Genetic diversity and structure of western white pine (Pinus monticola) in North America: a baseline study for conservation, restoration, and addressing impacts of climate change

Western white pine (Pinus monticola) is an economically and ecologically important species in western North America that has declined in prominence over the past several decades, mainly due to the introduction of Cronartium ribicola (cause of white pine blister rust) and reduced opportunities for regeneration. Amplified fragment length polymorphism (AFLP) markers were used to assess the genetic diversity and structure among populations at 15 sites (e.g., provenances) across the native range of western white pine. The level of genetic diversity was different among 15 populations tested using 66 polymorphic AFLP loci. Nei’s gene diversity (H _E) at the population level ranged from 0.187 to 0.316. Genetic differentiation (G _ST) indicated that 20.1% of detected genetic variation was explained by differences among populations. In general, populations below 45^oN latitude exhibited a higher level of genetic diversity than higher latitude populations. Genetic distance analysis revealed two major clades between northern and southern populations, but other well-supported relationships are also apparent within each of the two clades. The complex relationships among populations are likely derived from multiple factors including migration, adaptation, and multiple glacial refugia, especially in higher latitudes. Genetic diversity and structure revealed by this study will aid recognition and selection of western white pine populations for species management and conservation programs, especially in consideration of current and future climate changes.     

Genetic issues in aquatic species management: the shortnose sturgeon (Acipenser brevirostrum) in the southeastern United States

Anadromous species occupy multiple freshwater,estuarine, and marine habitats, which posesspecial challenges in wildlife management. Inparticular, the level of immigration betweendrainages can be a critical factor in thedefinition of management units and the designof population-specific stocking programs. Thesemi-anadromous shortnose sturgeon (Acipenser brevirostrum) is listed as anendangered species under the Endangered SpeciesAct in the United States. To assess populationstructure in this species as a guide toeffective management, tissue samples werecollected from adult specimens (N = 198) fromfive river systems in the southeastern U.S.,and augmented with a sample from New Brunswick,Canada (N = 13), the extreme northern end ofthe species' range. Comparisons of mtDNAcontrol region sequences reveal a shallow genegenealogy and modest, but significant,population structuring (_st = 4.3%or _st = 17.7% when Canadian samplesare included). Populations inhabiting riversystems in the southeastern U.S. are closelyrelated, a pattern consistent with more recentdivergences along evolutionary timeframes. Haplotype diversity is moderate to high in mostdrainages (h = 0.383–1.000), exceptfor the Savannah and Edisto rivers, indicatingthat historical levels of mtDNA diversity mightbe largely intact outside of these drainages. Low mtDNA diversity in the Savannah andneighboring Edisto drainages might stem from anexperimental stocking effort during 1984–1992 that depressed overall genetic diversityin the Savannah and established or augmentedthe Edisto River population with a relativelylimited number of matrilines.     

Genetic Analysis of Selected Strains of Eastern Oyster (Crassostrea virginica Gmelin) Using AFLP and Microsatellite Markers

Amplified fragment length polymorphisms (AFLPs) and microsatellite markers were used to examine genetic variation and divergence in 4 selected strains (DBH, NEH, FMF, and CTS) and 1 wild population (DBW) of the eastern oyster Crassostrea virginica Gmelin. Eighty-six AFLP markers (from 3 primer pairs) and 5 microsatellite loci were used for the analysis of 30 oysters from each of the 5 populations. Microsatellite loci were considerably more variable than AFLPs. The observed heterozygosity ranged from 0.560 to 0.640 across populations for microsatellites, and from 0.186 to 0.207 for AFLPs. Both F_st and _PT of microsatellite data and _PT statistics of AFLP data revealed significant divergence between all pairs of populations. There was no significant reduction in heterozygosity in all 4 selected strains; however, the number of alleles per locus was considerably lower in the selected strains than in the wild population. Two strains subjected to long-term selection for disease resistance shared frequency shifts at a few loci, which deserve further analysis to determine if they are linked to disease-resistance genes.     

