MDH-polymorphism in Drosophila subobscura: I. Selection and hitch-hiking in laboratory populations

Summary The interrelation of genic polymorphism at the Malate dehydrogenase (Mdh)-locus and chromosomal polymorphism for superimposed gene arrangements was studied in 5 experimental populations of Drosophila subobscura. It could be shown that chromosomal polymorphism is maintained by balancing selection in favour of the heterozygotes. In contrast, selection at the Mdh-locus itself seems to be of minor importance. The changes of frequency observed for the Mdh-alleles are most probably due to hitch-hiking on the gene blocks enclosed by the chromosomal structures or to selection for tightly linked genes rather than to fitness differences between Mdh-genotypes. The results may be considered as a model for the situation found in natural populations of D. subobscura.     

Selection and genetic drift in captive versus wild populations: an assessment of neutral and adaptive (MHC-linked) genetic variation in wild and hatchery brown trout (<Emphasis Type="Italic">Salmo trutta</Emphasis>) populations

Captive bred individuals are often released into natural environments to supplement resident populations. Captive bred salmonid fishes often exhibit lower survival rates than their wild brethren and stocking measures may have a negative influence on the overall fitness of natural populations. Stocked fish often stem from a different evolutionary lineage than the resident population and thus may be maladapted for life in the wild, but this phenomenon has also been linked to genetic changes that occur in captivity. In addition to overall loss of genetic diversity via captive breeding, adaptation to captivity has become a major concern. Altered selection pressure in captivity may favour alleles at adaptive loci like the Major Histocompatibility Complex (MHC) that are maladaptive in natural environments. We investigated neutral and MHC-linked genetic variation in three autochthonous and three hatchery populations of Austrian brown trout (Salmo trutta). We confirm a positive selection pressure acting on the MHC II β locus, whereby the signal for positive selection was stronger in hatchery versus wild populations. Additionally, diversity at the MHC II β locus was higher, and more uniform among hatchery samples compared to wild populations, despite equal levels of diversity at neutral loci. We postulate that this stems from a combination of stronger genetic drift and a weakening of positive selection at this locus in wild populations that already have well adapted alleles for their specific environments.     

The albumin polymorphism and bilirubin binding in rhesus monkeys (Macaca mulatta)

Family studies confirmed a codominant mode of inheritance of albumin phenotypes identified in Macaca mulatta. Preliminary epidemiological and bilirubin binding studies suggest that selection favors the variant allele by reducing risks of hyperbilirubinemia and might be responsible for this otherwise rare polymorphism.     

(C-A) and (G-A) anchored simple sequence repeats (ASSRs) generated polymorphism in soybean, Glycine max (L.) Merr.

 We used thirty simple sequence repeats (SSRs) with a variable 2–4 base ‘anchor’ at their 5′ ends (ASSRs) independently or with arbitrary primers in analysis of soybean germplasm and the intercross of ‘Essex’ and PI 437654. (AG)₆, (GA)₆ or (CT)₆ and (GT)₆, (TG)₆ or (CA)₆ were efficient in the detection of (G-A) and (C-A) ASSR-generated polymorphisms, respectively. DNA sequence analysis of the ASSR-amplified fragments confirmed the presence of SSR sequences. (A-T) ASSRs failed to give amplification or generated fewer number of fragments. Only one of the four tested decamer primers altered ASSR banding patterns in the soybean. All the six long primers (18–20 mer) tested changed ASSR banding profiles. On average, seven polymorphic fragments per ASSR primer were produced in soybean germplasm and four in the intraspecific cross of ‘Essex’ and PI 437654. The grouping of 48 genotypes in UPGMA analysis using four (C-A) and four (G-A) ASSR primers was consistent with the classification obtained with RFLP markers. Seventy-seven (91%) ASSR markers were dominant, while the remaining 8 (9%) showed codominant segregation. Fifty-eight ASSR markers were mapped onto 18 RAPD/RFLP linkage groups, which covered approximately 50% of the soybean genome. Of the (G-A) ASSR-derived markers 49% remained unlinked compared with 17% of (C-A) ASSR markers at LOD 3.0. Map linkage information showed that the assigned (C-A) polymorphisms had a biased distribution, whereas (G-A) polymorphisms were randomly dispersed. Received: 24 July 1997 / Accepted: 17 November 1997     

