THE EVOLVING GENETIC HISTORY OF A POPULATION OF LATHYRUS SYLVESTRIS: EVIDENCE FROM TEMPORAL AND SPATIAL GENETIC STRUCTURE

We analyze patterns of genetic microdifferentiation within a natural population of Lathyrus sylvestris, a perennial herb with both sexual reproduction and clonal growth. In a population from the northern foothills of the Pyrénées in southwestern France, a combined demographic and genetic investigation enabled the study not only of spatial genetic structure of the population, but also of the history of the population's spatial genetic structure over time. Excavation of all individuals allowed identification of clonemates. Age of each individual was determined by counting annual growth rings in the taproot, a method tested with individuals of known age planted in experimental gardens. Each individual was mapped, and genotypes of all individuals were determined using allozyme markers for a number of polymorphic loci. Distribution patterns and spatial genetic structure, both for all individuals and for different age classes, were analyzed using spatial autocorrelation statistics (Geary's Index, Moran's Index). Patterns of gene flow within the population were also studied using F-statistics and tests for random associations of alleles. Because age, allele frequencies, and location were known for each individual, it was possible to study how spatial genetic structure changed over time. Results from all these diverse approaches are consistent with one another, and clearly show the following: (1) founder effects, with the study transect being first colonized by individuals at either end of the transect that were homozygous for different alleles at one marker locus; (2) a difference in spatial distribution of individuals originated from sexual reproduction (seedlings) and from clonal growth (connected individuals); (3) restricted gene flow, due to inbreeding among related, clumped individuals; and (4) increase in heterozygote deficit within the youngest cohort of individuals. The results indicate that genetic differentiation in time was much less marked than differentiation in space. Nevertheless, the results revealed that the studied population is experiencing demographic and genetic variation in time, suggesting that it is not at equilibrium. On the one hand, spatial structuring is becoming less marked due to the recombination of founder genotypes; on the other hand, as establishment of new individuals increases, a new spatial structure emerges due to mating between relatives.     

FINE-GRAINED SPATIAL AND TEMPORAL VARIATION IN SELECTION DOES NOT MAINTAIN GENETIC VARIATION IN ERIGERON ANNUUS

Because interactions among plants are spatially local, the scale of environmental heterogeneity can have large effects on evolutionary dynamics. However, very little is known about the spatial patterns of variation in fitness and the relative magnitude of spatial and temporal variation in selection. Replicates of 12 genotypes of Erigeron annuus (Asteraceae) were planted in 288 locations within a field, separated by distances of 0.1 to 30.0 m, and replicated in two years. In a given year, most spatial variation in relative fitness (genotype-environment [G × E] interactions for fitness) occurred over distances of only 50 cm. Year effects were as large or larger than the spatial variation in fitness; in particular there was a large, three-way, genotype-year-environment interaction at the smallest spatial scale. The genetic correlation of fitness across years at a given location was near zero, 0.03. Thus, the relative fitness of genotypes is spatially unpredictable and a map of the selective environment has constantly shifting locations of peaks and valleys. Including measurements of soil nutrients as covariates in the analysis removed most of the spatial G × E interaction. Vegetation and microtopography had no effect on the G × E terms, suggesting that differential response to soil nutrients is the cause of spatial variation in fitness. However, the slope of response to NH₄ and P0₄ was negative; therefore the soil nutrients are probably just indicators of other, unknown, environmental factors. We explored via simulation the evolutionary consequences of spatial and temporal variation in fitness and showed that, for this system, the spatial scale of variation was too fine grained (by a factor of 3 to 5) to be a powerful force maintaining genetic variation in the population. The inclusion of both spatial and temporal variation in fitness actually reduced the coexistence of genotypes compared to pure spatial models. Thus the presence of spatial or temporal variation in selection does not guarantee that it is an effective evolutionary force maintaining diversity. Instead the pattern of selection favors generalist genotypes.     