Multiple-population versus hierarchical conifer breeding programs: a comparison of genetic diversity levels

Advanced-generation domestication programs for forest-tree species has raised some concerns about the maintenance of genetic diversity in forest-tree breeding programs. Genetic diversity in natural stands was compared with two genetic conservation options for a third-generation elite Pinus taeda breeding population. The breeding population was subdivided either on the basis of geographic origin and selection goals (multiple-population or MPBS option) or stratified according to genetic value (hierarchical or HOPE option). Most allelic diversity in the natural stands of loblolly pine is present in the domesticated breeding populations. This was true at the aggregate level for both multiple-population (MPBS) and the hierarchical (HOPE) populations. Individual subpopulations within each option had less genetic diversity but it did not decline as generations of improvement increased. Genetic differentiation within the subdivided breeding populations ranged from 1 to 5%, genetic variability is within each subpopulation rather than among subpopulations for both MPBS (>95%) and the HOPE approaches (>98%). Nei's G_st estimates for amongpopulation differentiation were biased upwards relative to estimates of from Weir and Cockerham (1984).     

The low-light reduction in the quantum yield of photosynthesis: potential errors and biases when calculating the maximum quantum yield

Photosynthesis-irradiance (P-E) curves are widely used to describe photosynthetic efficiency and potential. Contemporary models assume maximal photosynthetic quantum yield () at low irradiances. But P-E observations made with both oxygen evolution and carbon uptake techniques show that this is not always the case. Using new and published data in conjunction with modeling exercises, we demonstrate that regardless of the mechanism there can be reductions in at low irradiances that are not readily observable using conventional P-E analyses. We also show that analytical errors, such as inaccurate estimation of dark oxygen consumption or carbon uptake, can markedly affect the structure of -E curves with negligible effect on P-E curve structure. Whether from respiration `corrections' or other mechanisms, these deviations in at low light levels from the maximum quantum yield of photosynthesis (_max) can lead to significant errors (> 50%) in the estimation of the linear portion of the P-E curve and ultimately _max. Non-linear models of P-E, such as the rectangular hyperbola, quadratic, exponential and hyperbolic tangent that are commonly used to estimate the initial slope () of the P-E curve assume that is maximal at low light levels and therefore can err in the estimation of _max when is reduced at low light levels. Using a diverse data set of 622 P-E curves with a total of 7623 points, we show that although model skills are high (r ² = 0.96 ± 0.05, 0.97 ± 0.04, 0.97 ± 0.04 and 0.97 ± 0.04, respectively), a large fraction of the model-predicted _max differ by greater than 10% from true _max values (91%, 50%, 82% and 46%, respectively). Data from these observations and modeling exercises lead us to suggest that _max be determined by directly estimating the true maximum of a -E curve rather than using the more conventional methodology employing the initial slope of the P-E curve.This revised version was published online in October 2005 with corrections to the Cover Date.     

