results differ from the population genetic models of short-term evolution in two aspects: (1) a polymorphism that is population genetically stable may be invaded by a new mutant allele and, as a consequence, the population may fall back to monomorphism, (2) long-term evolution by allele substitutions may lead from a population where polymorphism is not possible into one where polymorphism is possible.     

THE EFFECT OF NICHE PREFERENCE ON POLYMORPHISM PROTECTION IN A HETEROGENEOUS ENVIRONMENT

A model is proposed in which niche choice precedes natural selection in an individual's life span; the panmictic population occupies an environment divided into niches, each contributing a constant proportion of parents to the next generation. Niche choice and fitness within the niche are controlled either by the same locus or by two linked loci. The conditions for a protected polymorphism are derived. It is shown that niche preferences can increase the protective effect of natural selection over a polymorphism. This effect depends on the existence of a positive correlation between preference for a niche and relative fitness within that niche, and also on the relative size of the niches. In the absence of within-niche fitness differences, alleles that cause preference for different niches can still be protected. Alleles that determine preference for niches contributing little to the total population can be eliminated by the ability to choose between niches.     

INTENSE SELECTION OF MITE CLONES IN A HETEROGENEOUS ENVIRONMENT

Genetic diversity within obligatorily parthenogenic species can be high, contrary to common views of the evolutionary consequences of this reproductive system. How this clonal diversity is maintained within populations is not clearly understood. Previous studies showing high clonal diversity have used parthenogenic organisms with known sexual forms or relatives. Here we report significant spatial and temporal clonal diversity within two populations of an obligatorily parthenogenic mite, Penthaleus major, which has no known sexual form or close sexual relative. Fitness estimates from temporal sampling at two sites and manipulated field plots reveal intense natural selection acting on ecologically different clones. We propose that environmental heterogeneity contributes to the maintenance of clonal diversity within populations of P. major and that selection is strong enough to overcome the problems of relative niche size.     

PARALLEL EVOLUTION AT MULTIPLE LEVELS IN THE ORIGIN OF HUMMINGBIRD POLLINATED FLOWERS IN IPOMOEA

A transition in flower color accompanying a shift in pollinator guilds is a prominent and repeated adaptation in angiosperms. In many cases, shifts to similar pollinators are associated with similar flower‐color transitions. The extent to which this parallelism at the phenotypic level results from parallel changes at the biochemical, developmental, and genetic levels, however, remains an open question. There have been few attempts to determine whether parallelism at these lower levels results from mutation bias or fixation bias of different classes of mutation. We address these issues by examining the biochemical, developmental, and genetic changes that have occurred in red‐flowering species of the Mina lineage of morning glories (Ipomoea) and compare these to the changes reported for I. horsfalliae, which has independently evolved red flowers. Using transgenic techniques, we demonstrate that the transition from blue to red flowers in Mina species is due primarily to down‐regulation of the enzyme flavonol‐3′‐hydroxylase (F3′H) in flowers but not in vegetative tissues, and that this down‐regulation is at least partly due to cis‐regulatory change in the gene for F3′H. These changes are similar to those exhibited by I. horsfalliae, indicating parallelism at the biochemical and developmental levels, and possibly at the genetic level.     

EVIDENCE FOR BATESIAN MIMICRY IN A POLYMORPHIC HOVERFLY

Palatable Batesian mimics are avoided by predators because they resemble noxious or defended species. The striking resemblance of many hoverflies to noxious Hymenoptera is a “textbook” example of Batesian mimicry, but evidence that selection by predators has shaped the evolution of hoverfly patterns is weak. We looked for geographical and temporal trends in frequencies of morphs of the polymorphic hoverfly Volucella bombylans that would support the hypothesis that these morphs are Batesian mimics of different bumblebee species. The frequency of the black and yellow hoverfly morph was significantly positively related to the frequency of black and yellow bumblebees across 52 sites. Similarly, the frequency of the red‐tailed hoverfly morph was positively related to the frequency of red‐tailed bumblebees. However, the frequencies of hoverfly morphs were positively spatially autocorrelated, and after controlling for this, only one of the two common hoverfly morphs showed a significant positive relationship with its putative model. We conclude that the distribution of V. bombylans morphs probably reflects geographical variation in selection by predators resulting from differences in the frequencies of noxious bumblebee species.     

