Spatial ecology of multiple parasitoids of a patchily‐distributed host: implications for species coexistence

1. The coexistence of multiple species sharing similar but spatially fragmented resources (e.g. parasitoids sharing a host species) may depend on their relative competitive and dispersal abilities, or on fine‐scale resource partitioning. Four generalist and one specialist parasitoid species associated with the holly leaf miner, Phytomyza ilicis, in a woodland network of 127 holly trees were investigated. 2. To understand coexistence and persistence of these potential competitors, patterns of occurrence in relation to patch size and isolation, vertical stratum within patches, and incidence and abundance of potential competitors were documented. Field experiments creating empty habitat patches suggested that dispersal rather than local demographic processes determines abundance and incidence. 3. Parasitoids showed species‐specific responses to patch properties, with the incidence of species determined mostly by patch size. Parasitism rates were less clearly related to patch characteristics, but parasitism rates for most species were lower in patches where the numerically dominant parasitoid species, Chrysocharis gemma, was present. No evidence of vertical stratification was found in species composition or abundance within patches, making it unlikely that coexistence is enhanced by fine‐scale resource division. 4. Overall, the patterns detected may be attributed to the distribution of C. gemma and differences in species' ecology other than dispersal ability. The life history of C. gemma may allow it to pre‐emptively exploit a large fraction of the available hosts, avoiding direct competition with other parasitoids. In contrast, direct competition is more likely among the pupal parasitoids Cyrtogaster vulgaris, Chrysocharis pubicornis, and Sphegigaster flavicornis which have a similar biology and phenology. For these species, coexistence may be facilitated by contrasting incidence in relation to patch size and isolation.     

Fine-scale spatial genetic structure in the brooding sea urchin <Emphasis Type="Italic">Abatus cordatus</Emphasis> suggests vulnerability of the Southern Ocean marine invertebrates facing global change

The Southern Ocean benthic communities are characterized by their levels of endemism and their diversity of invertebrate brooding species. Overall, biological processes acting within these species remain poorly understood despite their importance to understand impacts of ongoing global change. We take part in filling this gap by studying the genetic structure over different spatial scales (from centimeters to tens of kilometers) in Abatus cordatus, an endemic and brooding sea urchin from the Kerguelen Islands. We developed three microsatellites and two exon-primed intron crossing markers and conducted a two-scale sampling scheme (from individuals to patches) within two dense localities of Abatus cordatus. Between patches, all pairwise comparisons, covering distances from few meters (between patches within locality) to 25 km (between localities), revealed significant genetic differentiation, a higher proportion of the molecular variance being explained by the comparisons between localities than within localities, in agreement with an isolation by distance model. Within patches, we found no significant correlation between individual pairwise spatial and genetic distances, except for the most polymorphic locus in the patch where the largest range of geographical distances had been analyzed. This study provides an estimation of the dispersal capacities of Abatus cordatus and highlights its low recolonization ability. Similar low recolonization capacities are thus expected in other Antarctic and Subantarctic brooding invertebrate species and suggest a high vulnerability of these species facing global change.     

Genetic diversity and biogeography of the traditional Chinese medicine, Gardenia jasminoides, based on AFLP markers

Gardenia jasminoides Ellis is used in traditional Chinese medicine (TCM) in China. Levels of genetic variation and patterns of population structure within and among eight wild or cultivated populations of G. jasminoides Ellis in China were investigated using amplified fragment length polymorphism (AFLP) markers. Of the 11 primers screened, four produced highly reproducible AFLP bands. Using these primers, 244 discernible DNA fragments were generated with 165 bands (67.6%), were polymorphic, indicating considerable genetic variation at the species level. In contrast, there were relatively low levels of polymorphism at the population level with the percentage of polymorphic bands (PPB) ranging from 36.89% to 59.43%. Genetic diversity within populations ranged from 0.2086 to 0.3108, averaging 0.2392 at the species level. A high level of genetic differentiation among populations was detected based on Nei's genetic diversity analysis (76.59%), Shannon's index analysis (64.8%) and AMOVA analysis (72.75%). No significant statistical differences (analysis of molecular variance [AMOVA], p = 0.0639) in AFLP variation were found between regions. However, the variance among populations and within populations differed significantly (p < 0.001). An indirect estimate of historical levels of gene flow (N_m = 1.7448) was consistent with the high mean genetic identity (mean I = 0.9263) found among populations. There is an association between geographic and genetic distances between populations. Presently gene change exists between populations.     

