FINE-SCALE GENETIC AND PHENOTYPIC STRUCTURE IN NATURAL POPULATIONS OF BACILLUS SUBTILIS AND BACILLUS LICHENIFORMIS: IMPLICATIONS FOR BACTERIAL EVOLUTION AND SPECIATION

The genetic and phenotypic structure of sympatric populations of wild bacteria traditionally identified as Bacillus subtilis and B. licheniformis was analyzed. Small soil samples were taken from a single, tiny site in the Sonoran Desert of Arizona, USA, to provide a true population analysis, in contrast to many analyses of genetic structure using bacterial strain collections of widely heterogeneous origin. Genetic analyses of isolates used multilocus enzyme electrophoresis, mismatches in restriction fragment length polymorphism, and variants from Southern hybridization with B. subtilis DNA probes. Phenotypic analyses of isolates used the API test system for detection of growth and acid production on specific carbon sources. The two species were distinct both phenotypically and genetically, despite their known potential for genetic exchange in laboratory experiments. Genic and genotypic diversity were high in both species, and only 16% of observed allozyme variants might possibly be common to both species. Hence, there is probably modest genetic exchange, if any, between the species in nature. Clear hierarchies of population-genetic structure were found for both species. Different types of genetic data yield concordant population structures for B. subtilis. For both species, two-locus and multilocus statistical analyses of linkage demonstrated modest to strong disequilibrium at the species level but truly panmictic subunits within each species. The evidence for extensive genetic recombination within these fine-scale subdivisions is unequivocal, indicating that the sexuality of these bacteria can be well expressed in nature. The relation of these results to processes of bacterial evolution and speciation is discussed.     

武汉东湖磷细菌种群结构的研究

对东湖磷细菌种群结构研究结果表明,至少有芽孢杆菌属,微球菌属,不动细菌属,气单胞菌属,产碱杆菌属,贝内克氏菌属,无色杆菌属,短杆菌属,肠杆菌属,黄杆菌尾,微杆菌 ,假单胞菌属,沙雷氏菌属,黄单胞菌属和发酵单胞菌属15个属的不同菌株以其对有机磷,无机磷化合物的分解能力推动着湖泊中磷的循环,其中以芽孢杆菌属和微球菌属居于明显的优势。磷细菌在湖泊中的分布随水质的污染程度和磷化合物含量的不同而表现有明显的判别：在数量上,湖泥高于湖水,污染较重的I站高于水质较好的II站,在调查期间,有机磷细菌数在多数情况下亦高于无机磷细菌,在种群结构上,1站湖水中种类最多,Ⅲ站次之,II站最少。     

RECOMBINATION AND MIGRATION RATES IN NATURAL POPULATIONS OF BACILLUS SUBTILIS AND BACILLUS MOJAVENSIS

We have investigated the rates of recombination and migration in native populations of two closely related, naturally competent Bacillus species. Native soil isolates of Bacillus subtilis and Bacillus mojavensis were obtained from three continents and, within North America, from populations at a range of geographical distances from one another. The rate of recombination within populations of each species was estimated from restriction-site data for three genes. Recombination was shown to occur within each species at about the same rate as neutral mutation, whatever the geographical scale or phylogenetic scale over which strains were sampled. The rate of migration between populations was estimated by a cladistic analysis and was shown to be high (i.e., Nm > 1), even among populations on different continents. The level of migration within each species is sufficient to prevent neutral geographical divergence within species.     

GENETIC RELATEDNESS OF TWO POPULATIONS OF THE SOUTHERN ELEPHANT SEAL, MIROUNGA LEONINA

Blood samples from southern elephant seals ( Mirounga leonina ) from Heard and Macquarie Islands were surveyed electrophoretically for protein variation. Thirty proteins encoded by a minimum of 35 loci were screened, four of which were found to be polymorphic. Statistically significant differences in allele frequencies were found between the two populations at three loci. Heterozygosity estimates for the Heard and Macquarie island populations were 0.034 ± 0.020 (mean ± standard error) and O.029 ± 0.017 respectively, with a Nei distance of 0.007. The findings suggest that the two populations may have diverged genetically and very limited gene flow exists between the islands, a finding consistent with limited information from mark-recapture studies.     