GENE DISPERSAL AND SPATIAL GENETIC STRUCTURE

Spatial autocorrelation statistics have been studied in theoretical population genetic models and widely used in experimental studies of spatial structure in many plant and animal populations. However, the statistical properties of spatial autocorrelation statistics have remained uncharacterized. Little is known about how values of spatial autocorrelation statistics in population samples depend on the level of dispersal and scheme of sampling. In this paper, we characterize the statistical properties of join-count spatial autocorrelation statistics for population genetic surveys under various conditions of dispersal and sampling. The results indicate generally high statistical power. These results can provide a method to estimate gene dispersal based on standing spatial patterns of genetic variation observed within populations.     

稻瘟病菌群体的分子遗传学研究II.广东省2000年度稻瘟病菌群体遗传结构的时空特性

利用随机扩增多态性DNA技术对广东省2000年度稻瘟病菌群体的遗传结构进行了分析。以相似性系数为0.62阀值时,可将采集于广东省三大生态稻作区、早稻和晚稻生长季节的104个菌株划分为14个遗传宗谱;其中宗谱5和宗谱8的菌株数各占总数的25%,为优势宗谱;宗谱4和12的菌株数各占总数的14.4%和9.6%,为亚优势宗谱;其余的29个菌株,分别归属于其它10个宗谱,其中有5个宗谱是单菌株宗谱。本年度稻瘟病菌群体的遗传结构呈现明显的区域性特性:遗传结构呈由北向南多样化的趋势;各个稻作区甚或亚区有其特异性的宗谱。本年度稻瘟病菌群体的遗传结构也显示分明的生长季节特性:来源于早稻和晚稻生长季节的菌株完全分属于宗谱图的上下两个半区,彼此之间不存在共同的宗谱;而且后者的遗传宗谱要比前者的复杂、多样。研究还表明,虽然稻瘟病菌群体的遗传结构2000年度与1998-1999年度的比较存在较大的差异,但是两者仍然具有良好的相承性和可比性。如何进一步验证和把握稻瘟病菌群体的遗传结构所表现出的时空特性是值得我们进一步探讨的重要课题。     

SPECIATION AND POPULATION GENETIC STRUCTURE IN TROPICAL PACIFIC SEA URCHINS

Unlike populations of many terrestrial species, marine populations often are not separated by obvious, permanent barriers to gene flow. When species have high dispersal potential and few barriers to gene flow, allopatric divergence is slow. Nevertheless, many marine species are of recent origin, even in taxa with high dispersal potential. To understand the relationship between genetic structure and recent species formation in high dispersal taxa, we examined population genetic structure among four species of sea urchins in the tropical Indo-West Pacific that have speciated within the past one to three million years. Despite high potential for gene flow, mtDNA sequence variation among 200 individuals of four species in the urchin genus Echinometra shows a signal of strong geographic effects. These effects include (1) substantial population heterogeneity; (2) lower genetic variation in peripheral populations; and (3) isolation by distance. These geographic patterns are especially strong across scales of 5000-10,000 km, and are weaker over scales of 2500-5000 km. As a result, strong geographic patterns would not have been readily visible except over the wide expanse of the tropical Pacific. Surface currents in the Pacific do not explain patterns of gene flow any better than do patterns of simple spatial proximity. Finally, populations of each species tend to group into large mtDNA regions with similar mtDNA haplotypes, but these regional boundaries are not concordant in different species. These results show that all four species have accumulated mtDNA differences over similar spatial and temporal scales but that the precise geographic pattern of genetic differentiation varies for each species. These geographic patterns appear much less deterministic than in other well-known coastal marine systems and may be driven by chance and historical accident.     

SPATIAL DISTRIBUTION OF A PRIMITIVELY SOCIAL BEE: DOES GENETIC POPULATION STRUCTURE FACILITATE ALTRUISM?