Genetic Variation and Clonal Diversity of Psammochloa villosa(Poaceae) Detected by ISSR Markers

Genetic variation and clonal diversity of seven Psammochloavillosa(Poaceae) populations from northwest China were investigatedusing inter simple sequence repeat (ISSR) markers. Of the 84primers screened, 12 produced highly reproducible ISSR bands.Using these primers, 173 discernible DNA fragments were generatedwith 122 (70.5%) being polymorphic, indicating considerablegenetic variation at the species level. In contrast, there wererelatively low levels of polymorphism at the population levelwith the percentage of polymorphic bands (PPB) ranging from6.1 to 26.8. Analysis of molecular variance (AMOVA) showed thata large proportion of genetic variation (87.46%) resided amongpopulations, while only 12.54% resided among individuals withinpopulations. Clonal diversity was also high with 98 genets beingdetected from among 157 individuals using 12 ISSR primers. Theevenness of distribution of genotypes in P. villosa populationsvaried greatly, with all of the genotypes being local ones.No significant differences in genetic or clonal diversity werefound between populations in mobile or fixed dunes. The mainfactor responsible for the high level of differentiation amongpopulations and the low level of diversity within populationsis probably the clonal nature of this species, although selfingmay also affect the population genetic structure to some extent.The efficiency of ISSRs in identifying genetic individuals wasmuch higher than that of allozymes. An approximately asymptoticcorrelation was found between the number of genets detectedand the number of polymorphic loci used, suggesting that useof a high number of polymorphic bands is critical in genet identification.Copyright 2001 Annals of Botany Company Psammochloa villosa, ISSRs, genetic variation, clonal diversity     

Low genetic variation detected within the widespread mangrove species Nypa fruticans (Palmae) from Southeast Asia

Genetic diversity within and among six natural populations of Nypa fruticans from China, Vietnam, and Thailand was assessed using SSR and ISSR analysis. Our results showed an extremely low level of genetic diversity of N. fruticans (at the species level, P = 11.76% and 2.88%, He = 0.0279 and 0.0113, I = 0.0470 and 0.0167 by SSRs and ISSRs, respectively) across a total of 183 individuals. No genetic variation was detected within any population except for the Thailand population by SSRs (P = 11.76%, He = 0.0417; I = 0.0622). The bottlenecks during glacial epochs, founder effects, and propagation pattern may be responsible for the extremely low level of genetic diversity of N. fruticans.     

Genetic diversity and population structure analysis of Qinghai-Tibetan plateau schizothoracine fish (Gymnocypris dobula) based on mtDNA D-loop sequences

Gymnocypris dobula is a commercially important fishery species and mostly distributed in the freshwater of Tibetan plateau. In this study, genetic diversity and intraspecific population differentiation were examined by using mitochondrial DNA D-loop sequences in 97 individuals sampled from three localities (Pali, Lasa and Yanghu) in the Tibetan plateau. Two hundred and fourteen polymorphic sites and 50 haplotypes were defined among the three localities. Genetic diversity analysis showed that the highest genetic diversity level was found in Pali population. Phylogenetic relationships analysis results indicated that closer phylogenetic relationships were found between the Yanghu and Lasa populations. Genetic population differentiation analysis indicated that the majority of variation (84.91%) was attributed to variations among populations and the largest differentiation was found between Pali and Lasa localities (F_st = 0.874, Nm = 0.036). Moreover, the historical demographic events were assessed by implementing the mismatch distribution analysis, Fajima's D test and Fu's Fs test. The results indicated that the Pali population had undergone a demographic expansion, possibly within the last 0.163 MYA (Million Years Ago). Our study firstly identified the population genetic structure of the G. dobula, which could be helpful for artificial breeding, fishery stock identification and resource conservation for this species.     