GENETIC POPULATION STRUCTURE OF PSEUDO‐NITZSCHIA PUNGENS (BACILLARIOPHYCEAE) FROM THE PACIFIC NORTHWEST AND THE NORTH SEA1

Several species of the diatom Pseudo‐nitzschia produce the neurotoxin domoic acid (DA). Consumption of fish and shellfish that have accumulated this potent excitotoxin has resulted in severe illness and even death in humans, marine mammals, and seabirds. Pseudo‐nitzschia pungens (Grunow ex Cleve) Hasle is a cosmopolitan diatom commonly occurring in the waters of the Pacific Northwest (PNW) and the eastern North Atlantic, including the North Sea. However, genetic and physiological relationships among populations throughout this large geographic distribution have not been assessed. Population genetic parameters (e.g., Hardy–Weinberg equilibrium, linkage equilibrium, F_ST) calculated for P. pungens collected from the Juan de Fuca eddy region in the PNW indicated the presence of two distinct groups that were more divergent from each other than either was from a P. pungens sample from the North Sea. Geographic heterogeneity was also detected within each of the two PNW groups. These results suggested that the populations of P. pungens recently mixed in the Juan de Fuca eddy region (a seasonally retentive feature off the coasts of Washington State, USA, and Vancouver Island, Canada) but did not exchange genetic material by sexual reproduction. Alternatively, these two groups may be cryptic (morphologically identical, but reproductively isolated) species. Identifying cryptic diversity in Pseudo‐nitzschia is important for bloom prediction and aiding the identification of molecular markers that can be used for rapid detection assay development.     

龙牙木天然种群遗传多样性及遗传分化

采用水平切片淀粉凝胶电泳测定东北地区分布的4 个龙牙木种群的遗传结构。统计分析了10 个酶系统的11 个位点, 结果表明:龙牙木种群内存在着丰富的遗传变异, 多态位点百分率72.73%, 等位基因平均数为1.72, 期望杂合度为0.5617, 遗传一致度和遗传距离为0.9865和0.0133。这个结果与前文对其形态特征和薄层色谱特征分析所得结果基本一致, 充分说明区域龙牙木属于同一种物种的两个变种。     

THE EFFECT OF DELAYED POPULATION GROWTH ON THE GENETIC DIFFERENTIATION OF LOCAL POPULATIONS SUBJECT TO FREQUENT EXTINCTIONS AND RECOLONIZATIONS

I investigated the effects of delayed population growth on the genetic differentiation among populations subjected to local extinction and recolonization, for two different migration functions; (1) a constant migration rate, and (2) a constant number of migrants. A delayed period of population growth reduces the size of the newly founded populations for one or several generations. Whether this increases differentiation among local populations depends on the actual pattern of migration. With a constant migration rate, fewer migrants move into small populations than into large, thus providing ample opportunity for drift to act within a population. A prolonged period of population growth thus makes the conditions for enhanced differentiation between local populations less restrictive and also inflates the actual levels of differentiation. The effect depends on the relative magnitudes of k_e, the effective number of colonizers and k, the actual number of colonizers. When there is a constant number of migrants into a population per generation, migration into small populations is increased. This increase of migration in small populations counteracts the effects of genetic drift due to small population size. It increases the rate by which populations approach equilibrium, as small populations are swamped by migrants from larger populations closer to genetic equilibrium, and overall levels of differentiation are thus reduced. I also discuss situations for which the results of this paper are relevant.     

HOST POPULATION STRUCTURE AND THE EVOLUTION OF VIRULENCE: A “LAW OF DIMINISHING RETURNS”

Structure in a population of host individuals, whether spatial or temporal, can have important effects on the transmission and evolutionary dynamics of its pathogens. One of these is to limit dispersal of pathogens and thus increase the amount of contact between a given pair or within a small group of host individuals. We introduce a “law of diminishing returns” that predicts an evolutionary decline of pathogen virulence whenever there are on average more possibilities of pathogen transmission between the same pair of hosts. Thus, the effect of repeated contact between hosts will be to shift the balance of any trade-off between virulence and transmissibility toward lower virulence.     