THE ORIGIN OF AFRO-ARABIAN 'DIDELPHIMORPH' MARSUPIALS

Abstract:  New specimens of Peratherium africanum from Early Oligocene deposits of the Fayum, Egypt, provide key information on the relationships of the species. These include the first maxilla to be found and two additional dentaries. The maxilla can be demonstrated to belong to the same species as the holotype dentary by study of the occlusal relationships of upper and lower molars. It can be shown by several synapomorphies that P. africanum is the sister species to European Bartonian–Rupelian Peratherium lavergnense . P. africanum therefore belongs to the 'didelphimorph' family Herpetotheriidae, not to the peradectimorph family Peradectidae. The genus Qatranitherium , previously erected for this species alone, is here synonymized with Peratherium . Comparison with 'didelphimorphian' taxa from early Paleogene deposits of South America suggests more remote relationships, indicating an origin for P. africanum by dispersal from Europe as originally envisaged. The more precise relationships deduced here help to constrain the time interval for dispersal to Afro-Arabia, probably during the earliest Oligocene sea-level low.     

猫耳蜗电图中N_2波起源的分析

在35只猫进行了耳蜗电图、听觉脑干电反应及耳蜗核局部电位的同时描记,将普鲁卡因或海人酸微量注入耳蜗核内,观察电位的变化,以分析耳蜗电图中N_2 波的起源。实验结果表明:猫的 N_2 波来源于外周第一级神经元冲动的成分和耳蜗核电活动的成分。     

THE EVOLUTION OF BIPEDALISM IN JERBOAS (RODENTIA: DIPODOIDEA): ORIGIN IN HUMID AND FORESTED ENVIRONMENTS

Mammalian bipedalism has long been thought to have arisen in response to arid and open environments. Here, we tested whether bipedalism coevolved with environmental changes using molecular and paleontological data from the rodent superfamily Dipodoidea and statistical methods for reconstructing ancestral characteristics and past climates. Our results show that the post‐Late Miocene aridification exerted selective pressures on tooth shape, but not on leg length of bipedal jerboas. Cheek tooth crown height has increased since the Late Miocene, but the hind limb/head‐body length ratios remained stable and high despite the environmental change from humid and forested to arid and open conditions, rather than increasing from low to high as predicted by the arid‐bipedalism hypothesis. The decoupling of locomotor and dental character evolution indicates that bipedalism evolved under selective pressure different from that of dental hypsodonty in jerboas. We reconstructed the habitats of early jerboas using floral and faunal data, and the results show that the environments in which bipedalism evolved were forested. Our results suggest that bipedalism evolved as an adaptation to humid woodlands or forests for vertical jumping. Running at high speeds is likely a by‐product of selection for jumping, which became advantageous in open environments later on.     

铅致大鼠记忆受损及海马AChE阳性纤维的改变

本实验采用明暗箱被动回避反应和穿梭箱条件性回避反应测定, 探讨了铅对大鼠学习与记忆的损害作用。应用酶组织化学方法观察了铅致大鼠海马的结构改变。行为测定显示, 大鼠饮１０％ 醋酸铅水液之前和之后, 其进洞潜伏期 （ｓｔｏｐｔｈｒｏｕｇｈ ｌａｔｅｎｃｙ, ＳＴＬ） 与对照组无明显差异, 而饮醋酸铅水液后, 其条件性回避反应率（ｃｏｎｄｉｔｉｏｎｅｄ ａｖｏｉｄａｎｃｅ ｒｅｓｐｏｎｓｅ ｒａｔｅ, ＣＡＲＲ） 较对照组明显下降（ｄ４－ ｄ１４, Ｐ＜ ００５）, 表明大鼠饮醋酸铅水液后长时记忆不受影响, 而短时记忆受到损伤。组织化学反应染色显示实验组海马腔隙分子层内ＡＣｈＥ阳性纤维面积百分数较对照组明显减少（Ｐ＜ ００５）, 且纤维变细, 膨体减少。推测铅致大鼠短时记忆能力下降, 可能与海马内胆碱能纤维减少有关。     

EVOLUTIONARY RESCUE OF SEXUAL AND ASEXUAL POPULATIONS IN A DETERIORATING ENVIRONMENT

The environmental change experienced by many contemporary populations of organisms poses a serious risk to their survival. From the theory of evolutionary rescue, we predict that the combination of sex and genetic diversity should increase the probability of survival by increasing variation and thereby the probability of generating a type that can tolerate the stressful environment. We tested this prediction by comparing experimental populations of Chlamydomonas reinhardtii that differ in sexuality and in the initial amount of genetic diversity. The lines were serially propagated in an environment where the level of stress caused by salt increased over time from fresh water to the limits of marine conditions. In the long term, the combination of high diversity and obligate sexuality was most effective in supporting evolutionary rescue. Most of the adaptation to high‐salt environments in the obligate sexual‐high diversity lines had occurred by midway through the experiment, indicating that positive genetic correlations of adaptation to lethal stress with adaptation to sublethal stress greatly increased the probability of evolutionary rescue. The evolutionary rescue events observed in this study provide evidence that major shifts in ways of life can arise within short time frames through the action of natural selection in sexual populations.     