Degenerate oligonucleotide gene shuffling (DOGS) and random drift mutagenesis (RNDM): two complementary techniques for enzyme evolution

Improvement of the biochemical characteristics of enzymes has been aided by misincorporation mutagenesis and DNA shuffling. Many gene shuffling techniques result predominantly in the regeneration of unshuffled (parental) molecules. We describe a procedure for gene shuffling using degenerate primers that allows control of the relative levels of recombination between the genes that are shuffled, and reduces the regeneration of unshuffled parental genes. This shuffling procedure avoids the use of endonucleases for gene fragmentation prior to shuffling and allows the inclusion of random mutagenesis of selected portions of the chimeric genes as part of the procedure. We illustrate the use of the shuffling technique with a family of beta-xylanase genes that possess widely different G+C contents. In addition, we introduce a new method (RNDM) for rapid screening of mutants from libraries where no adaptive selection has been imposed on the cells. They are identified only by their retention of enzymatic activity. The combination of RNDM followed by DOGS allows a comprehensive exploration of a protein's functional sequence space.     

Balancing selection, random genetic drift, and genetic variation at the major histocompatibility complex in two wild populations of guppies (Poecilia reticulata)

Our understanding of the evolution of genes of the major histocompatibility complex (MHC) is rapidly increasing, but there are still enigmatic questions remaining, particularly regarding the maintenance of high levels of MHC polymorphisms in small, isolated populations. Here, we analyze the genetic variation at eight microsatellite loci and sequence variation at exon 2 of the MHC class IIB (DAB) genes in two wild populations of the Trinidadian guppy, Poecilia reticulata. We compare the genetic variation of a small (Ne, 100) and relatively isolated upland population to that of its much larger (Ne approximately 2400) downstream counterpart. As predicted, microsatellite diversity in the upland population is significantly lower and highly differentiated from the population further downstream. Surprisingly, however, these guppy populations are not differentiated by MHC genetic variation and show very similar levels of allelic richness. Computer simulations indicate that the observed level of genetic variation can be maintained with overdominant selection acting at three DAB loci. The selection coefficients differ dramatically between the upland (s > or = 0.2) and lowland (s < or = 0.01) populations. Parasitological analysis on wild-caught fish shows that parasite load is significantly higher on upland than on lowland fish, which suggests that large differences in selection intensity may indeed exist between populations. Based on the infection intensity, a substantial proportion of the upland fish would have suffered direct or indirect fitness consequences as a result of their high parasite loads. Selection by parasites plays a particularly important role in the evolution of guppies in the upland habitat, which has resulted in high levels of MHC diversity being maintained in this population despite considerable genetic drift.     

Middle ear structure and bone conduction in Spalax,Eospalax, and Tachyoryctes mole‐rats (Rodentia: Spalacidae)

There is evidence that spalacine, tachyoryctine, and myospalacine mole‐rats all communicate with conspecifics through a form of seismic signaling, but the route for the detection of these signals is disputed. It has been proposed that two unusual anatomical adaptations in Spalax allow jaw vibrations to pass to the inner ear via the incus and stapes: a pseudoglenoid (=postglenoid) fossa which accomodates the condylar process of the mandible, and a bony cup, supported by a periotic lamina, through which the incus articulates with the skull. In this study, a combination of dissection and computed tomography was used to examine the ear region in more detail in both Spalax and its subterranean relatives Tachyoryctes and Eospalax, about which much less is known. Tachyoryctes was found to lack a pseudoglenoid fossa, while Eospalax lacks a periotic lamina and bony cup. This shows that these structures need not simultaneously be present for the detection of ground vibrations in mole‐rats. Based on the observed anatomy, three hypothetical modes of bone conduction are argued to represent more likely mechanisms through which mole‐rats can detect ground vibrations: ossicular inertial bone conduction, a pathway involving sound radiation into the external auditory meatus, and a newly‐described fluid pathway between pseudoglenoid fossa and cranial cavity. The caudolateral extension of the tympanic cavity and the presence of a bony cup might represent synapomorphies uniting Spalax and Tachyoryctes, while the loss of the tensor tympani muscle in Spalax and Eospalax may be convergently derived. J. Morphol., 2010. © 2009 Wiley‐Liss, Inc.     