HOST-PATHOGEN INTERACTIONS IN NATURAL POPULATIONS OF LINUM MARGINALE AND MELAMPSORA LINI: II. LOCAL AND REGIONAL VARIATION IN PATTERNS OF RESISTANCE AND RACIAL STRUCTURE

Spatial variation in the resistance structure of Linum marginale (wild flax) populations to the rust fungus Melampsora lini, and in the racial structure of this pathogen, was investigated by sampling 10 populations distributed throughout the Kosciusko National Park, New South Wales, Australia. Considerable differences were found among populations in the structure of both host and pathogen. Host populations were divided into three broad categories: (1) populations susceptible to all testing races; (2) populations containing a strictly limited number of resistant phenotypes; and (3) populations with a considerable diversity of resistant phenotypes. The pathogen populations also showed a range of diversity. The major differences between these populations were determined by the occurrence and frequency of four common races of pathogen (races A, E, K, and N). These differences were apparent both at a regional spatial scale (over the 100 km separation of the most distant populations) and at a local scale where major differences were detected between two populations only 300 m apart. The distribution of the four common races of M. lini was consistent with the hypothesis that a fitness cost was associated with unnecessary virulence. In general, however, differences in the structure of pathogen populations from genetically very similar host populations implied that, in addition to host resistance genes, other evolutionary forces are also important in determining the genetic structure of individual pathogen populations.     

中华猕猴桃和美味猕猴桃自然居群遗传结构及其种间杂交渐渗

 利用9对SSR引物对中华猕猴桃（Actinidia chinensis）和美味猕猴桃（A. deliciosa）两近缘种的5个同域分布复合体和各自1个非同域分布居群进行了遗传多样性、居群遗传结构的分析以及种间杂交渐渗的探讨。结果表明：1）两物种共有等位基因比例高达81.13%,物种特有等位基因较少（中华猕猴桃：13.27%,美味猕猴桃：5.61%）,但共享等位基因表型频率在两近缘种间存在差异,而且与各同域复合体中两物种样本的交错程度或间距存在关联;2）两种猕猴桃均具有极高遗传多样性,美味猕猴桃的遗传多样性（H_o＝0 .749, PIC＝0.818）都略高于中华猕猴桃（H_o＝0.686,PIC＝0.799）;3）两猕 猴桃物种均具有较低的Nei’s居群遗传分化度,但AMOVA分析结果揭示种内异域居群间（F_ST=0.091 5）和同域复合体种间（F_ST=0.111 5）均存在一定程度的遗传分化;中华猕猴桃居群遗传分化（G_ST=0.086; F_ST=0.212 1）高于美味猕猴桃（G_ST= 0.080;F_ST=0.142 0）;4）同域分布复合体两物种间的遗传分化（G_ST=0.020）低于物种内异域居群间的遗传分化（中华猕猴桃：G_ST=0.086; 美味猕猴桃：G_ST=0.080）,同域复合体物种间的基因流（N_m＝7.89 -29.75）远远高于 同种异域居群间（中华猕猴桃：N_m =2.663; 美味猕猴桃：N_m=2.880）; 5）居群UPGMA聚类揭示在同一地域的居群优先聚类,个体聚类结果显示多数个体聚在各自居群组内,但各地理居群并不按地理距离的远近聚类,这与Mantel相关性检测所揭示的居群间遗传距离与地理距离没有显著性相关的结果一致。进一步分析表明两种猕猴桃的遗传多样性和居群遗传结构不仅受其广域分布、远交、晚期分化等生活史特性的影响,同时还与猕猴桃的染色体基数高 (x=29)、倍性复杂和种间杂交等因素密切相关,其中两种猕猴桃的共享祖先多态性和同域分布种间杂交基因渗透对两猕猴桃的居群遗传结构产生了重要影响。     

ESTERASE VARIATION AND GENETIC STRUCTURE IN THREE GEOGRAPHIC POPULATIONS OF SITODIPLOSIS MOSELLANA (GEHIN) (DIPTERA: CECIDOMYIIDAE) IN WESTERN CHINA*