Genetic structure of Rhamnus glaucophylla Sommier endemic to Tuscany

Rhamnus glaucophylla Sommier is an endemic small tree or shrub belonging to sect. Eurhamnus. It grows on cliffs of the Apuan Alps and of the Lucca Apennine in Tuscany (Italy). ISSRs were used to detect the genetic diversity within and among five populations representative of the species distribution range. Moderate levels of genetic diversity were revealed as percentage of polymorphic band (PPB) both at the population (PPB?=?22.63%, H _S?=?0.0801, Sh?=?0.1193) and species level (PPB?=?48.68%, H _T?=?0.1155, Sh?=?0.1837). The correlation between genetic and geographic distances was not significant (Mantel test, r?=?0.17; P?<?0.26). The information on the pattern of genetic variation obtained in this work bears important implications for conservation management, and in particular for ex situ conservation programmes involving long-term seed storage in seed banks.     

Patch size drives colonization by aquatic insects,with minor priority effects of a cohabitant

Patch size is one of the most important factors affecting the distribution and abundance of species, and recent research has shown that patch size is an important niche dimension affecting community structure in aquatic insects. Building on this result, we examined the impact of patch size in conjunction with presence of larval anurans on colonization by aquatic insects. Hyla chrysoscelis (Cope''s gray treefrog) larvae are abundant and early colonists in fishless lentic habitats, and these larvae can fill multiple ecological roles. By establishing larvae in mesocosms prior to colonization, we were able to assess whether H. chrysoscelis larvae have priority effects on aquatic insect assemblages. We conducted a series of three experiments in naturally colonized experimental landscapes to test whether (1) H. chrysoscelis larval density affects insect colonization, (2) variation in patch size affects insect colonization, and (3) the presence and larval density of H. chrysoscelis shift colonization of insects between patches of different size. Larval density independently had almost no effect on colonization, while patch size had species‐specific effects consistent with prior work. When larvae and patch size were tested in conjunction, patch size had numerous, often strong, species‐specific effects on colonization; larval density had effects largely limited to the assemblages of colonizing beetles and water bugs, with few effects on individual species. Higher larval densities in large mesocosms shifted some insect colonization to smaller patches, resulting in higher beta diversity among small patches in proximity to high density large mesocosms. This indicates establishing H. chrysoscelis larvae prior to insect colonization can likely create priority effects that slightly shape insect communities. Our results support the importance of patch size in studying species abundances and distributions and also indicate that colonization order plays an important role in determining the communities found within habitat patches.     

Hygrochastic capsule dehiscence supports safe site strategies in New Zealand alpine Veronica (Plantaginaceae)

Background and Aims

Hygrochasy is a capsule-opening mechanism predominantly associated with plants in arid habitats, where it facilitates spatially and temporally restricted dispersal. Recently, hygrochastic capsules were described in detail for the first time in alpine Veronica in New Zealand. The aim of the present study was to investigate whether hygrochastic capsules are an adaptation of alpine Veronica to achieve directed dispersal to safe sites. We expect that by limiting dispersal to rainfall events, distances travelled by seeds are short and confine them to small habitat patches where both seedlings and adults have a greater chance of survival.

Methods

Dispersal distances of five hygrochastic Veronica were measured under laboratory and field conditions and the seed shadow was analysed. Habitat patch size of hygrochastic Veronica and related non-hygrochastic species were estimated and compared.

Key Results

Dispersal distances achieved by dispersal with raindrops did not exceed 1 m but weather conditions could influence the even distribution of seeds around the parent plant. Compared with related Veronica species, hygrochastic Veronica mostly grow in small, restricted habitat patches surrounded by distinctly different habitats. These habitat patches provide safe sites for seeds due to their microtopography and occurrence of adult cushion plants. Non-hygrochastic Veronica can be predominantly found in large habitats without clearly defined borders and can be spread over long distances along rivers.