The relationship between CO2 assimilation and electron transport in leaves

The inter-relationships between the quantum efficiencies of photosystems I (_I) and II (_II) and the quantum yield of CO₂ fixation % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabeA8aMnaaBa% aaleaacaWGdbGaam4tamaaBaaameaacaaIYaaaleqaaaqabaaaaa!3BD3!\[\phi _{CO_2 } \] were investigated in pea (Pisum sativum (L)) leaves with differing rates of photosynthesis using both photorespiratory and non-photorespiratory conditions, and in a leaf of Hedera helix (L) under photorespiratory conditions. The results indicate that under photorespiratory conditions the relationship between % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabeA8aMnaaBa% aaleaacaWGdbGaam4tamaaBaaameaacaaIYaaaleqaaaqabaaaaa!3BD3!\[\phi _{CO_2 } \] and both _I and _II is non-linear and variable. The relationship between _I and _II under these circumstances remains predominantly linear. Under non-photorespiratory conditions, leaves with a low rate of photosynthesis due to sink limitation exhibit a non-linear relationship between _I and _II, though the relationship between _I and _II remains linear suggesting a close relationship between linear electron flow and CO₂ fixation. Leaves irradiated at the CO₂ compensation point also exhibit a non-linear relationship between _I and _II. These results suggest that for leaves in air linear electron flow is the predominant source of energy for metabolism. The role of cyclic electron transport is considered when the requirement for the products of linear electron transport is depressed.Abbreviations q_p the coefficient for photochemical quenching of chlorophyll fluorescence - _exe the quantum efficiency of excitation energy capture by open PS II traps - _II the quantum efficiency for electron transport by PS II - _I the quantum efficiency (for electron transport) by PS I - % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabeA8aMnaaBa% aaleaacaWGdbGaam4tamaaBaaameaacaaIYaaaleqaaaqabaaaaa!3BD3!\[\phi _{CO_2 } \] the quantum yield for CO₂ fixation (obtained as the gross rate of CO₂ fixation divided by the irradiance) - % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGak0dh9WrFfpC0xh9vqqj-hEeeu0xXdbba9frFj0-OqFf% ea0dXdd9vqaq-JfrVkFHe9pgea0dXdar-Jb9hs0dXdbPYxe9vr0-vr% 0-vqpWqaaeaabaGaciaacaqabeaadaqaaqaaaOqaaiabgs5aenaaBa% aaleaacqaH8oqBdaWgaaadbaGaamisamaaCaaabeqaaiabgUcaRaaa% aeqaaaWcbeaaaaa!3CB0!\[\Delta _{\mu _{H^ + } } \] trans-thylakoid proton potential difference - PAQF photosynthetically active quantum flux     

Photon yield of O2 evolution and chlorophyll fluorescence characteristics at 77 K among vascular plants of diverse origins

Photon yields of oxygen evolution at saturating CO₂ were determined for 44 species of vascular plants, representing widely diverse taxa, habitats, life forms and growth conditions. The photonyield values on the basis of absorbed light ( ^a) were remarkably constant among plants possessing the same pathway of photosynthetic CO₂ fixation, provided the plants had not been subjected to environmental stress. The mean ^a value ±SE for 37 C₃ species was 0.106±0.001 O₂·photon^-1. The five C₄ species exhibited lower photon yields and greater variation than the C₃ species ( ^a=0.0692±0.004). The ^a values for the two Crassulaceanacid-metabolism species were similar to those of C₃ species. Leaf chlorophyll content had little influence on ^a over the range found in normal, healthy leaves. Chlorophyll fluorescence characteristics at 77 K were determined for the same leaves as used for the photon-yield measurements. Considerable variation in fluorescence emission both at 692 nm and at 734 nm, was found 1) among the different species; 2) between the upper and lower surfaces of the same leaves; and 3) between sun and shade leaves of the same species. By contrast, the ratio of variable to maximum fluorescence emission at 692 nm (F_v/F_M, 692) remained remarkably constant (The mean value for the C₃ species was 0.832±0.004). High-light treatments of shade leaves resulted in a reduction in both ^a and the F_v/F_M, 692 ratio. The extent of the reductions increased with time of exposure to bright light. A linear relationship was obtained when ^a was plotted against F_v/F_M, 692. The results show that determinations of the photon yield of O₂ evolution and the F_v/F_M, 692 ratio can serve as excellent quantitative measures of photoinhibition of overall photosynthetic energy-conversion system and of photochemistry of photosystem II, respectively. This is especially valuable in field work where it is often impossible to obtain appropriate controls.Abbreviations and symbols CAM Crassulacean acid metabolism - PFD photon flux density (photon fluence rate) - PSI, PSII photosystem I, II - F_o, F_M, F_v instantaneous, maximum, variable fluorescence emission - absorptance - ^a photon yield (absorbed light) - ⁱ photon yield (incident light) C.I.W.-D.P.B. Publication No. 923     

Isolation and characterization of lytic phages fromBacterioides ruminicola ssbrevis