Abstract This paper investigates the esterase variation and genetic structure in three geographic populations of Sitodiplosis mosellana (Géhin) in western China by PAGE. The localities surveyed are Gaolan (36.3°N, 103.9°E) and Wuwei (37.9°N, 102.6°E) in spring wheat region and Chang'an (34. 1°N, 108.9° E) in winter wheat region. The results suggest that the esterase is coded by two loci: Est‐1 and Est‐2. Est‐1 is coded by a plastogene producing only one band that is the fastest on the gel among all bands. The Est‐2 is duplicated loci with 8 alleles, namely, a, b, c, d, e, f, g, h, which produce altogether 8 bands in all the populations and 1–4 bands in individual samples. There are 19 zymogram types observed in the three geographic populations. Seventeen zymogram types emerge in Chang'an population, but 5 and 4 zymogram types are found respectively in Gaolan and Wuwei populations. II₂ zymogram type is the commonest in all the populations. The alleles that had the highest frequencies in all the populations are d, e, g. All 8 alleles at the Est2 were observed in Chang'an population, but only total 3 alleles‐d, e, g at the Est‐2 appeared in Gaolan and Wuwei populations. The analysis of genetic identity and cluster (UPGMA) on the alloenzyme indicates that the relationship between the two populations of spring wheat region seems to be closer, as compared with the relationship between spring wheat population and winter wheat population. It is evident that there exists some infraspecific variation caused mainly by genetic drift in S. mosellana and the gene flow among the populations possibly took place to some extent.     

GENETIC VARIANCE FOR RATE OF POPULATION INCREASE IN NATURAL POPULATIONS OF FLOUR BEETLES,TRIBOLIUM SPP.

The genetic and ecological effects of population subdividsion were investigated for two wild strains of Tribolium castaneum and two wild strains of T. confusum and compared with the effects of population subdivision on the synthetic laboratory strain of T. castaneum (c-SM), used extensively in earlier experiments. For the c-SM strain, it has been shown repeatedly, for a variety of different population structures (different combinations of effective numbers, N_e, and migration rates, m), that large heritable differences in population growth rate arise among demes during 10 to 15 generations of population subdivision. Because this laboratory strain was synthesized by mass mating several “inbred” strains in 1973 (80 to 100 generations ago), it is possible that it has genetic variation for fitness (measured as the heritable variance among demes in the rate of population increase) unusually large compared to natural populations of flour beetles. In this paper, I report that natural populations of flour beetle exhibit as much or more phenotypic and genetic variation in the effects of population structure on fitness than the laboratory strain, c-SM. The observation of substantial heritable variation for fitness in natural populations is unexpected under additive theory and may be indicative of nonadditive genetic variance.     

HOMOGENEITY OF THE GENETIC VARIANCE-COVARIANCE MATRIX FOR ANTIPREDATOR TRAITS IN TWO NATURAL POPULATIONS OF THE GARTER SNAKE THAMNOPHIS ORDINOIDES

Quantitative genetic models of evolution rely on the genetic variance-covariance matrix to predict the phenotypic response to selection. Both prospective and retrospective studies of phenotypic evolution across generations rely on assumptions about the constancy of patterns of genetic covariance through time. In the absence of robust theoretical predictions about the stability of genetic covariances, this assumption must be tested with empirical comparisons of genetic parameters among populations and species. Genetic variance-covariance matrices were estimated for a suite of antipredator traits in two populations of the northwestern garter snake, Thamnophis ordinoides. The characters studied include color pattern and antipredator behaviors that interact to facilitate escape from predators. Significant heritabilities for all traits were detected in both populations. Genetic correlations and covariances were found among behaviors in both populations and between color pattern and behavior in one of the populations. Phenotypic means differed among populations, but pairwise comparisons revealed no heterogeneity of genetic parameters between the populations. The structure of the genetic variance-covariance matrix has apparently not changed significantly during the divergence of these two populations.     

EPISTASIS AND THE EVOLUTION OF ADDITIVE GENETIC VARIANCE IN POPULATIONS THAT PASS THROUGH A BOTTLENECK

Traditional models of genetic drift predict a linear decrease in additive genetic variance for populations passing through a bottleneck. This perceived lack of heritable variance limits the scope of founder-effect models of speciation. We produced 55 replicate bottleneck populations maintained at two male-female pairs through four generations of inbreeding (average F = 0.39). These populations were formed from an F₂ intercross of the LG/J and SM/J inbred mouse strains. Two contemporaneous control strains maintained with more than 60 mating pairs per generation were formed from this same source population. The average level of within-strain additive genetic variance for adult body weight was compared between the control and experimental lines. Additive genetic variance for adult body weight within experimental bottleneck strains was significantly higher than expected under an additive genetic model This enhancement of additive genetic variance under inbreeding is likely to be due to epistasis, which retards or reverses the loss of additive genetic variance under inbreeding for adult body weight in this population. Therefore, founder-effect speciation processes may not be constrained by a loss of heritable variance due to population bottlenecks.     

