Genetic and chromosomal polymorphism in hybridizing populations of the grasshopper Podisma pedestris

The grasshopper Podisma pedistris occurs in two chromosome races, which have XO/XX and neo-XY sex chromosome systems. We have studied chromosomal and electrophoretic variation in populations where these two races meet and hybridize, in an area near the town of Seyne, Alpes Maritimes, southern France. Allozyme variation, at 21 loci in 11 populations, does not seem to show any relationship to the underlying cline in the frequency of the two chromosome types. This indicates that the chromosomal cline does not offer a strong barrier to the flow of genes at other loci. The XO/XX race in this area occurs on a single plateau, isolated from other populations with the same karyotype. It is suggested that this form is only able to persist here because the introgression of neo-XY chromosomes is inhibited by steep cliffs, which tend to keep the two races apart.     

Chromosome polymorphism and C-banding variation of the brachypterous grasshopper Podisma sapporensis Shir. (Orthoptera, Acrididae) in Hokkaido, northern Japan

The grasshopper Podisma sapporensis consists of two main chromosome races in Hokkaido. The western group of populations of P. sapporensis, belonging to the XO race, has a diploid number of chromosomes 2n = 23 in the male and 2n = 24 in the female (sex determination XO male/XX female). The eastern group of populations of this species, belonging to the XY race, differs from the western one as a result of Robertsonian translocation between the originally acrocentric X chromosome and M5 autosome in homozygous state, having resulted in the forming of chromosome sex determination neo-XY male/neo-XX female (2n = 22). These races are geographically isolated by the mountainous system consisting of the Mts Daisetsu and Hidaka range, occupying the central part of the island. The hybrid zones between the races have not so far been discovered. Various levels of polymorphism for the pericentric inversions and C-banding variation exist in different chromosomes throughout populations in both chromosome races. In some solitary populations (the population at the summit of Mt Yotei, populations in the vicinity of Naganuma, Oketo, and Tanno) pericentric inversions are fixed in some pairs of chromosomes, which enables marking of the discrete karyomorphes. In the Mt Daisengen population all chromosomes are two-armed as a result of fixing the pericentric inversions. These facts contradict karyotypical conservatism of the tribe Podismini. The level of diversity of P. sapporensis karyotypes could provide a new perspective on the evolutionary process of different karyotype in Orthoptera. The considerable occurrence of polymorphism in chromosomes suggests that karyotypic diversification is undergoing in P. sapporensis. The authors also proposed that P. sapporensis would be divided into four chromosome subraces in the XO chromosome race and two chromosome subraces in the XY race, on the basis of karyotypic features. These races may have been established by fundamental climatic changes during the glacial epoch.     

东北地区亚洲飞蝗染色体核型分析

采用常规压片法对吉林省亚洲飞蝗Locusta migratoria migratoria(L.)的染色体核型进行分析.研究结果表明:亚洲飞蝗性别决定机制为X0型,染色体数目为2n♂=23,染色体组式4L+4M+3S+X,全部为端部着丝粒染色体,NF=23.染色体中最长(L1)与最短(S11)染色体之比大于4:1,臂比大...     

Chromosome variation in British populations of Oncopsis (Hemiptera:Cicadellidae)

The British species of Oncopsis fall into two groups with respect to male karyotype. Four species—O. avallanae, O. carpini and O. subangulata, together with an undescribed new species close to carpini—consistently show ten autosome pairs and a single X-chromosome (2n=10AII+XO). In O. tristis too the XO state predominates but single neo-sex chromosome variants with nine and eight autosome pairs respectively have also been found. The two remaining species-O. alni and O. flavicollis—both regularly include derived neo-XY states involving the incorporation of autosomal material onto the X(2n=9AII+XY). The single population of O. alni studied was entirely neo-XY but the situation in O. flavicollis proved more complex. Montane populations which occur on Betula pubescens are XO-monomorphic whereas populations in lowland woodlands are polymorphic, including both XO and neo-XY types. In such situations the XO forms are found predominantly on B. pubescens whereas the XY morphs predominate on a second species of birch, B. pendula, which is absent from montane woodlands. In all three species where the neo-XY state is present, different autosomes have been involved in the fusion process.     