Conclusions

The results suggest that hygrochasy is a very effective mechanism of restricting seed dispersal to rainfall events and ensuring short-distance dispersal within a small habitat patch. It appears that it is an adaptation for directed dispersal to safe sites that only exist within the parent habitat.     

Integrative taxonomy of a poorly known Sahelian rodent, Gerbillus nancillus (Muridae,Gerbillinae)

Aridification processes that affected the Sahelian area of West Africa during the last decades have induced significant changes in plant and animal communities of this region. In rodents, the genus Gerbillus characteristic of North African and Asian arid habitats has been affected by this climatic and environmental trends. Several species of this genus showed a southward range expansion in recent years into the Sahelian bioclimatic zone. Recent sampling in several localities of West Africa (Mali, Niger and Senegal) enabled us to collect numerous specimens of small gerbils. An integrative study of these samples using molecular, morphological and cytogenetical methods revealed that many of them were attributable to Gerbillus nancillus, a secretive and poorly known species. Gerbillus nancillus appears characterized by a well differentiated karyotype with 2n = 56 chromosomes, and to represent a unique genetic lineage within this genus. Body and skull measurements of G. nancillus were compared with those of the morphologically similar Gerbillus henleyi, which provided diagnostic clues between them. These new data significantly expand the distribution area of G. nancillus, which now ranges from Sudan in the East to Senegal in the West. G. nancillus is here reported from numerous new localities in Niger and Mali, and for the first time in Senegal, which raises questions about the origin of its presence and the colonization routes it followed to get there. We also show that G. henleyi and G. nancillus are sympatric and apparently often syntopic in the sub-Saharan part of the distribution of G. henleyi.     

Phylogenetic analysis of nuclear and mitochondrial DNA reveals a complex of cryptic species in Crassicutis cichlasomae (Digenea: Apocreadiidae), a parasite of Middle-American cichlids

We obtained nuclear ITS-1 and mitochondrial cox1 sequences from 225 Crassicutis cichlasomae adults collected in 12 species of cichlids from 32 localities to prospect for the presence of cryptic species. This trematode is commonly found in species of cichlids over a wide geographic range in Middle-America. Population-level phylogenetic analyses of ITS-1 and cox1, assessments of genetic and haplotype diversity, and morphological observations revealed that C. cichlasomae represents a complex of seven cryptic species for which no morphological diagnostic characters have been discovered thus far. Bayesian and Maximum Likelihood analyses of concatenated datasets (906 bp) recovered eight lineages of C. cichlasomae, all with high posterior probabilities and bootstrap branch support. Values of genetic divergence between clades ranged from 1.0% to 5.2% for ITS-1, and from 7.2% to 30.0% for cox1. Morphological study of more than 300 individuals did not reveal structural diagnostic traits for the species defined using molecular evidence. These observations indicate that some traditional morphological characters (e.g., testes position) have substantial intra-specific variation, and should be used with caution when classifying C. cichlasomae and their sister taxa. Additionally, phylogenetic analyses did not reveal a strict correlation between these cryptic species and their host species or geographic distribution, however it appears that genetic distinctiveness of these cryptic species was influenced by the diversification and biogeographical history of Middle-American cichlids.     