Two bacteriophages (Brb01 and Brb02), lytic toBacteroides ruminicola ssbrevis AR20, were isolated from sewage water. Both phages possessed polyhedral heads and long noncontractile tails, and were classified as Siphoviridae of morphotype B1. Bacteria resistant to phages Brb01 and Brb02 arose following lysis of broth cultures. Survivors of Brb01 infection were capsulated but remained susceptible to Brb02 infection. Survivors of Brb02 infection were noncapsulated and were resistant to attack by both Brb01 and Brb02. Neither phage lysogenized the host. Both phages contained double-stranded DNA, and their restriction endonuclease digestion patterns indicated that the phage genomes were circularly permuted and terminally redundant. Phage Brb01 genome was examined in greater detail and confirmed to be circularly permuted, of size 33 kb, with a terminal redundancy of 2 kb, or 6% of the length of the genome. Circularly permuted genomes in phages of rumen bacteria do not appear to have been reported previously.At present, there is considerable interest in the genetic manipulation of rumen bacteria. The characterization of the phages described herein provides the basic information required for their use in the construction of vectors for the transfer of genetic material.     

Genetic assessment of <Emphasis Type="Italic">Abies koreana</Emphasis> (Pinaceae), the endangered Korean fir,and conservation implications

To establish a management plan for endangered and rare species, genetic assessment must first be conducted. The genetic characteristics of plant species are affected by demographic history, reproductive strategy, and distributional range as well as anthropological effects. Abies koreana E. H. Wilson (Pinaceae), Korean fir, is endemic to Korea and found only in sub-alpine areas of the southern Korean Peninsula and Jejudo Island. This species has been designated as critically endangered by the International Union for Conservation of Nature due to a continuous decline in its range and population fragmentation. We genotyped 176 individuals from seven natural populations and two afforested populations on the Korean Peninsula using 19 microsatellite loci. STRUCTURE analysis revealed two genetic clusters in natural populations (F _st  = 0.040 and R _st  = 0.040) despite low differentiation. We did not detect a significant reduction in genetic diversity or the signature of a genetic bottleneck despite population fragmentation and small population size. We deduced that this species exhibits a metapopulation structure, with the population on Jirisan Mountain acting as a source of genetic diversity for other local small populations on the Korean Peninsula, through contemporary asymmetric gene flow. However, the majority of afforested individuals on the Korean Peninsula originated from a different gene cluster. Thus, we recommend a conservation strategy that maintains two genetically unique clusters.     

Genetic Variation and Differentiation of Peat-Bog and Dry-Meadow Populations of the Dwarf Mountain Pine Pinus mugo Turra in the Highlands of the Ukrainian Carpathians

Based on electrophoretic analysis of 21 isozyme loci controlling 10 enzyme systems, the intra- and interpopulation variation was studied in two peat-bog and three dry-meadow populations of the dwarf mountain pine Pinus mugoTurra from the highlands of the Ukrainian Carpathians. In the studied samples (a total of 164 trees), on average 62% of the studied genes were polymorphic; the mean heterozygosity was 21.3%. The dry-meadow populations differed from the peat-bog populations by allele and genotype diversity and by heterozygosity although the indices characterizing population heterogeneity (F _st and G _st) were small (0.027 and 0.032, respectively). Nei's genetic distances between the populations ranged of 0.011 to 0.032 with the mean of 0.018.     

Genetic diversity in Orobanche crenata populations from southern Spain

The pattern of genetic variation within and among natural populations of broomrape (Orobanche crenata Forsk.) from southern Spain was analysed by RAPD markers. Hierarchical analysis of phenotypic diversity using AMOVA was performed to analyse the partitioning of the variation among populations and among individuals. Although most of the genetic diversity was attributable to differences among individuals within a population (94.29%), significant φ_st values among populations suggested the existence of phenotypic differentiation. Moreover, corresponding HOMOVA analysis revealed that molecular variances were significantly heterogeneous among populations although no clear grouping pattern could be established. These results are to be expected considering the predominant outcrossing behaviour of O. crenata. Received: 10 January 2001 / Accepted: 12 February 2001     

