Cytogenetics of the Anopheles gambiae complex in Sudan, with special reference to An. arabiensis: relationships with East and West African populations

The species composition of malaria vector mosquitoes belonging to the Anopheles gambiae complex (Diptera: Culicidae) from >40 localities in Sudan, representing most ecological situations, was determined by analysis of ovarian polytene chromosomes. Of 2162 females, 93% were identified as An. arabiensis Patton and 7% were An. gambiae Giles sensu stricto. No hybrids were found between the two species. Anopheles arabiensis occurred in all but two sites, whereas An. gambiae s.s. was effectively limited to the southernmost, more humid localities. For chromosomal paracentric inversions, the degree of polymorphism was low in An. gambiae s.s. (inversions 2La, 2Rb and 2Rd), higher in An. arabiensis (inversions Xe, 2Ra, b, bc, d1, s; 3Ra, d). Anopheles gambiae samples from Sudan were all apparently panmictic, i.e. they did not show restricted gene flow such as observed among West African populations (interpreted as incipient speciation). Chromosomal inversion patterns of An. gambiae in southern Sudan showed characteristics of intergrading Savanna/Forest populations similar to those observed in comparable eco-climatic situations of West Africa. Anopheles arabiensis was polymorphic for inversion systems recorded in West Africa (2Ra, 2Rb, 2Rdl, 3Ra) and for a novel 2Rs polymorphism, overlapping with inversion systems 2Rb and 2Rd1. Samples carrying the 2Rs inversion were mostly from Khashm-el-Girba area in central-eastern Sudan. In the great majority of the samples all polymorphic inversions were found to be in Hardy-Weinberg equilibrium. Sudan populations of An. arabiensis should therefore be considered as generally panmictic. Anopheles arabiensis shows more inversion polymorphism in west than in east African populations. Sudan populations have more evident similarities with those from westwards than those from eastwards of the Great Rift Valley. The possible influence of the Rift on evolution of An. arabiensis is discussed.     

Geographic and breed distribution of an Msp I PCR-RFLP in the bovine growth hormone (bGH) gene

Information is presented on the frequency of the Msp I (-) allele in the third intron of the bovine growth hormone gene in a large number of cattle breeds. Consideration of the breed frequencies in relation to their geographic origin shows a low frequency for breeds originating in Northern Europe, moderate frequencies for breeds originating in Eastern Europe or the countries surrounding the Mediterranean basin, and very high frequencies for breeds originating in the Indian subcontinent. Consideration of breed frequencies in relation to breed type, shows low to moderate frequencies for the humpless breeds, high frequencies for the humped breeds. Various explanations for this distribution are discussed, among them the possibility that the Msp I (-) allele originated in the Bos indicus breeds of the Indian subcontinent, from which it diffused through the humpless Bos taurus breeds of Eastern Europe, the Mediterranean basin, eventually reaching Western, Northern Europe, Western Africa in low frequencies.     

杨树基因工程进展

本文重点对杨树的遗传转化技术进行了概述,讨论了影响农杆菌转化效果的主要因素,并综述了杨树在抗除草剂、抗虫和抗病等方面基因工程进展状况。     

Protected polymorphism in the two-locus haploid model with unpredictable fitnesses

 In an unpredictable environment, the distributions of alleles from which polymorphism can be maintained forever belong to a certain set, the C-viability kernel. Such a set is calculated in the two-locus haploid model, as well as the corresponding fitnesses at any time which make this maintenance possible. The dependence of the C-viability kernel on the set U of admissible fitnesses and on the recombination rate r is studied. Notably, the C-viability kernel varies rapidly in the neighborhood of equal fitness of AB and ab; it becomes empty when ab has a fitness below a certain function, which is delineated, of the recombination rate. The properties of the two-locus model under constraints, out of equilibrium and with unpredictable selection are thus presented. Received: 20 May 1999     

Chloroplast DNA polymorphism reveals little geographical structure in Castanea sativa Mill. (Fagaceae) throughout southern European countries