Genetic Diversity and Spatial Genetic Structure of an Epiphytic Bromeliad in Costa Rican Montane Secondary Forest Patches

Information on genetic variation and its distribution in tropical plant populations relies mainly on studies of ground‐rooted species, while genetic information of epiphytic plants is still limited. Particularly, the effect of forest successional condition on genetic diversity and structure of epiphytes is scanty in the literature. We evaluated the genetic variation and spatial genetic structure of the epiphytic bromeliad Guzmania monostachia (Bromeliaceae, Tillandsioideae) in montane secondary forest patches in Costa Rica. The sampling design included plants on the same trees (i.e., populations), populations within forest patches and patches within secondary forest at two different successional stages (early vs. mid‐succession). Six microsatellites revealed low levels of population genetic variation (A = 2.06, A_E = 1.61, H_E = 0.348), a marked deficiency of heterozygotes (H_O = 0.031) and high inbreeding (f = 0.908). Genetic differentiation was negligible among populations within the same forest patch, but moderate (G_ST = 0.123 ± 0.043) among forest patches. Genetic relatedness between individuals was significantly higher for plants located within the same forest patch and separated by <60 m and decreased as distance between plants increased, becoming significantly negative at distances >400 m. An analysis of molecular variance (AMOVA) showed significant genetic variation between forest patches, but non‐significant variation between successional stages. The selfing breeding system and limited seed dispersal capabilities in G. monostachia could explain the observed levels and partitioning of genetic diversity at this geographic scale. However, these results also suggest that forest fragmentation is likely to influence the degree of local genetic structuring of epiphytic plants by limiting gene flow.     

Endangered species in small habitat patches can possess high genetic diversity: the case of the Tana River red colobus and mangabey

We used mtDNA sequence data from the Tana River red colobus and mangabey to determine how their population genetic structure was influenced by dispersal and habitat fragmentation. The colobus and mangabey are critically endangered primates endemic to gallery forests in eastern Kenya. The forests are a Pliocene–Pleistocene refugium that has recently undergone significant habitat loss and fragmentation due to human activities. We expected both primates to exhibit low levels of genetic diversity due to elevated genetic drift in their small populations, and to show a strong correspondence between genetic and geographic distance due to disruption of gene flow between forests by habitat fragmentation. Additionally, because mangabey females are philopatric, we expected their mtDNA variation to be homogeneous within forest patches but to be heterogeneous between patches. In contrast, colobus have a female-biased dispersal and so we expected their mtDNA variation to be homogeneous within and between forest patches. We found high levels of haplotype and nucleotide diversity as well as high levels of sequence divergence between haplotype groups in both species. The red colobus had significantly higher genetic variation than the mangabey did. Most of the genetic variation in both primates was found within forest fragments. Although both species showed strong inter-forest patch genetic structure we found no correspondence between genetic and geographic distances for the two primates. We attributed the high genetic diversity to recent high effective population size, and high sequence divergence and strong genetic structures to long-term habitat changes in the landscape.     

Genetic diversity of an endangered aquatic plant, Potamogeton lucens subspecies sinicus

The genetic diversity of the species, Potamogeton lucens subsp. sinicus var. teganumensis, which is critically endangered in Japan, was investigated. This species now occurs in only two known localities in Japan. One is a native population (Oitoike population), but the other (Teganuma-Okahotto population) is found in a small artificial pond that was dug in 1998. It is considered that the Teganuma-Okahotto population grew from a soil seed bank. Based on RAPD variation, we compared the genetic diversity of the two populations of P. lucens var. teganumensis in Japan and one population of P. lucens subsp. sinicus var. sinicus in China. The Teganuma-Okahotto population showed RAPD variation, suggesting that it may be derived from more than one seed buried in old sediments. This population also had the highest value of Shannon's Information Index among the three study populations. This finding suggests that seeds buried in sediments can contain genetic variability, and may be used to conserve the genetic diversity of rare and endangered plants.     