Isolation and biodiversity in Cytisus villosus Pourret (Fabaceae,Genisteae): enzyme polymorphism in disjunct populations

ABSTRACT

The genetic diversity of isolated populations of Cytisus villosus has been studied by means of enzyme polymorphism analysis. Two types of isolated populations were studied: “terrestrial islands” in Sicily, and “true islands” in the Aeolian archipelago. In the populations of “true islands” the number of alleles and the heterozygosity are lower than in “terrestrial islands”. Isolation amongst Sicilian populations seems to be more recent than isolation of the Aeolian populations, and may be attributed to climatic changes which occurred during the Holocene and/or to human activities. The disjunction of the Aeolian populations seems much more recent than the origin of the isles themselves; the colonization of the archipelago is attributed to a single, recent dispersal event not followed by local evolution. In view of the biological structure of the Aeolian populations, C. villosus must be regarded as a locally endangered species.     

Chromosomal polymorphism of populations of Anopheles arabiensis (Diptera: Culicidae) from Reunion island and cross-fertility among continental African populations

Cytological examination of a sample of Anopheles gambiae complex mosquitoes from Reunion island revealed the presence of An. arabiensis only. Chromosomal polymorphisms were observed only for inversion 3Ra, the standard homozygote form being predominant. Cross-mating experiments with laboratory specimens originating from continental Africa produced viable and fertile offspring with no chromosomal asynapsis observed in the F1 female progeny. There was no evidence for speciation of the Reunion island populations. The results are discussed with regard to the behaviour of the vector and its influence on the vectorial capacity of this species, and the history of malaria and malaria control in the South-West islands of the Indian Ocean and on Reunion island in particular.     

Genetic modelling: an analysis of a colour polymorphism in the Snow Goose (Anser caerulescens)

Previous studies of colour polymorphism in the Snow Goose [ Anser caerulescens ; failed to consider and reject alternative hypotheses to that of a single locus with incomplete dominance. Utilizing data from a long-term study of a wild population, we test the validity of these earlier results by considering two alternatives: (1) a single locus with multiple allelism and (2) a threshold polygenic system. Our analyses corroborate the original model, but emphasize the importance of testing all plausible hypotheses.     

Selection and drift in subdivided populations: a straightforward method for deriving diffusion approximations and applications involving dominance, selfing and local extinctions

Population structure affects the relative influence of selection and drift on the change in allele frequencies. Several models have been proposed recently, using diffusion approximations to calculate fixation probabilities, fixation times, and equilibrium properties of subdivided populations. We propose here a simple method to construct diffusion approximations in structured populations; it relies on general expressions for the expectation and variance in allele frequency change over one generation, in terms of partial derivatives of a "fitness function" and probabilities of genetic identity evaluated in a neutral model. In the limit of a very large number of demes, these probabilities can be expressed as functions of average allele frequencies in the metapopulation, provided that coalescence occurs on two different timescales, which is the case in the island model. We then use the method to derive expressions for the probability of fixation of new mutations, as a function of their dominance coefficient, the rate of partial selfing, and the rate of deme extinction. We obtain more precise approximations than those derived by recent work, in particular (but not only) when deme sizes are small. Comparisons with simulations show that the method gives good results as long as migration is stronger than selection.     

Epigenetic polymorphism in populations of Apodemus sylvaticus and Apodemus flavicollis (Rodentia, Muridae)

The main object of this study was to use discontinuous variation as a means of measuring divergence between populations of A. sylvaticus (L.) and A. flavicollis (Melchior) from Britain and Eurasia. Samples from 17 populations (671 skulls) were each classified for 20 discontinuous morphological variants. The investigation has shown that certain epigenetic variants have a characteristic incidence in populations of the two species and that the relative incidence of the variants could be used as an aid to specific separation. The calculation of degree of divergence between the populations has made it possible to suggest the order in which populations are related to each other.     