大鼠尾壳核的GABA能传递在条件性行为调控中的作用

本工作采用了行为和脑内注射相结合的方法研究了大鼠尾壳核的 GABA 能传递在条件性行为调控中的作用。在分辨学习的基础上训练大鼠完成条件性回避任务,以比较药物对分辨学习和条件性回避的不同效应。实验结果表明,于大鼠双侧尾壳核内分别注入 γ-氨基丁酸(GABA)(每侧100μg/μl)和 GABA 受体激动剂蝇蕈醇(Muscimol)(每侧0.1μg/μl)后可暂时抑制条件性回避反应的出现,但分辨学习无明显影响。作为对照,于尾壳核内注入等量的生理盐水则既不影响条件性回避反应,也不影响分辨学习。在条件性回避反应被 Muscimol抑制后于尾壳核内再注入 GABA 受体阻断剂印防己毒素(PTX)(每侧0.1μg/μl)则可拮抗Muscimol 的行为抑制效应,即条件反应的出现率可恢复到或接近注射前水平。实验结果表明,大鼠尾壳核的 GABA 能传递在条件性行为调控中的重要作用。     

扬子区中、晚奥陶世和兰多维利世四射珊瑚起源、扩散及生物古地理关系

扬子区中奥陶世—志留纪兰多维利世四射珊瑚产出丰富 ,尤以兰多维利世最为繁盛。目前已报道的1 2 3属 (包括中奥陶世 4属 ,晚奥陶世 2 5属 ,兰多维利世 94属 )中 ,有 30属最早出现在扬子区 ,尔后扩散到欧洲、北美和其它邻区。例如 :Calostylis最早出现在川南的中奥陶世 (Llandeilo)地层中 ;Aphyllum和Cantrillia最早出现在浙西的晚奥陶世中Ashgill地层 ;扭心珊瑚类Briantelasma ,Pycnactis和Tunguselasma等 ,最早出现在黔东北晚Ruddanian地层 ;柱珊瑚类Ceriaster、Stauria、Amplexoides、Synamplexoides等 ,泡沫珊湖类Maikottia ,Rhizophyl lum等均最早出现在黔东北的中兰多维利统 (MiddleLlandovery)。根据这些资料 ,我们认为扬子区应该是奥陶纪和志留纪四射珊瑚起源中心之一。文中论述扬子区中奥陶世—兰多维利世四射珊瑚动物群特征及其生物古地理关系。该区中奥陶世珊瑚以穿孔珊瑚类的Calostylis和Yohophyllum为特征。下扬子区浙西晚奥陶世三衢山组 (中Ashgill)四射珊瑚群有某些澳大利亚分子Hillophyllum和Bowanophyllum ;而上扬子区晚Ashgill观音桥层的四射珊瑚动物群与北欧同期珊瑚群有高度的相似性。这表明扬子区当时与欧洲具有较密切的古生物地理关系。扬子区兰多维利世四射珊瑚群与西伯利亚。     

豚鼠听觉诱发电位中潜伏期反应Pa波起源的研究

将豚鼠大脑皮层划分为五个区,采用大脑皮层吸除破坏与海人酸局部浸润破坏相结合的方法,研究了MLR的起源以及皮层各区域与MLR的关系。结果表明,破坏听区以外各区域,对MLR影响不大,一旦破坏听区皮层,MLR主波Pa立即消失,并且不再恢复。海人酸局部浸润破坏不仅进一步证明Pa波来源于听区皮层,而且还表明Pa波主要来源于听区皮层内神经无电活动,与皮层下听觉中枢及其向听区皮层投射的纤维无关。     

DENSITY DEPENDENCE AND UNPREDICTABLE SELECTION IN A HETEROGENEOUS ENVIRONMENT: COMPROMISE AND POLYMORPHISM IN THE ESS REACTION NORM

In quantitative genetic models of the evolution of reaction norms, an individual is selected in the habitat in which it develops; as a consequence, selection leads to the optimum phenotype in each habitat. Here, individuals are assumed to experience unpredictable habitat change between development and selection, so that the environment in which an individual is selected may differ from the environment in which it developed. The model reveals that unpredictability of the selection an individual actually faces leads to the evolutionarily stable bet-hedging reaction norm constituting a compromise between the phenotypic optima in the different patches. We also examine the effect of local density regulation before selection, in the patches in which the individuals develop, and after selection, in the patches in which they are selected. Density regulation before selection has a much lower influence on the evolution of the reaction norm than density regulation after selection. The source-sink structure of the environment caused by differential productivity of patches strongly affects how the compromise bet-hedging strategy weighs the different phenotypic optima and might compromise the local evolutionary stability of the evolved reaction norm. If the strength and variability among patches of density regulation after selection is sufficiently large, no single reaction norm is evolutionary stable: Polymorphic reaction norms constitute the evolutionarily stable population. We also show that a polymorphic reaction norm is more likely to be observed in a less productive habitat. The relations between the present model and the Dempster and the Levene models are discussed.     