Electrophoretic analysis of a chromosomal hybrid zone in the grasshopper Podisma pedestris

We have made an extensive allozyme survey of 21 enzyme and protein loci in populations of the alpine grasshopper Podisma pedestris. This species occurs in two races, differing by a chromosomal fusion which separates the ancestral XO/XX race from a derived neo-XY race. These races also differ in DNA content, and hybrids between them have reduced viability. Electrophoresis reveals that the amount of genetic differentiation between these races is no greater than the variation among populations within each race. Both larger-scale surveys and a detailed survey of an area where the races hybridize, show that the chromosomal change is not correlated with gene frequency changes at any of the 21 loci studied. These findings are consistent with recently developed theory concerning the strength of the barrier to gene flow posed by a hybrid zone with characteristics such as those measured experimentally in Podisma. It is argued that hybrid zones in other species which involve allozymic differences do so because of stronger selection against hybrids rather than through mating isolation.     

Fission or fusion? Mitochondrial DNA phylogenetics of the chromosome races of Hemideina crassidens (Orthoptera: Anostostomatidae)

The weta Hemideina crassidens has two chromosomal races that differ by two centric fusions or fissions. The mitochondrial DNA of weta from both chromosomal races and a sister species were sequenced for a 750-bp region of the gene coding for cytochrome oxidase I. The average pairwise genetic distance among the 15 (XO)-chromosome race weta was almost four times greater than the average distance among the 19 (XO)-chromosome race weta. The weta from the 19-chromosome race formed a well-supported monophyletic clade in all shortest maximum parsimony trees. Maximum likelihood and neighbor-joining trees suggested that the 15-chromosome karyotype was paraphyletic with respect to the 19-chromosome karyotype, but this was not supported by maximum parsimony analyses. Although phylogenetic analysis could not exclude chromosome fusion as the rearrangement responsible for the karyotype differentiation, the level of sequence variation and pattern of distribution appear to implicate fission as the more likely event.     

Gene flow between chromosomal forms of the malaria vector Anopheles funestus in Cameroon, Central Africa, and its relevance in malaria fighting

Knowledge of population structure in a major vector species is fundamental to an understanding of malaria epidemiology and becomes crucial in the context of genetic control strategies that are being developed. Despite its epidemiological importance, the major African malaria vector Anopheles funestus has received far less attention than members of the Anopheles gambiae complex. Previous chromosomal data have shown a high degree of structuring within populations from West Africa and have led to the characterization of two chromosomal forms, "Kiribina" and "Folonzo." In Central Africa, few data were available. We thus undertook assessment of genetic structure of An. funestus populations from Cameroon using chromosomal inversions and microsatellite markers. Microsatellite markers revealed no particular departure from panmixia within each local population and a genetic structure consistent with isolation by distance. However, cytogenetic studies demonstrated high levels of chromosomal heterogeneity, both within and between populations. Distribution of chromosomal inversions was not random and a cline of frequency was observed, according to ecotypic conditions. Strong deficiency of heterokaryotypes was found in certain localities in the transition area, indicating a subdivision of An. funestus in chromosomal forms. An. funestus microsatellite genetic markers located within the breakpoints of inversions are not differentiated in populations, whereas in An. gambiae inversions can affect gene flow at marker loci. These results are relevant to strategies for control of malaria by introduction of transgenes into populations of vectors.     

Rapid micro-evolution and loss of chromosomal diversity in Drosophila in response to climate warming

Concern regarding the ecological impact of rapid global warming has encouraged research on climate-induced changes in biological systems. Critical problems, still poorly understood, are the potential for rapid adaptive responses and their genetic costs to populations. The O chromosomal polymorphisms of Drosophila subobscura have been monitored at a southern Palearctic locality experiencing sustained climate warming since the mid-1970s. Observations suggest that the population is rapidly evolving in response to the new environmental conditions, and has lost a significant amount of chromosomal diversity (18.3% in 16 years). These findings are consistent with results from another population of D. subobscura, which is also undergoing climate warming, and are in accord with what would be expected from latitudinal and seasonal patterns of the various inversions. In addition, data on the O chromosomal polymorphisms from other localities throughout t he range of this species suggest that other populations vary similarly.     