Divergent selection along climatic gradients in a rare central European endemic species,Saxifraga sponhemica

Background and Aims The effects of habitat fragmentation on quantitative genetic variation in plant populations are still poorly known. Saxifraga sponhemica is a rare endemic of Central Europe with a disjunct distribution, and a stable and specialized habitat of treeless screes and cliffs. This study therefore used S. sponhemica as a model species to compare quantitative and molecular variation in order to explore (1) the relative importance of drift and selection in shaping the distribution of quantitative genetic variation along climatic gradients; (2) the relationship between plant fitness, quantitative genetic variation, molecular genetic variation and population size; and (3) the relationship between the differentiation of a trait among populations and its evolvability.Methods Genetic variation within and among 22 populations from the whole distribution area of S. sponhemica was studied using RAPD (random amplified polymorphic DNA) markers, and climatic variables were obtained for each site. Seeds were collected from each population and germinated, and seedlings were transplanted into a common garden for determination of variation in plant traits.Key Results In contrast to previous results from rare plant species, strong evidence was found for divergent selection. Most population trait means of S. sponhemica were significantly related to climate gradients, indicating adaptation. Quantitative genetic differentiation increased with geographical distance, even when neutral molecular divergence was controlled for, and Q_ST exceeded F_ST for some traits. The evolvability of traits was negatively correlated with the degree of differentiation among populations (Q_ST), i.e. traits under strong selection showed little genetic variation within populations. The evolutionary potential of a population was not related to its size, the performance of the population or its neutral genetic diversity. However, performance in the common garden was lower for plants from populations with reduced molecular genetic variation, suggesting inbreeding depression due to genetic erosion.Conclusions The findings suggest that studies of molecular and quantitative genetic variation may provide complementary insights important for the conservation of rare species. The strong differentiation of quantitative traits among populations shows that selection can be an important force for structuring variation in evolutionarily important traits even for rare endemic species restricted to very specific habitats.     

Identification of endangered Mediterranean cyprinodontiform fish by means of DNA inter-simple sequence repeats (ISSRs)

We used inter-simple sequence repeat (ISSR) technique to obtain species-specific molecular markers for the cyprinodontiform fish Valencia hispanica, Valencia letourneuxi and Aphanius fasciatus. The aims were (i) to assess the effectiveness of ISSRs in discriminating the three species and (ii) to identify tissues of two unidentified fish suspected to belong to one of the three above species by comparing ISSR genotypes. Nine ISSR primers produced 101 scorable polymorphic loci with a large number of species-specific bands. UPGMA cluster analysis and nonmetric multidimensional scaling of inter-individual genetic dissimilarities were consistent in separating the three species and attributing the unidentified individuals to V. hispanica. This result was corroborated by the assignment test. ISSRs represent an attractive tool for specimen and species identification. We recommend their use, coupled with appropriate statistical analyses, in species identification when only small portions of the organisms are available.     

Fine-scale spatial genetic structure of an endangered marsh herb, Caldesia grandis (Alismataceae)

The endangered marsh herb, Caldesia grandis, is native to China. We investigated the spatial structure of the genetic variation of three populations of C. grandis using RAPD markers and spatial autocorrelation analysis, based on the method of equal distance interval. A total of 157 individuals were sampled from four patches collected from the region of Hunan and Yunnan Provinces, China. Among the polymorphic bands generated by seven selective primers, polymorphic bands with frequencies ranging from 20 to 80% were used to calculate Moran's I spatial autocorrelation coefficient for each patch. We found significant spatial structure of genetic variation in the three patches in Bei Hai (BH) (patches BH-1 and BH-2) and Guai Hu (GH) (patch GH-1) populations of C. grandis (with significant positive autocorrelation within the short distance class). In contrast, the genetic variation in the Lang Pan Hu (LPH) population (patch LPH-1) was found to be randomly distributed. The different spatial distribution patterns may be attributed to environment differences. These results have implications for the conservation and management of this species, especially for sampling strategies for ex situ conservation.     

