华山新麦草开花物候期观测和自然种群基因流的间接估测

华山新麦草（Psathyrostachys huashanica keng)为我国特有种,仅分布在陕西华山。通过对自然种群传粉物候期观测发现,华山新麦草的传粉高峰期与海拔有一定关系。海拔每升高200m,传粉物候期就推迟2－3d;海拔差异超过500m的亚种群,传粉物候期就不会出现重叠。同时应用等位酶分析技术和E－统计分析间接估测华山新麦草自然种群的基因流（Nm）,其值较一般风媒传粉植物低。通过华山新麦草资源状况、生长环境的调查以及移栽试验的观察,认为该物种稀的主要原因之一 可能是其生存竞争能力弱,最终被迫生长在其它物种所不能生存的环境。     

GENETIC MODELS OF ADAPTATION AND GENE FLOW IN PERIPHERAL POPULATIONS

We investigate the interplay between gene flow and adaptation in peripheral populations of a widespread species. Models are developed for the evolution of a quantitative trait under clinally varying selection in a species whose density decreases from the center of the range to its periphery. Two major results emerge. First, gene flow from populations at the range center can be a strong force that inhibits peripheral populations from evolving to their local ecological optima. As a result, peripheral populations experience persistent directional selection. Second, response to local selection pressures can cause rapid and substantial evolution when a peripheral population is isolated from gene flow. The amount of evolutionary change depends on gene flow, selection, the ecological gradient, and the trait's heritability. Rapid divergence can also occur between the two halves of a formerly continuous population that is divided by a vicariant event. A general conclusion is that disruption of gene flow can cause evolutionary divergence, perhaps leading to speciation, in the absence of contributions from random genetic drift.     

GENE EXCHANGE IN LOBLOLLY PINE: THE RELATION BETWEEN POLLINATION MECHANISM,FEMALE RECEPTIVITY AND POLLEN AVAILABILITY

The pollination process in loblolly pine has been examined over several years, both in the field (seed orchards) and experimentally on greenhouse-grown material. Female strobili are receptive to pollination for periods of a wk or more. Initially, background pollen from outside the seed orchard is the main source of pollen but as peak receptivity approaches, pollen from the stand itself predominates especially in older orchards. Consequently, strobili can receive pollen both from outside the orchard as well as from within. The pollen lands on the micropylar horns where it is transferred through the micropyle onto the nucellus by either rainfall or the pollen drop, whichever comes first. Since the pollen drop does not occur until the latter part of the receptive period, rainfall is the most likely transfer agent and pollen flotation is vital if rain occurs. Early arriving pollen does not appear to have an advantage over later arriving pollen for uptake onto the nucellus, even if rain follows the first pollination immediately. Therefore, total pollination of the strobilus can result from both distant and nearby pollen sources.     

GENETIC DIFFERENTIATION AND HETEROZYGOSITY IN PINYON PINE ASSOCIATED WITH RESISTANCE TO HERBIVORY AND ENVIRONMENTAL STRESS

Arizona's Sunset Crater began erupting in 1064 AD and for the next 200 years buried over 2,000 km² in ash, cinders, and lava. Soil analyses indicate that pinyon pines (Pinus edulis) currently colonizing the cinder fields are faced with a highly stressful environment. Many of these pinyons suffer chronic, intense insect herbivory that reduces plant growth and eliminates female cone production. In contrast, herbivory among pinyons growing in neighboring sandy-loam soils is minimal. Furthermore, numerous trees within the heavily infested cinder field population suffer relatively low herbivory and maintain normal growth and reproduction. We used four polymorphic enzymes to examine the relationship between herbivore attack, environmental stress and genotypes of the adjacent cinder field, and sandy-loam soil pinyon populations. Our results demonstrate that 1) resistant trees display significant genetic differences and are more heterozygous for two enzymes associated with herbivory than susceptible trees; and 2) the cinder-soil pinyons exhibit significant genetic differences and are more heterozygous for an enzyme associated with environmental stress than the neighboring sandy-loam soil pinyons. We conclude that heterozygosity of specific or closely linked loci may facilitate pinyon resistance to herbivory and environmental stress, and that strong selection across narrow geographic boundaries resulted in rapid genetic differentiation of pinyon populations.     