Exoneura bicolor is a univoltine, facultatively social bee exhibiting a solitary/quasisocial/semisocial colony polymorphism (Schwarz, 1986, 1987). Intracolony relatedness in semisocial colonies has been previously estimated at 0.49 ± 0.06 (Schwarz, 1987), although the crucial relatedness between altruists and the brood that they rear will be about half this value. This value is unlikely to be increased by the preferential rearing of only close relatives (Schwarz, 1988a) and no known morphological specializations preclude workers from reproducing in this species. Hamilton (1972, 1975) suggested that relatedness may be increased through population subdivision, if this leads to significant inbreeding and increased between-colony genetic variance. The same process may also operate at higher levels of population structure (e.g., Wade, 1978). Population structure and intracolony relatedness in E. bicolor were investigated in seven localities in southern Victoria, Australia, to determine if inbreeding at any level of population structure was contributing to relatedness between altruists and beneficiaries within these colonies. Population structure was described using hierarchical F-statistics and an identity by descent measure, developed by Queller and Goodnight (1989), was used to estimate intracolony relatedness. It was found that inbreeding was not contributing to between-group genetic variance, at any level, in a consistent manner across localities. Therefore relatedness, considered in isolation, does not seem sufficient to account for the presence of worker behavior. It is suggested that large benefits for group living may be responsible for maintaining altruistic behavior, in part, in this species. Significant heterogeneity among localities for all F-statistics estimated in our analysis was found and this may be attributable to stochastic elements such as cofounding behavior and the low percentage of males in the brood. The possible consequences of such heterogeneity in population structure for the maintenance of altruism in E. bicolor are discussed.     

园林昆虫群落时间结构及动态研究

调查了４种不同园林植物类型内昆虫群落在１年中的组成及数量变化,应用主分量分析方法及最优分割方法分析了昆虫群落在时间过程中的主导因素和时间格局。结果表明,构成复杂的植物类型内,昆虫群落构成亦复杂,在时间过程中,昆虫群落结构变化波动小,具有明显的主导因素和时间格局;在单纯的植物类型中,昆虫群落组成简单,随时间变化,群落剧烈波动,且主导因素及时间格局分化不明显。     

不同生境三裂叶豚草种群的遗传结构

采用水平切片淀粉凝胶电泳测定了东北地区分布的不同生态环境条件下7个三裂叶豚草种群的遗传结构。统计分析了10个酶系统的10个位点,结果表明:三裂叶豚草种群内存在着丰富的遗传变异。多态位点百分率为80.00%,等位基因平均数为1.80,期望杂合度为0.375,遗传一致度和遗传距离为0.959和0.042。     

GENETIC POPULATION STRUCTURE AND MATING SYSTEM IN CHONDRUS CRISPUS (RHODOPHYTA)1

Chondrus crispus Stackh. has been intensely studied, yet no study to date has elucidated its population structure or mating system despite many populations in which there was a haploid bias and lack of male gametophytes. Therefore, 12 nuclear microsatellite loci were identified in this red alga. Microsatellite markers were developed and tested against a panel of specimens collected from two shore levels at two sites in Brittany, France: Pointe de Primel and Pointe de la Jument, Concarneau. Single locus genetic determinism was verified at eight polymorphic loci, as only one band was observed for haploid genotypes, whereas one or two bands were observed for diploids. These markers enabled the detection of unique genotypes within sampled populations, indicating that very few fronds shared the same multilocus genotype. This finding suggests that asexual reproduction was not the prevailing mode of reproduction. In addition, we explored the hierarchical population structure showing that gene flow is restricted at small spatial scales (<50 m) between upper and lower Chondrus‐range populations within a shore. Sexual reproduction predominated in the populations of C. crispus studied, but probably due to fine‐scale spatial substructuring, inbreeding was also significant. In conclusion, this study reveals that fine‐scale genetic variation is of major importance in C. crispus, suggesting that differences between microhabitats should be essential in understanding evolutionary processes in this species.     