The distribution of haplotypic diversity of 38 European chestnut (Castanea sativa Mill.) populations was investigated by PCR/RFLP analysis of regions of the chloroplast and mitochondrial genomes in order to shed light on the history of this heavily managed species. The rapid expansion of chestnut starting from 3000 years ago is strongly related to human activities such as agricultural practice. This demonstrates the importance of human impact, which lasted some thousands of years, on the present-day distribution of the species. No polymorphism was detected for the single mitochondrial analysed region, while a total of 11 different chloroplast (cp) haplotypes were scored. The distribution of the cpDNA haplotypes revealed low geographical structure of the genetic diversity. The value of population subdivision, as measured by GSTc, is strikingly lower than in the other species of the family Fagaceae investigated. The actual distribution of haplotypic diversity may be explained by the strong human impact on this species, particularly during the Roman civilization of the continent, and to the long period of cultivation experienced during the last thousand years.     

Genetic diversity in an endangered alpine plant, Eryngium alpinum L. (Apiaceae), inferred from amplified fragment length polymorphism markers

Eryngium alpinum L. is an endangered species found across the European Alps. In order to obtain base-line data for the conservation of this species, we investigated levels of genetic diversity within and among 14 populations from the French Alps. We used the amplified fragment length polymorphism (AFLP) technique with three primer pairs and scored a total of 62 unambiguous, polymorphic markers in 327 individuals. Because AFLP markers are dominant, within-population genetic structure (e.g. FIS) could not be assessed. Analyses based either on the assumption of random-mating or on complete selfing lead to very similar results. Diversity levels within populations were relatively high (mean Nei's expected heterozygosity = 0.198; mean Shannon index = 0.283), and a positive correlation was detected between both genetic diversity measurements and population size (Spearman rank correlation: P = 0. 005 and P = 0.002, respectively). Moreover, FST values and exact tests of differentiation revealed high differentiation among populations (mean pairwise FST = 0.40), which appeared to be independent of geographical distance (nonsignificant Mantel test). Founder events during postglacial colonizations and/or bottlenecks are proposed to explain this high but random genetic differentiation. By contrast, we detected a pattern of isolation by distance within populations and valleys. Predominant local gene flow by pollen or seed is probably responsible for this pattern. Concerning the management of E. alpinum, the high genetic differentiation leads us to recommend the conservation of a maximum number of populations. This study demonstrates that AFLP markers enable a quick and reliable assessment of intraspecific genetic variability in conservation genetics.     

Genomic diversity among Vibrio cholerae O139 strains isolated in Bangladesh and India between 1992 and 1998

In order to assess the extent of genomic diversity among Vibrio cholerae O139 strains, restriction fragment length polymorphisms in two genetic loci, rrn and ctx, were studied. Analysis of 144 strains isolated from different regions of Bangladesh and India between 1992 and 1998 revealed the presence of at least six distinct ribotypes (B-I through B-VI) of which three were new ribotypes, and one of these was represented by a nontoxigenic O139 strain. Strains of ribotypes B-I through B-V shared 11 different CTX genotypes (A through K). Antimicrobial resistance patterns of the strains varied independently of their ribotypes and CTX genotypes. Results of this study suggest that V. cholerae O139 is undergoing rapid genetic changes leading to the origination of new variants, and temporal changes in antimicrobial resistance patterns may be contributing to the selection of different variants.     

Molecular evidence for selection on female color polymorphism in the damselfly Ischnura graellsii

The significance of female color polymorphism in Odonata remains controversial despite many field studies. The importance of random factors (founder effects, genetic drift and migration) versus selective forces for the maintenance of this polymorphism is still discussed. In this study, we specifically test whether the female color polymorphism of Ischnura graellsii (Odonata, Coenagrionidae) is under selection in the wild. We compared the degree of genetic differentiation based on RAPD markers (assumed to be neutral) with the degree of differentiation based on color alleles. Weir and Cockerham's theta values showed a significant degree of population differentiation for both sets of loci (RAPD and color alleles) but the estimated degree of population differentiation (theta) was significantly greater for the set of RAPD loci. This result shows that some sort of selection contributes to the maintenance of similar color morph frequencies across the studied populations. Our results combined with those of previous field studies suggest that at least in some I. graellsii populations, density-dependent mechanisms might help to prevent the loss of this polymorphism but cannot explain the similarity in morph frequencies among populations.