Genetic variability and structure of natural and domesticated populations of Caribbean pine (Pinus caribaea Morelet)

 Isozyme analysis of seed samples derived from natural and managed populations of the tropical pine Pinus caribaea vars ‘bahamensis’ and ‘caribaea’ was used to assess population genetic structure in its native range and to detect changes occurring during early domestication of the species. Baseline data from natural populations of the two varieties showed that populations sampled as seed are characterized by high gene diversity (mean H_e=0.26) and a low level of inbreeding ( mean F_is=0.15). A UPGMA tree of genetic relatedness among populations indicates that the two varieties represent distinct evolutionary units. Within each variety there is significant differentiation among populations, and this is greater for the more fragmented populations of var ‘bahamensis’ (F_st=0.08) than for var ‘caribaea’ (F_st=0.02). Seed from a seed orchard population of var ‘caribaea’ established within its natural range showed no change in genetic diversity but did show a reduced inbreeding coefficient (F_is=0.09) compared with its progenitor populations, suggesting a decrease in selfing and/or biparental inbreeding. A bulked seed sample from an exotic plantation of var ‘bahamensis’ in Australia displayed a large increase in the inbreeding coefficient (F_is=0.324) compared with that found in natural populations, possibly due to elevated self-fertilization. Finally, a bulked seed sample from an exotic plantation population of var ‘caribaea’ from China showed enhanced genetic diversity, an increase in the inbreeding coefficient and more linkage disequilibrium than its presumed progenitor populations. It was also genetically divergent from them. RFLP analysis of chloroplast DNA variation in the Chinese sample suggested that seeds of the related taxa P. elliottii and P. taeda, or seeds derived from hybridization with these taxa growing in the seed production area, had been included in the seed crop during harvesting. We conclude that monitoring of appropriate genetic markers may be an effective means of identifying potentially deleterious genetic changes occurring during forest tree domestication. Received: 10 August 1998 / Accepted: 8 September 1998     

Colonization History,Host Distribution,Anthropogenic Influence and Landscape Features Shape Populations of White Pine Blister Rust,an Invasive Alien Tree Pathogen

White pine blister rust is caused by the fungal pathogen Cronartium ribicola J.C. Fisch (Basidiomycota, Pucciniales). This invasive alien pathogen was introduced into North America at the beginning of the 20th century on pine seedlings imported from Europe and has caused serious economic and ecological impacts. In this study, we applied a population and landscape genetics approach to understand the patterns of introduction and colonization as well as population structure and migration of C. ribicola. We characterized 1,292 samples of C. ribicola from 66 geographic locations in North America using single nucleotide polymorphisms (SNPs) and evaluated the effect of landscape features, host distribution, and colonization history on the structure of these pathogen populations. We identified eastern and western genetic populations in North America that are strongly differentiated. Genetic diversity is two to five times higher in eastern populations than in western ones, which can be explained by the repeated accidental introductions of the pathogen into northeastern North America compared with a single documented introduction into western North America. These distinct genetic populations are maintained by a barrier to gene flow that corresponds to a region where host connectivity is interrupted. Furthermore, additional cryptic spatial differentiation was identified in western populations. This differentiation corresponds to landscape features, such as mountain ranges, and also to host connectivity. We also detected genetic differentiation between the pathogen populations in natural stands and plantations, an indication that anthropogenic movement of this pathogen still takes place. These results highlight the importance of monitoring this invasive alien tree pathogen to prevent admixture of eastern and western populations where different pathogen races occur.     

Investigation of genetic diversity in fragmented geometric tortoise (Psammobates geometricus) populations

Microsatellite DNA was used to investigatelevels of genetic variability in severelyfragmented populations of the geometrictortoise, Psammobates geometricus, themost endangered tortoise on mainland Africa.Eight microsatellite markers were used toassess genetic variability within and betweenthree naturally occurring populations of P. geometricus. These populations areseparated by the Cape Fold mountain ranges inthe western Cape region of South Africa. Levels of variability were quantified usingallelic diversity, genotypic frequencies andheterozygosity. Evidence for populationsub-structuring was examined using Fstatistics, R_st and ².High levels of variability were found in allthree populations. Low levels of populationdifferentiation were found suggestingsignificant gene flow between the populations.     