UNDERSTANDING THE POPULATION GENETIC STRUCTURE OF GLEDITSIA TRIACANTHOS L.: THE SCALE AND PATTERN OF POLLEN GENE FLOW

Spatial and temporal patterns of gene flow determine the extent to which populations can differentiate from one another as a result of natural selection or genetic drift. In this study, we investigated pollen-mediated gene flow in two eastern Kansas populations of the subdioecious tree species, Gleditsia triacanthos L. (Leguminosae), or honeylocust. In 2 yr at each site, we used paternity-exclusion analysis to estimate the proportion of seeds sired by immigrant pollen. We also used a single-parent and parent-pair exclusion analysis on naturally established seedlings and saplings to estimate gene flow into one site over a 12-yr period and into the second site over a 22-yr period. Results of both analyses showed high minimum estimates of pollen gene flow into each site (17%–30%). In each population, we found significantly less gene flow in years of high fruit production than in years of low fruit production, but in one population, we observed little variation in gene-flow rates among age classes of seedlings and saplings. The level of pollen gene flow showed weak negative dependence on the relative isolation distances of the maternal trees sampled (140–240 m at one site vs. 85–120 m at the second site), and gene-flow estimates from naturally established juveniles were very similar at the two sites. Within populations, a multiple regression model showed that maximum-likelihood estimates of male fertility were negatively associated with distances between mates and positively associated with male size as measured by stem diameter. In neither population, however, did the regression explain more than 16% of the total variation in male fertilities.     

HIGH LEVELS OF GENETIC DIVERSITY AND LOW LEVELS OF GENETIC DIFFERENTIATION IN NORTH SEA PSEUDO‐NITZSCHIA PUNGENS (BACILLARIOPHYCEAE) POPULATIONS1

Six microsatellite markers were used to investigate the genetic structure of North Sea Pseudo‐nitzschia pungens (Grunow ex P. T. Cleve) Hasle populations. Isolates were collected on 42 separate occasions from waters surrounding the German islands of Helgoland and Sylt over the course of three sampling periods: spring 2002, spring 2003, and autumn 2003. In total, 464 isolates were genotyped, of which 453 were different (i.e. clonal diversity was 98%). The numbers of alleles per locus ranged from 6 to 24 and the observed heterozygosities from 0.59 to 0.87 (mean H_o and H_e were 0.73); there were no significant departures from Hardy‐Weinberg equilibrium at any of the six loci. Sexual reproduction therefore appears to be important in the production of genetic variation. Over the temporal and spatial scales sampled (18 months and 100 km), weak genetic differentiation was detected both within and between sampling periods (significant F_ST values ranged from 0.0018 to 0.0389), suggesting that the German North Sea supports a single largely unstructured population of P. pungens.     

MOVEMENTS AND POPULATION STRUCTURE OF HUMPBACK WHALES IN THE NORTH PACIFIC

Despite the extensive use of photographic identification methods to investigate humpback whales in the North Pacific, few quantitative analyses have been conducted. We report on a comprehensive analysis of interchange in the North Pacific among three wintering regions (Mexico, Hawaii, and Japan) each with two to three subareas, and feeding areas that extended from southern California to the Aleutian Islands. Of the 6,413 identification photographs of humpback whales obtained by 16 independent research groups between 1990 and 1993 and examined for this study, 3,650 photographs were determined to be of suitable quality. A total of 1,241 matches was found by two independent matching teams, identifying 2,712 unique whales in the sample (seen one to five times). Site fidelity was greatest at feeding areas where there was a high rate of resightings in the same area in different years and a low rate of interchange among different areas. Migrations between winter regions and feeding areas did not follow a simple pattern, although highest match rates were found for whales that moved between Hawaii and southeastern Alaska, and between mainland and Baja Mexico and California. Interchange among subareas of the three primary wintering regions was extensive for Hawaii, variable (depending on subareas) for Mexico, and low for Japan and reflected the relative distances among subareas. Interchange among these primary wintering regions was rare. This study provides the first quantitative assessment of the migratory structure of humpback whales in the entire North Pacific basin.