SPATIAL SCALE OF GENETIC STRUCTURE AND AN INDIRECT ESTIMATE OF GENE FLOW IN EELGRASS,ZOSTERA MARINA

In this study, the first investigation of population structure in an aquatic angiosperm, I show that populations of a marine angiosperm (eelgrass, Zostera marina) are genetically differentiated at a number of spatial scales. I find also that there is no correspondence between geographic and genetic distances separating subpopulations, an increasingly common result in spatially stratified studies of genetic structure in marine invertebrates. F-statistics, calculated for two years from electrophoretic variation at five polymorphic allozyme loci, indicate significant genetic differentiation among sampling quadrats within each of two bays (θ = 0.064-0.208), between tide zones within a bay (θ = 0.025-0.157) and between bays (θ = 0.079). Spatial autocorrelation analysis was used to explore genetic differentiation at smaller spatial scales; estimated patch sizes (within which genetic individuals are randomly associated) indicated no appeciable genetic structure at scales less than 20 m × 20 m. Calculated values of F-statistics were a function of the spatial scale from which samples were drawn: increasing the size of the “subpopulation” included in calculation of fixation indices for the same “total” sample resulted in an increase in the magnitude of f (e.g., from 0.092 to 0.181) and a decrease in θ (e.g., from 0.186 to 0.025). On the basis of the best estimate of the spatial scale of subpopulations, the effective number of migrants per generation (N_em) ranges from 1.1 to 2.8. Genetic consequences of the disturbance regime in the eelgrass habitat sampled were extreme variation between years in the allele richness and proportion of heterozygotes in a sample and a positive relationship between the extinction probability of patches and the genetic variance among them. The changes in F-statistics as a function of sampling scale and the observation that θ among sampled quadrats was positively associated with the probability of extinction among quadrats indicated that indirect estimates of gene flow (N_em) calculated from θ should be cautiously interpreted in populations that may not yet be in drift-migration equilibrium.     

HISTORICAL SEPARATION AND PRESENT GENE FLOW THROUGH A ZONE OF SECONDARY CONTACT IN PONDEROSA PINE

I examined the effects of historical division and secondary contact between eastern and western varieties of ponderosa pine (Pinus ponderosa Laws Pinaceae) on extant patterns of genetic variation. Fossil and biogeographic evidence both indicate that the current point of contact between these two varieties represents secondary contact following historical separation during the Wisconsin glaciation. Current gene flow was assessed by observing the degree of introgression of paternally inherited cpDNA and maternally inherited mtDNA polymorphisms. Both seeds and pollen are wind dispersed in ponderosa pine. Introgression was primarily from west to east, the direction of the prevailing wind, for both organelles, but introgression of cpDNA far exceeded that of mtDNA. Thus pollen is the main agent of contemporary gene flow between the two varieties. Neither seeds nor pollen showed enough introgression since secondary contact to have homogenized the two gene pools. However, allozyme differentiation was minimal. This calls into question assumptions of selective neutrality for at least some of the markers. Theory predicts that nuclear markers will show a high locus-to-locus variance of F_ST following historical separation. This prediction is confirmed by the allozyme data for ponderosa pine, and may provide a useful means of identifying historical separations from allele frequency data.     

INTROGRESSION IN THE DROSOPHILA SUBOBSCURA—D. MADEIRENSIS SISTER SPECIES: EVIDENCE OF GENE FLOW IN NUCLEAR GENES DESPITE MITOCHONDRIAL DIFFERENTIATION

Species hybridization, and thus the potential for gene flow, was once viewed as reproductive mistake. However, recent analysis based on large datasets and newly developed models suggest that gene exchange is not as rare as originally suspected. To investigate the history and speciation of the closely related species Drosophila subobscura, D. madeirensis, and D. guanche, we obtained polymorphism and divergence data for 26 regions throughout the genome, including the Y chromosome and mitochondrial DNA. We found that the D. subobscura X/autosome ratio of silent nucleotide diversity is significantly smaller than the 0.75 expected under neutrality. This pattern, if held genomewide, may reflect a faster accumulation of beneficial mutations on the X chromosome than on autosomes. We also detected evidence of gene flow in autosomal regions, while sex chromosomes remain distinct. This is consistent with the large X effect on hybrid male sterility seen in this system and the presence of two X chromosome inversions fixed between species. Overall, our data conform to chromosomal speciation models in which rearrangements are proposed to serve as gene flow barriers. Contrary to other observations in Drosophila, the mitochondrial genome appears resilient to gene flow in the presence of nuclear exchange.     