Growth form distribution and genetic relationships in tree clusters of Pinus flexilis,a bird-dispersed pine

Seed dispersal by the Clark's nutcracker (Nucifraga columbiana Wilson) may markedly influence the growth form and genetic population structure of limber pine (Pinus flexilis James). The nutcracker buries clusters of seeds in subterranean caches; germination of clustered seeds often results in a growth form characterized by two or more genetically distinct trees with fused or contiguous trunks (tree clusters). The occurrence of a morphologically similar form, the multi-trunk tree (a single genet branched near the base), as well as the typical single-trunked tree, complicates the study of limber pine populations. We examined growth form distribution and genetic relationships in tree clusters in limber pine populations at four elevations (from 2585 m to 3460 m) in the Colorado Front Range. At three study areas, relative occurrence of limber pine growth forms, as well as that of associated pines, was examined by a point-centered quarter survey. From the four study areas, we collected foliage from each trunk from a total of 74 clumps (combined tree clusters and multi-trunk trees) in order to differentiate the two growth forms using starch gel protein electrophoresis. Tree clumps were significantly more common in limber pine than in ponderosa or lodgepole pine (P<0.010). Although single-trunk limber pine was the most common growth form, except at the highest elevation, both multi-trunk trees and tree clusters were present in each stand. Tree clusters were estimated to comprise about 20% of the tree sites in each limber pine stand; the estimated proportion of multi-trunk trees varied by site from 5% to 77%. Trees in clusters were related, on average, as half to full siblings (mean r=0.43), but were unrelated to trees in other clusters (mean r=0.01). Electrophoretic analysis suggests possible genetic differentiation in limber pine that may be the result of different selection pressures on the growth forms.     

Genetic diversity and structure of <Emphasis Type="Italic">Capparis spinosa</Emphasis> L. in Iran as revealed by ISSR markers

Capparis spinosa L. (caper bush) is an economically and ecologically important perennial shrub that grows across different regions of Iran. In this study, the genetic diversity and population structure of Iranian genepool of C. spinosa is evaluated using Inter Simple Sequence Repeat (ISSR) markers. Using 10 ISSR primers, 387 DNA fragments (bands) were amplified from the genomic DNA of 92 individuals belonging to twenty-one populations of C. spinosa, of which 378 (97.7%) were polymorphic. High level of genetic diversity (percentage of polymorphic loci = 98.2%, h = 0.1382, I = 0.243), high genetic differentiation (G_st = 0.5234) and low gene flow (Nm = 0.4553) among populations were observed. Caper bush populations were divided into 4 groups in the dendrogram, PCoA plot and Bayesian clustering results, mostly corresponded to their geographic regions. The results showed that there are value in sampling Iranian caper bush populations to look for valuable alleles for use in plant breeding programs.     

中国东部沿海水仙归化群体的遗传多样性

为揭示我国东部归化水仙(Narcissus tazetta var. chinensis)的群体遗传多样性,利用2个叶绿体基因mat K和trn H-psb A片段对采自沪、浙、闽的5个代表群体的49株水仙进行了评估。结果表明,双基因联合序列的总长为1443bp,共定义6个单倍型,各归化群体的遗传多样性水平为DLSYPTDNJD=ZZCMD。AMOVA分析表明,群体内变异为遗传变异的主要来源(91.98%),群体间的遗传分化较低(Fst=0.080 22)。群体物种水平上的谱系结构不显著(Nst=0.020Gst=0.031;P0.05)。Mantel检验表明水仙群体间的遗传距离与地理距离呈显著的线性相关(r=0.929,P=0.02 0.05)。中性检验和错配分布分析结果均暗示水仙群体背离了快速扩张模型的假设。单倍型分布的中介网络图结合系统发育NJ树均将所有群体划分为2大分支。因此,我国东部沿海水仙归化群体整体遗传多样性水平较低,各群体间遗传分化较弱,遗传变异主要来自群体内,物种近期未经历扩张事件,可能是基因流受海岛隔离、自身生物学特征、生境异质性与及人为干扰的综合作用影响。     