Life on the edge: conspecific attraction and recruitment of populations to disturbed habitats

Variation in the recruitment of benthic marine invertebrates is often attributed to the interaction of the supply of new individuals to a habitat and the availability of space for colonisation when they arrive. Also important in determining variation in recruitment is the response of the larvae to the characteristics of the habitat. Larvae of many benthic marine invertebrates have shown great specificity of requirements in setting their limits of distribution at the time of selection of a habitat. The tubeworm Galeolaria caespitosa shows great variation in recruitment from place to place on rocky intertidal seashores and is a gregarious animal with larvae showing directed responses to conspecific adults on the substratum. I hypothesised that, if variation in recruitment of G. caespitosa were independent of conditions on the substratum, the magnitude of recruitment in patches of the same shape but different sizes cleared within continuous mats of conspecific adults would be directly related to the area available for colonisation in the patch. Alternatively, if variation in recruitment were due to the response of larvae to conspecific adults on the substratum, the magnitude of recruitment would be a function of the perimeter of the patch, which, given patches of the same shape, is a measure of the influence of conspecific adults in that patch. To distinguish between these alternatives, small (area = 25 cm²; perimeter = 20 cm) and large (area = 225 cm²; perimeter = 60 cm) square patches were cleared within continuous mats of conspecific adults at four sites and recruitment of G. caespitosa was monitored over two seasons of recruitment. The density of recruits per unit area was, on average, almost three times greater in small than in large patches, indicating that recruitment of G. caespitosa is not directly related to the area of the patch. In contrast, the density of recruits per unit perimeter was not significantly different between small and large patches, indicating that recruitment of G. caespitosa is related to the proximity of conspecific adults in the patch. Therefore, at a given site, the perimeter of patches within mats of G. caespitosa is a better predictor of the relative magnitude of recruitment among patches than that provided by their areas. These results are contrary to many models of invertebrate recruitment that assume close linkage between available space on the substratum and settlement. Moreover, they highlight the importance of behavioural responses of animals at the time of selection of habitat in accounting for variation in recruitment. For populations of organisms that display gregarious behaviour at settlement, or conspecific attraction, this direct relationship between the perimeter of patches and recruitment could be used as a tool in restoring populations to disturbed habitats. The added benefit of such facilitative interactions in restoring populations is that they provide increasing returns to the population for a given supply of potential colonists to a habitat. Received: 1 November 1996 / Accepted: 20 January 1997     

The genetic architecture of sexually selected traits in two natural populations of Drosophila montana

We investigated the genetic architecture of courtship song and cuticular hydrocarbon traits in two phygenetically distinct populations of Drosophila montana. To study natural variation in these two important traits, we analysed within-population crosses among individuals sampled from the wild. Hence, the genetic variation analysed should represent that available for natural and sexual selection to act upon. In contrast to previous between-population crosses in this species, no major quantitative trait loci (QTLs) were detected, perhaps because the between-population QTLs were due to fixed differences between the populations. Partitioning the trait variation to chromosomes suggested a broadly polygenic genetic architecture of within-population variation, although some chromosomes explained more variation in one population compared with the other. Studies of natural variation provide an important contrast to crosses between species or divergent lines, but our analysis highlights recent concerns that segregating variation within populations for important quantitative ecological traits may largely consist of small effect alleles, difficult to detect with studies of moderate power.     

浙江天台山甜槠种群遗传结构的空间自相关分析

采用空间自相关分析方法对浙江天台山亚热带常绿阔叶林优势种甜槠种群全部个体及不同年龄级个体的小尺度空间遗传结构进行了分析,以探讨甜槠种群内遗传变异的分布特征及其形成机制。根据11个ISSR引物所提供的多态位点,经GenAlEx 6软件计算地理坐标和遗传距离矩阵在10个距离等级下的空间自相关系数。在样地内,甜槠种群内个体在空间距离小于10 m时存在显著的正空间遗传结构,其X-轴截矩为9.945。甜槠种群的空间遗传结构与其种子短距离传播和广泛的花粉传播有关。Ⅰ年龄级、Ⅱ年龄级和III年龄级个体在空间距离小于10 m时存在显著的正空间遗传结构,其X-轴截矩分别为11.820、9.746和9.792。当距离等级为5 m时,其空间自相关系数r分别为0.068、0.054和0.070。Ⅳ年龄级个体在所有空间距离等级中均不存在显著的空间遗传结构。甜槠是多年生、长寿命植物,自疏作用是导致IV年龄级个体空间遗传结构消失的主要原因。