THE INFLUENCE OF DENSITY AND SEX ON PATTERNS OF FINE-SCALE GENETIC STRUCTURE

Natal philopatry is expected to limit gene flow and give rise to fine-scale spatial genetic structure (SGS). The banner-tailed kangaroo rat ( Dipodomys spectabilis ) is unusual among mammals because both sexes are philopatric. This provides an opportunity to study patterns of local SGS faced by philopatric and dispersing animals. We evaluated SGS using spatial genetic autocorrelation in two D. spectabilis populations (Rucker and Portal) over a 14-year temporal series that covered low, medium, and high population densities. Significantly positive autocorrelation values exist up to 800 m at Rucker and 400 m at Portal. Density was negatively associated with SGS (low >medium >high), and suggests that increases in density are accompanied by greater spatial overlap of kin clusters. With regard to sex-bias, we find a small but significant increase in the SGS level of males over females, which matches the greater dispersal distances observed in females. We observed variation in SGS over the ecological time scale of this study, indicating genetic structure is temporally labile. Our study is the first temporal exploration of the influence of density and sex on spatial genetic autocorrelation in vertebrate populations. Because few organisms maintain discreet kin clusters, we predict that density will be negatively associated with SGS in other species.     

CONTRIBUTION OF TEMPORAL AND SPATIAL COMPONENTS TO MORPHOLOGICAL VARIATION IN THE KELP ECKLONIA (LAMINARIALES)1

Environmental conditions that are known to cause morphological variation in algae (e.g., wave exposure) often vary in both space and time and are superimposed onto the distinct seasonal growth cycles of most temperate macroalgae. We tested the hypothesis that the morphology of the small kelp Ecklonia radiata (C. Agardh) J. Agardh is the product of an interaction between site (five reefs of different wave exposure) and the time of year that sampling occurs (summer vs. winter 2004). We determined that wave exposure had a strong directional effect on kelp morphology, with “Reefs” accounting for up to 43.4% of variation in individual morphological characters. “Times” had a narrowly nonsignificant effect on overall morphology but accounted for up to 31% of variation in individual characters. Many characters were affected by wave exposure, whereas only a few were (strongly) affected by time (e.g., thallus biomass). Interactive effects between “Reefs” and “Times” were generally small, accounting for 15.8% of variation in lamina thickness, but much less for most other characters. We conclude that wave exposure exerts a strong control over the morphology of E. radiata, but that the nature of the effect depends on the magnitude of wave exposure. We also conclude that most of the effects of wave exposure are consistent through time and do not interact with cycles of growth and pruning in any major way.     

THE EFFECTS OF HOST GENOTYPE AND SPATIAL DISTRIBUTION ON TREMATODE PARASITISM IN A BIVALVE POPULATION

A basic assumption underlying models of host-parasite coevolution is the existence of additive genetic variation among hosts for resistance to parasites. However, estimates of additive genetic variation are lacking for natural populations of invertebrates. Testing this assumption is especially important in view of current models that suggest parasites may be responsible for the evolution of sex, such as the Red Queen hypothesis. This hypothesis suggests that the twofold reproductive disadvantage of sex relative to parthenogenesis can be overcome by the more rapid production of rare genotypes resistant to parasites. Here I present evidence of significant levels of additive genetic variance in parasite resistance for an invertebrate host-parasite system in nature. Using families of the bivalve mollusc, Transennella tantilla, cultured in the laboratory, then exposed to parasites in the field, I quantified heritable variation in parasite resistance under natural conditions. The spatial distribution of outplanted hosts was also varied to determine environmental contributions to levels of parasite infection and to estimate potential interactions of host genotype with environment. The results show moderate but significant levels of heritability for resistance to parasites (h² = 0.36). The spatial distribution of hosts also significantly influenced parasite prevalence such that increased host aggregation resulted in decreased levels of parasite infection. Family mean correlations across environments were positive, indicating no genotype-environment interaction. Therefore, these results provide support for important assumptions underlying coevolutionary models of host-parasite systems.     