Phenogenetic Characterization of a Group of Giant φKZ-like Bacteriophages of Pseudomonas aeruginosa

A comparative study was made of a group ofPseudomonas aeruginosa virulent giant DNA bacteriophages similar to phage KZ in several genetic and phenotypic properties (particle size, particle morphology, genome size, appearance of negative colonies, high productivity, broad spectrum of lytic activity, ability to overcome the suppressing effect of plasmids, absence of several DNA restriction sites, capability of general transduction, pseudolysogeny). We have recently sequenced the phage KZ genome (288 334 bp) [J. Mol. Biol., 2002, vol. 317, pp. 1–19]. By DNA homology, the phages were assigned to three species (represented by phages KZ, Lin68, and EL, respectively) and two new genera (KZ and EL). Restriction enzyme analysis revealed the mosaic genome structure in four phages of the KZ species (KZ, Lin21, NN, and PTB80) and two phages of the EL species (EL and RU). Comparisons with respect to phage particle size, number of structural proteins, and the N-terminal sequences of the major capsid protein confirmed the phylogenetic relatedness of the phages belonging to the KZ genus. The origin and evolution of the KZ-like phages are discussed. Analysis of protein sequences encoded by the phage KZ genome made it possible to assume wide migration of the KZ-like phages (wandering phages) among various prokaryotes and possibly eukaryotes. Since the phage KZ genome codes for potentially toxic proteins, caution must be exercised in the employment of large bacteriophages in phage therapy.     

基于叶绿体DNA非编码序列的蒙古扁桃谱系地理学研究

该研究选取中国西北干旱区第三纪孑遗植物蒙古扁桃（Amygdalus mongolica）,基于叶绿体DNA非编码trnH psbA序列对蒙古扁桃17个居群324个个体进行了谱系地理学研究。结果表明：（1）蒙古扁桃trnH psbA序列长度350 bp,变异位点63个,共有9种单倍型,居群间总遗传多样性为（H_t）为0.758,居群内平均遗传多样性为（H_s）为0.203,贺兰山东麓及阴山南麓边缘的居群具有较高的单倍型多样性及核苷酸多样性并固定较多特有单倍型,推测这2个地区是蒙古扁桃在第四纪冰期时的重要避难所。（2）AMOVA分析表明,居群间的遗传变异为83.84%,居群内的遗传变异为16.16%,居群间遗传分化系数N_st>G_st（N_st=0.733, G_st=0.655, P>0.05）,表明蒙古扁桃不存在明显的谱系地理结构;根据单倍型地理分布及网络关系图,把蒙古扁桃自然地理居群分为东、西两大地理组群,而且东、西地理组群没有共享单倍型;居群遗传结构分析表明,两大地理组群遗传分化较大。（3）蒙古扁桃居群在间冰期或冰期后经历了近期的居群扩张,由于奠基者效应使得多数居群只固定了单一的单倍型。     

Global asymptotic stability of the size distribution in probabilistic models of the cell cycle

Probabilistic models of the cell cycle maintain that cell generation time is a random variable given by some distribution function, and that the probability of cell division per unit time is a function only of cell age (and not, for instance, of cell size). Given the probability density, f(t), for time spent in the random compartment of the cell cycle, we derive a recursion relation for _n(x), the probability density for cell size at birth in a sample of cells in generation n. For the case of exponential growth of cells, the recursion relation has no steady-state solution. For the case of linear cell growth, we show that there exists a unique, globally asymptotically stable, steady-state birth size distribution, _*(x). For the special case of the transition probability model, we display _*(x) explicitly.This work was supported by the National Science Foundation under grants MCS8301104 (to J.J.T.) and MCS8300559 (to K.B.H.), and by the National Institutes of Health under grant GM27629 (to J.J.T.).