STRUCTURE AND POPULATION GENETICS OF THE BREAKPOINTS OF A POLYMORPHIC INVERSION IN DROSOPHILA SUBOBSCURA

Drosophila subobscura is a paleartic species of the obscura group with a rich chromosomal polymorphism. To further our understanding on the origin of inversions and on how they regain variation, we have identified and sequenced the two breakpoints of a polymorphic inversion of D. subobscura—inversion 3 of the O chromosome—in a population sample. The breakpoints could be identified as two rather short fragments (～300 bp and 60 bp long) with no similarity to any known transposable element family or repetitive sequence. The presence of the ～300‐bp fragment at the two breakpoints of inverted chromosomes implies its duplication, an indication of the inversion origin via staggered double‐strand breaks. Present results and previous findings support that the mode of origin of inversions is neither related to the inversion age nor species‐group specific. The breakpoint regions do not consistently exhibit the lower level of variation within and stronger genetic differentiation between arrangements than more internal regions that would be expected, even in moderately small inversions, if gene conversion were greatly restricted at inversion breakpoints. Comparison of the proximal breakpoint region in species of the obscura group shows that this breakpoint lies in a small high‐turnover fragment within a long collinear region (～300 kb).     

竞争对马尾松和木荷觅取异质分布养分行为的影响

为了研究邻株竞争对马尾松(Pinus massoniana)和木荷(Schima superba)觅取异质分布养分机理和行为的影响, 我们设计单植、双株纯栽和两株混植3种栽植方式, 构建了同质和异质养分环境开展盆栽实验。结果表明: 单植时, 马尾松和木荷苗木生长对斑块养分反应敏感, 与同质养分环境相比, 两树种在异质养分环境中具有苗高生长量大、干物质积累量高、根系在富养斑块中大量增生, 根系N、P含量和吸收效率高等特点。在异质养分环境中, 木荷与马尾松邻株竞争时的生长表现优于双株纯栽模式而与单植处理相近, 根系形态可塑性和生理可塑性在其觅取斑块养分中的作用显著增强; 与木荷邻株竞争时, 马尾松苗高生长也表现出较单植和双株纯栽模式一定的优势, 这与其根系的广布性、觅养精确性和反应敏感度变化较小及富养斑块中根系P素含量和吸收效率较高等有关。相反, 同种邻株竞争则使得异质养分环境中马尾松和木荷的根系广布性减小, 反应敏感度减弱, 富养斑块中根系N、P含量降低, 苗高生长量和干物质积累量减小。与马尾松相比, 同种邻株竞争对异质养分环境中木荷生长的负向影响更为强烈。建议在生产中采用混交造林的方式促进马尾松和木荷生长。若要营造人工纯林, 可通过适当降低初植密度或及时调控林分密度促进林木生长。     

THE DETECTION OF PLASTICITY GENES IN HETEROGENEOUS ENVIRONMENTS

The molecular genetic mechanisms for phenotypic plasticity across heterogeneous macro- and microenvironments were examined using the Populus genomic map constructed by DNA-based markers. Three hypotheses have been suggested to explain genetic variation in phenotypic response to varying environments (i.e., reaction norm): Lerner's homeostasis, allelic sensitivity, and gene regulation. The homeostasis hypothesis, which predicts that heterozygotes are less sensitive to the environment than homozygotes, was supported for phenotypic plasticity to unpredictable environments (microenvironmental plasticity) at the whole-genome level, but for phenotypic plasticity to predictable environments (macroenvironmental plasticity) the hypothesis was supported only at functioning quantitative trait loci (QTLs). For all growth traits studied, gene regulation was suggested to play a prevailing role in determining the norms of reaction to environments. Indirect evidence for gene regulation is that there tend to be more QTLs with larger effects on the phenotype in optimal growing conditions than suboptimal growing conditions because the expression of these QTLs identified is mediated by regulatory genes. Direct evidence for gene regulation is the identification of some loci that differ from QTLs for trait values within environments and exert an environmentally dependent control over structural gene expression. In this study, fewer environmentally sensitive QTLs were detected that display unparalleled allelic effects across environments. For stem height, there were more regulatory loci and more structural loci (whose expression is determined by gene regulation) affecting phenotypic plasticity than for basal area. It was found that microenvironmental plasticity was likely controlled by different genetic systems than those for macroenvironmental plasticity.