Variability in response of four populations of Amblyseius largoensis (Acari: Phytoseiidae) to Raoiella indica (Acari: Tenuipalpidae) and Tetranychus gloveri (Acari: Tetranychidae) eggs and larvae

Raoiella indica (Acari: Tenuipalpidae) is a phytophagous mite that recently invaded the Neotropical region. A predatory mite Amblyseius largoensis (Acari: Phytoseiidae) has been found associated with R. indica in Florida. This study evaluated A. largoensis by determining its likelihood of consuming eggs and larvae of R. indica and Tetranychus gloveri (Acari: Tetranychidae) under no-choice and choice conditions. To detect variations in the response of A. largoensis to R. indica, four populations of predators were examined: (1) predators reared exclusively on R. indica in the laboratory for 2 years, (2) predators reared on T. gloveri in the laboratory for 2 months but reared on R. indica for 2 years previously, (3) predators collected from a field infested with R. indica, and (4) predators collected from a field that had never been infested with R. indica. Results of this study suggest that A. largoensis is likely to accept and consume high numbers of R. indica eggs regardless of their previous feeding experience. In contrast, all populations consumed relatively fewer R. indica larvae than the other prey tested. Predators previously exposed to R. indica were more likely to consume R. indica larvae. By contrast, predators not previously exposed to R. indica showed the lowest likelihood of choosing to feed on this prey item. Plasticity in the response of A. largoensis to R. indica larvae could be associated to selection, learning, or a combination of both. The possible implications of the observed differences in terms of biological control of R. indica are discussed.     

Selection of light or darkness, locomotor, and stridulatory activities in the cricket, Gryllus bimaculatus Degeer (Orthoptera: Gryllidae)

Male crickets, Gryllus bimaculatus Degeer, turned lights on or off in a chamber by a seesaw device: (1) during a 12 h, and (2) during a 24 h day. The crickets in (1) and the last-instar nymph in (2) turned the lights on and off at irregular intervals and duration. The selection rate for darkness was greater than that for light by an average exceeding 80%. The locomotor activity of the nymph in (1) was arrhythmic. In (1) the adults stridulated and were active in continuous darkness during the 12 h, while, in contrast in (2) they turned on the light and stridulated without switching the light off. The locomotor and stridulatory activities of the adult crickets in (2) were free-running. These activities resulted in a free-running rhythm of selection for light or darkness in (2). Under the conditions of the present experiments, the circadian pacemaker functioned in the same way in light and dark cycles as in constant light conditions.     

Genetic drift vs. natural selection in a long-term small isolated population: major histocompatibility complex class II variation in the Gulf of California endemic porpoise (Phocoena sinus)

Although many studies confirm long-term small isolated populations (e.g. island endemics) commonly sustain low neutral genetic variation as a result of genetic drift, it is less clear how selection on adaptive or detrimental genes interplay with random forces. We investigated sequence variation at two major histocompatibility complex (Mhc) class II loci on a porpoise endemic to the upper Gulf of California, México (Phocoena sinus, or vaquita). Its unique declining population is estimated around 500 individuals. Single-strand conformation polymorphism analysis revealed one putative functional allele fixed at the locus DQB (n = 25). At the DRB locus, we found two presumed functional alleles (n = 29), differing by a single nonsynonymous nucleotide substitution that could increase the stability at the dimer interface of alphabeta-heterodimers on heterozygous individuals. Identical trans-specific DQB1 and DRB1 alleles were identified between P. sinus and its closest relative, the Burmeister's porpoise (Phocoena spinipinnis). Comparison with studies on four island endemic mammals suggests fixation of one allele, due to genetic drift, commonly occurs at the DQA or DQB loci (effectively neutral). Similarly, deleterious alleles of small effect are also effectively neutral and can become fixed; a high frequency of anatomical malformations on vaquita gave empirical support to this prediction. In contrast, retention of low but functional polymorphism at the DRB locus was consistent with higher selection intensity. These observations indicated natural selection could maintain (and likely also purge) some crucial alleles even in the face of strong and prolonged genetic drift and inbreeding, suggesting long-term small populations should display low inbreeding depression. Low levels of Mhc variation warn about a high susceptibility to novel pathogens and diseases in vaquita.     