MIGRATION-SELECTION BALANCE AND LOCAL ADAPTATION OF MITOCHONDRIAL HAPLOTYPES IN RUFOUS-COLLARED SPARROWS (ZONOTRICHIA CAPENSIS) ALONG AN ELEVATIONAL GRADIENT

Variable selection pressures across heterogeneous landscapes can lead to local adaptation of populations. The extent of local adaptation depends on the interplay between natural selection and gene flow, but the nature of this relationship is complex. Gene flow can constrain local adaptation by eroding differentiation driven by natural selection, or local adaptation can itself constrain gene flow through selection against maladapted immigrants. Here we test for evidence that natural selection constrains gene flow among populations of a widespread passerine bird ( Zonotrichia capensis ) that are distributed along an elevational gradient in the Peruvian Andes. Using multilocus sequences and microsatellites screened in 142 individuals collected along a series of replicate transects, we found that mitochondrial gene flow was significantly reduced along elevational transects relative to latitudinal control transects. Nuclear gene flow, however, was not similarly reduced. Clines in mitochondrial haplotype frequency were strongly associated with transitions in environmental variables along the elevational transects, but this association was not observed for the nuclear markers. These results suggest that natural selection constrains mitochondrial gene flow along elevational gradients and that the mitonuclear discrepancy may be due to local adaptation of mitochondrial haplotypes.     

EVIDENCE FOR GENETIC DIFFERENTIATION BETWEEN CHOKE-INDUCING AND ASYMPTOMATIC STRAINS OF THE EPICHLOË GRASS ENDOPHYTE FROM BRACHYPODIUM SYLVATICUM

Life cycle and breeding system variation in Epichloë grass endophytes (choke disease) is tightly linked to the degree of stroma formation. It is not known whether this variation results from differences in host resistance, fungal virulence, or environmental conditions. We found genetic differentiation between 173 asymptomatic (NS) and 93 stromata-forming (S) Epichloë strains isolated from one grass species, Brachypodium sylvaticum, based on 13 presumed allozyme loci, of which six were variable. The fungal strains originated from 10 sites in Switzerland, three sites of which were represented by both NS and S subpopulations. In total, 19 allozyme genotypes, that were nonrandomly distributed among S and NS were detected. Genetic variation measured as G_ST between S and NS strains isolated from the same site ranged from 0.73 to 0.98. Clonality, measured as linkage disequilibrium at one site, was significant in the NS subpopulation (P ? 0.001), but not in the S subpopulation (P = 0.21), implying asexual reproduction by NS strains as well as successful horizontal transmission of S strains. Since all seeds are usually infected vegetatively, horizontal transmission implies the occurrence of multiple host infections. Altogether, these results provide indirect evidence that NS and S strains do not belong to one panmictic population and that differentiation patterns of stroma formation found in nature are due to genetic differences among fungi in associations with their host plants. We discuss the direction of evolution of disease expression in this system. The distribution of genetic variability suggests that the asymptomatic strains were derived from stromata-forming populations.     

GENETIC STRUCTURE AND MALE-MEDIATED GENE FLOW IN THE GHOST BAT (MACRODERMA GIGAS)

The Australian ghost bat is a large, opportunistic carnivorous species that has undergone a marked range contraction toward more mesic, tropical sites over the past century. Comparison of mitochondrial DNA (mtDNA) control region sequences and six nuclear microsatellite loci in 217 ghost bats from nine populations across subtropical and tropical Australia revealed strong population subdivision (mtDNA φ_ST = 0.80; microsatellites UR_ST = 0.337). Low-latitude (tropical) populations had higher heterozygosity and less marked phylogeographic structure and lower subdivision among sites within regions (within Northern Territory [NT] and within North Queensland [NQ]) than did populations at higher latitudes (subtropical sites; central Queensland [CQ]), although sampling of geographically proximal breeding sites is unavoidably restricted for the latter. Gene flow among populations within each of the northern regions appears to be male biased in that the difference in population subdivision for mtDNA and microsatellites (NT φ_ST = 0.39, UR_ST = 0.02; NQ φ_ST = 0.60, UR_ST = ?0.03) is greater than expected from differences in the effective population size of haploid versus diploid loci. The high level of population subdivision across the range of the ghost bat contrasts with evidence for high gene flow in other chiropteran species and may be due to narrow physiological tolerances and consequent limited availability of roosts for ghost bats, particularly across the subtropical and relatively arid regions. This observation is consistent with the hypothesis that the contraction of the species' range is associated with late Holocene climate change. The extreme isolation among higher-latitude populations may predispose them to additional local extinctions if the processes responsible for the range contraction continue to operate.     

GENETIC VARIABILITY AND POPULATION DIFFERENTIATION INFERRED FROM DNA FINGERPRINTING IN SILVEREYES (AVES: ZOSTEROPIDAE)