POPULATION STRUCTURE OF DIPODOMYS INGENS (HETEROMYIDAE): THE ROLE OF SPATIAL HETEROGENEITY IN MAINTAINING GENETIC DIVERSITY

The giant kangaroo rat, Dipodomys ingens (Heteromyidae), is an endangered rodent that inhabits approximately 3% of its estimated historic range. Its current distribution is centered in two geographic areas, situated about 150 km apart, in south-central California. We sequenced a 293 base-pair fragment at the 5' end of the control region in 95 giant kangaroo rats from nine localities to examine the genetic structure of extant populations. We determine that mutations in this section of the control region follow a negative binominal distribution, rather than a Poisson. However, the distance between haplotypes is small enough that the difference between a tree that corrects for the non-Poisson distribution of mutations and one that does not, is minimal. This implies that the use of methods that assume a Poisson distribution of mutations, such as those based on coalescent theory, are justified. We find that the correlation between levels of genetic diversity and estimated census size is poor. This suggests that population sizes have fluctuated over time or that populations have not been isolated from one another, or both. We also examine the hierarchical structure of populations and find that the southern populations are not genetically subdivided but that there is significant subdivision between northern and southern populations and between some northern subpopulations. The phylogeographic relationship between northern and southern populations can primarily be attributed to isolation by distance, although the time since divergence between them appears to be less than the age of either. To examine the phylogeographic relationships in more detail we construct a minimum spanning tree based on Tamura-Nei gamma-corrected distances and superimpose on it the geographic position of haplotypes. This reveals that there is more genetic distance between some northern haplotypes than between any northern and southern haplotypes, despite the geographic distance separating north from south and the larger size of the southern population. It also reveals that one northern population, in the Panoche Valley, contains old allelic lineages and shares ancestral polymorphism with several other populations. It also shows that two, small, geographically remote populations contain a surprising amount of genetic diversity, but that different population/geographic processes have affected the structure of that diversity. We estimate the average migration rate among all populations to be 7.5 per generation, and conclude that a disproportionate number of migration events involve gene flow with one northern population, the Panoche Valley. We find evidence for the hypothesis that there has been an increase in population size in the remaining populations in the north and suggest that the Panoche Valley could play a role in these expansions. Finally we discuss the probabilitiy that the genetic structure of the southern populations has been affected by fluctuations in size. These results are briefly compared to other studies on the genetic structure of rodent populations.     

TEMPORAL PATTERNS IN POPULATION GENETIC DIVERSITY OF PROROCENTRUM MICANS (DINOPHYCEAE)1

We describe temporal changes in the genetic structure of populations of the dinoflagellate Prorocentrum micans Ehrenberg over a period of 2 years at Scripps Pier (La Jolla, CA, USA). We collected 12 water samples over the course of two blooms and analyzed 166 single‐cell isolates using randomly amplified polymorphic DNA analysis. Six PCR primers uncovered 27 polymorphic markers, allowing the identification of 40 unique haplotypes. Analysis of molecular variance demonstrated that >92% of the genetic variance was partitioned within water samples, providing evidence of high levels of genetic diversity and possibly sexual reproduction. Although the level of genetic diversity remained fairly stable over the sampled time interval, several populations (sampled in June 1998 and March 1999) exhibited significantly different genetic composition, demonstrating differences among bloom and nonbloom periods. About 40% of the isolates in each sample were identified as one haplotype, suggesting that a genetically distinct subgroup was a common member of the populations during the sampled periods. The composition of the remaining isolates was genetically diverse and changed over time, indicating rapid responses (days) to changing environmental conditions or extensive genetic spatial patchiness (kilometers). Within the limitations of our sampling, these two genetically distinct groups appear to exhibit different population dynamics (one stable and the other variable), suggesting that genetic diversity may be closely linked to the change in abundance of phytoplankton on ecological time scales.     

SPATIAL GENETIC STRUCTURE OF CLONAL AND SEXUAL REPRODUCTION IN POPULATIONS OF ADENOPHORA GRANDIFLORA (CAMPANULACEAE)

The spatial distribution of clonal versus sexual reproduction in plant populations should generally have differing effects on the levels of biparental inbreeding and the apparent selfing rate, produced via mating by proximity through limited pollen dispersal. We used allozyme loci, join-count statistics, and Moran's spatial autocorrelation statistics to separate the spatial genetic structure caused by clonal reproduction from that maintained in sexually reproduced individuals in two populations of Adenophora grandiflora, a perennial herb. Join-count statistics showed that there were statistically significant clustering of clonal genotypes within distances less than 4 m. Both the entire populations and the sets of sexually reproduced individuals exhibited significant spatial autocorrelation at less than about 12 m, and the sexually reproduced individuals are substantially structured in an isolation-by-distance manner, consistent with a neighborhood size of about 50.     