Alary polymorphism in the small brown planthopper Laodelphax striatellus (Homoptera: Delphacidae)

Environmental and genetic factors which influence alary polymorphism in Laodelphax striatellus (Fallén) were studied. It has been found that alary polymorphism in L. striatellus is determined by quality and quantity of food, and crowding. There is also a clear genetic basis for polymorphism.

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Zusammenfassung In dieser Studie werden einige Umweltfaktoren und genetische Faktoren untersucht, welche den Flügelpolymorphismus bei Laodelphax striatellus (Fallén) beeinflussen. Es wurde festgestellt, dass verschiedene Umweltfaktoren wie Qualität und Quantität des Futters und Crowding bestimmend wirken. Es besteht auch eine klare genetische Basis für Polymorphismus.

     

Diffusion approximations for one-locus multi-allele kin selection,mutation and random drift in group-structured populations: a unifying approach to selection models in population genetics

Diffusion approximations are ascertained from a two-time-scale argument in the case of a group-structured diploid population with scaled viability parameters depending on the individual genotype and the group type at a single multi-allelic locus under recurrent mutation, and applied to the case of random pairwise interactions within groups. The main step consists in proving global and uniform convergence of the distribution of the group types in an infinite population in the absence of selection and mutation, using a coalescent approach. An inclusive fitness formulation with coefficient of relatedness between a focal individual J affecting the reproductive success of an individual I, defined as the expected fraction of genes in I that are identical by descent to one or more genes in J in a neutral infinite population, given that J is allozygous or autozygous, yields the correct selection drift functions. These are analogous to the selection drift functions obtained with pure viability selection in a population with inbreeding. They give the changes of the allele frequencies in an infinite population without mutation that correspond to the replicator equation with fitness matrix expressed as a linear combination of a symmetric matrix for allozygous individuals and a rank-one matrix for autozygous individuals. In the case of no inbreeding, the mean inclusive fitness is a strict Lyapunov function with respect to this deterministic dynamics. Connections are made between dispersal with exact replacement (proportional dispersal), uniform dispersal, and local extinction and recolonization. The timing of dispersal (before or after selection, before or after mating) is shown to have an effect on group competition and the effective population size. In memory of Sam Karlin.     

极小种群野生植物抢救性保护面临的机遇与挑战

<正>过去50年,中国在生物多样性保护方面取得了一系列成就。但是,国家层面立法保护的植物仅有1999年国务院批准的《国家重点保护野生植物名录(第一批)》,之后并没有根据全国普查与各地调查报道的结果进行相应的更新。目前,最突出和最具有争议性的一个问题是,哪些物种才是最需要优先保护的?一些研究机构基于物种保护生物学的研究,认为应参考国际自然保护联盟(IUCN)对受威胁物种的等级划分标准来确定,但这些标准是基于全球尺度,并且大部分标准是依赖于对未来若干年的物     

Trans-species polymorphism and evidence of selection on class II MHC loci in tuco-tucos (Rodentia: Ctenomyidae)

Balancing selection acting over the evolutionary history of a lineage can result in the retention of alleles among species for longer than expected under neutral evolution. The associated pattern of trans-species polymorphism, in which similar or even identical alleles are shared among species, is often used to infer that balancing selection has occurred. The genes of the major histocompatibility complex (MHC) are thought to be subject to balancing selection that maintains alleles associated with response to specific pathogens. To explore the role of balancing selection in shaping MHC diversity in ctenomyid rodents, we examined allelic variability at the class II DRB and DQA loci in 18 species in the genus Ctenomys. Previous studies of four of these species had revealed significant within-population evidence of positive selection on MHC loci. The current study expands upon these analyses to (1) evaluate among-species evidence of positive selection and (2) explore the potential for balancing selection on MHC genes. Interspecific nucleotide sequence variation revealed significant evidence of positive selection on the DRB and DQA loci. At the same time, comparisons of phylogenetic trees for these MHC loci with a putative species tree based on mitochondrial sequence data revealed multiple examples of trans-specific polymorphism, including sharing of identical DRB and DQA alleles among distantly related species of Ctenomys. These findings suggest that MHC genes in these animals have historically been subject to balancing selection and yield new insights into the complex suite of forces shaping MHC diversity in free-living vertebrates.