This study evaluated DNA fingerprinting as a tool for estimating population genetic diversity and differentiation by comparing minisatellite variation in island and mainland populations of silvereyes (Aves: Zosterops lateralis). Three populations with different recent histories were compared: (1) Heron Island and neighboring islands, colonized 3000 to 4000 yr ago; (2) Lady Elliot Island, colonized within the past two decades; and (3) an adjacent mainland population, which presumably has existed for thousands of years. The degree of genetic variability within the three populations reflected both their size and the time since their colonization. Minisatellite diversity was highest in the mainland population, intermediate in the Capricorn Island group (which was shown to represent a single admixture), and lowest in the Lady Elliot Island population, possibly because of a recent population bottleneck during colonization. Mean band sharing between any two populations was less than the mean within either of those populations, and four fingerprint bands common to island birds were rare or absent in the fingerprints of mainland birds. In the absence of significant gene flow between the mainland and the islands, the populations have apparently become distinct at minisatellite loci, as evidenced by differences in both allelic diversity and in the frequencies of specific fragments. Within the Heron Island population, cohort analyses demonstrated the temporal stability of the fingerprint profile over 6 yr. This study demonstrates that length polymorphisms at minisatellite loci may be stable enough over time to retain information about recent historical and demographic effects on the relative genetic variability and differentiation of small, closely related populations.     

ADAPTIVE RADIATION AND GENETIC DIFFERENTIATION IN HAWAIIAN BIDENS

The genus Bidens (Asteraceae) has undergone extensive adaptive radiation on the Hawaiian Islands. The 19 species and eight subspecies endemic to Hawaii exhibit much more morphological and ecological differentiation than the continental members of the genus. However, the Hawaiian taxa have the same chromosome number and retain the capacity to interbreed in all possible combinations. Twenty-two populations of 15 Hawaiian taxa and four populations of American taxa were compared at 21 loci controlling eight enzyme systems. Populations of Hawaiian taxa are highly polymorphic. However, little genetic differentiation has occurred among taxa in spite of the high levels of genetic variability. Genetic identities calculated for pairs of populations show that populations of the same taxon are genetically more similar than are populations belonging to different taxa, but all values are high. The level of genetic differentiation that has occurred among the species of Hawaiian Bidens is comparable to the level of genetic differences found among populations within single continental plant species. Moreover, there is no correlation between the isozyme data and morphological data. No groups of taxa are evident in the genetic data, although morphological groups exist. Genetic differentiation at isozyme loci has not occurred at the same rate as the acquisition of presumably adaptive morphological and ecological characters in Hawaiian Bidens. Adaptive radiation may be limited to a few genes controlling morphological and ecological characters.     

猪CAST基因的遗传多态性及遗传效应分析

采用PCR—SSCP方法对猪cAST基因遗传多态性进行分析。并研究基因型与肉质性状和背膘厚的相关性。根据猪CAST基因的cDNA序列（M20160）设计7对引物。结果在F1／R1。F6／R6引物对扩增的片段上发现了多态性。并对纯合子进行测序。发现317位A—G突变。2042位G—C突变。基因型在不同猪种分布的多重比较结果表明。长白猪、大白猪和杜洛克猪与沂蒙黑猪和莱芜猪比较差异极显著（P〈0．01）。固定效应模型分析结果表明,嫩度及背膘厚基因型间差异显著（P〈0．05）,而pH值、温度及滴水损失基因型间差异不显著（P〉0．05）。最小二乘分析结果表明,不同猪种比较,屠宰12h和24h后肌肉温度、30min和1h后pH值及滴水损失差异显著（P〈0．05）;BBCC和BBDD单元型个体与其他单元型个体比较肌肉嫩度的差异显著（P〈0．01）。AACC和AADD单元型个体与其他单元型个体比较背膘厚的差异显著（P〈0．01）。因此,推测CAST基因对猪肉品质及背膘厚存在一定的影响。将CAST基因应用于猪育种过程中的标记辅助选择将可以改善猪肉品质。加快猪的育种进程。     

GENE FLOW AND SPECIES DELIMITATION: A CASE STUDY OF TWO PINE SPECIES WITH OVERLAPPING DISTRIBUTIONS IN SOUTHEAST CHINA

Species delimitation detected by molecular markers is complicated by introgression and incomplete lineage sorting between species. Recent modeling suggests that fixed genetic differences between species are highly related to rates of intraspecific gene flow. However, it remains unclear whether such differences are due to high levels of intraspecific gene flow overriding the spread of introgressed alleles or favoring rapid lineage sorting between species. In pines, chloroplast (cp) and mitochondrial (mt) DNAs are normally paternally and maternally inherited, respectively, and thus their relative rates of intraspecific gene flow are expected to be high and low, respectively. In this study, we used two pine species with overlapping geographical distributions in southeast China, P. massoniana and P. hwangshanensis, as a model system to examine the association between organelle gene flow and variation within and between species. We found that cpDNA variation across these two pine species is more species specific than mtDNA variation and almost delimits taxonomic boundaries. The shared mt/cp DNA genetic variation between species shows no bias in regard to parapatric versus allopatric species’ distributions. Our results therefore support the hypothesis that high intraspecific gene flow has accelerated cpDNA lineage sorting between these two pine species.