SELECTION ON THE COLOR POLYMORPHISM IN HAWAIIAN HAPPY-FACE SPIDERS: EVIDENCE FROM GENETIC STRUCTURE AND TEMPORAL FLUCTUATIONS

Throughout this century genetic polymorphisms for color have been widely used as a research tool to allow insights into key evolutionary processes. Although color variants can often be diverse within populations, frequencies of different morphs may be similar across populations, either as a result of balancing selection or gene flow. Under these circumstances selection can be extremely difficult to demonstrate. Here we test for balancing selection on the naturally occurring color forms of the Hawaiian happy-face spider, Theridion grallator with two approaches. First, allozyme loci are used to generate a null model against which to test selection. Frequencies of alleles involved in the color polymorphism of T. grallator are used to generate another estimate for comparison. The results suggest that statistically similar frequencies of color morphs among populations of T. grallator may be maintained by some form of balancing selection. Second, we make use of an unusual event in which the normally stable frequencies of unpatterned and patterned morphs within a population were found to have shifted toward an excess of unpatterned morphs. We scored offspring of all fertilized, unpatterned (bottom-recessive) females found during this period of skewed morph frequencies and also in a year when morph frequencies were normal to deduce paternal color phenotypes. Mating was found to be random in the normal year, but in the perturbed year females had mated with rare (patterned) males twice as frequently as expected on the basis of the frequency of this morph type in the population. Both of these results are consistent with selection operating on the color polymorphism, and we speculate that apostatic selection, perhaps mediated by bird predators, may provide the mechanism.     

新疆早寒武世Lapworthella一新种及本属有关问题探讨

西藏海相下第三系,广泛断续地出露于雅鲁藏布江断裂带南北。本文概略论述西藏海相下第三系,列举一些新的地层剖面,划分对比地层,简述沉积岩相古地理,提出双壳类化石组合特征并描述5个双壳类化石新种。它的研究对了解喜马拉雅地质历史的发展,海水退出该区的最后时限,以及探讨古地理、古气候等,都具有一定的理论与实际意义。     

中生代海相双壳类(软体动物)的漂浮与假漂浮

中生代很多海相足丝固着和粘结固着的表栖双壳类有着广泛的、甚至全球的地理分布,有些种能够穿越赤道,跨越特提斯海,进行南、北高纬度生物群的交流,有些种能够由东向西横渡浩瀚的古太平洋进行扩散,因为这些双壳类幼虫期的浮游异养型幼虫能营漂浮生活,而幼虫期后的个体又能挂系或粘结在其它漂浮或移动的物体上进行假漂浮迁移。     

地质统计学在昆虫种群空间结构研究中的应用概述

地质统计学是用来人间相关变量结构的统计学方法,地质统计学中的变差函数和变差图,可分析空间变量在不同方向或不同环境条件下的空间结构。     

浙江省松阳县黄山松种群的年龄结构与分布格局

黄山松在我国东南部海拔700—2000m中山带广泛分布,是该地区森林群落中重要的组成成分。黄山松种群在森林群落演替中起着先锋种群的作用。但在裸岩陡坡或峰岭梁岗等特殊生境中,黄山松种群亦能形成稳定的群落。本文在种群的水平上,分析了浙江省松阳县关山源地区黄山松种群在不同演替阶段群落中和不同生境中的年龄结构与种群空间分布格局,并探讨了它们的动态规律及其与环境之间的相互关系。黄山松种群的研究不仅有助于探讨植物生态学研究中的一些理论问题,而且对经营管理和保护利用具有一定的实践意义。