Long‐term changes in the chromosomal inversion polymorphism of Drosophila subobscura. II. European populations*

The chromosomal polymorphism of 13 European populations of Drosophila subobscura has been compared with that of the same populations collected 15–35 years ago. The chromosomal polymorphism of the old populations differs significantly from that of the new populations, mainly for chromosomes U and O. There is a very good agreement between the geographical space and the genetic space as shown by a graphical representation of the 26 statistical populations (13 old and 13 new) obtained by a principal coordinate analysis. This reflects both the existence of significant latitudinal clines for the frequencies of some chromosomal arrangements in the old and new samples and systematic changes that have taken place in these populations during the period that elapsed between the two surveys. An increase in the frequency of those arrangements typical of southern latitudes and a decrease for those common in northern latitudes is observed in all populations – Mediterranean, Atlantic and Central European. Furthermore, the genetic distances of the new populations to a southern population of reference have decreased in comparison with those of the old populations. These changes could be the result of climatic factors that are correlated with latitude. In particular, the assumption that global warming is responsible for all the changes observed appears rather likely. Whether these systematic changes of the chromosomal polymorphism are a consequence of local adaptations or have been produced by migration from the south remains an open question.     

Teratogen susceptibility of XO mothers and XO embryos in mice

We examined whether the chromosomal imbalance inherent in an XO constitution in mice is more susceptible to teratogenic influence of biotin deficiency using a newly established mouse colony with pure X monosomy. We hypothesized that XO mothers or XO embryos might be more susceptible to certain teratogens. Contrary to our expectation, the incidence of external malformations induced by biotin deficiency did not differ either between XX dams and XO dams or between XX fetuses and XO fetuses.     

A strategy for an improved separation of mammalian spermatids

A collection procedure has been developed to improve the homogeneity of mammalian spermatid populations separated by elutriation. Trypsinizied ram testis cells were elutriated at 18C. Every cell population was eluted by progressive changes in the flow rate and/or rotor speed, instead of by abrupt changes, to reduce the contamination by cells from the next population. Pure populations were collected alternating with mixed populations corresponding to the overlap between two adjacent pure populations. Furthermore, each pure population was collected into two subfractions, the second of which, contamined by cells from the following population, was pooled with the following fraction. In less than 2 hr after castration, three populations of at least 1 × 10⁸ viable round or elongated or elongating spermatids were obtained with respective purities of 95%, 82%, and 99% of the nucleated cells. In addition, two mixed populations containing only two adjacent spermatid types (round plus elongating spermatids: 98%; elongated plus elongating spermatids: 98%) were obtained, as well as a population containing around 60% pachytene spermatocytes.     

Selective sweeps and intercontinental migration in the cosmopolitan moss Ceratodon purpureus (Hedw.) Brid

The moss Ceratodon purpureus has long been used as a model system in plant development and physiology. However, the molecular population genetics of the species remains virtually unexplored. In this study, we used population genetic analyses of DNA sequence data from three unlinked loci (atpB-rbcL spacer, adk, and phy2) to examine biogeographical patterns in a global sample of this species. The three loci differed significantly in mutation frequency spectra and implied population structure. Pairs of haplotypes from single populations were frequently more divergent than haplotypes sampled from widely disjunct populations. In the atpB-rbcL spacer and adk samples, Australasian haplotypes were more closely related to Northern Hemisphere haplotypes than to haplotypes found in the equatorial regions. In contrast, the phy2 sample showed that the north and south temperate regions were genetically divergent, with the equatorial regions intermediate. Maximum-likelihood estimates (MLE) of the rates of migration between the two hemispheres were significantly different for the two nuclear genes. The frequency spectra of mutations indicated that differences in implied population structure among the three loci resulted from directional selection on the chloroplast genome and on the chromosomal segment containing adk. Collectively, these data suggest that long-distance migration within the Northern Hemisphere and Australasian regions is common (relative to the mutation rate) and that migration between these two regions, potentially via equatorial populations, is more frequent than migration among equatorial populations.     

Analysis of genetic structure in soil populations of Rhizobium leguminosarum recovered from the USA and the UK

Although many studies have shown that animal-associated bacterial species exhibit linkage disequilibrium at chromosomal loci, recent studies indicate that both animal-associated and soil-borne bacterial species can display a nonclonal genetic structure in which alleles at chromosomal loci are in linkage equilibrium. To examine the situation in soil-borne species further, we compared genetic structure in two soil populations of Rhizobium leguminosarum bv. trifolii and two populations of R. leguminosarum bv. viciae from two sites in Oregon, with genetic structure in R. leguminosarum bv. viciae populations recovered from peas grown at a site in Washington, USA, and at a site in Norfolk, UK. A total of 234 chromosomal types (ET) were identified among 682 strains analysed for allelic variation at 13 enzyme-encoding chromosomal loci by multilocus enzyme electrophoresis (MLEE). Chi-square tests for heterogeneity of allele frequencies showed that the populations were not genetically uniform. A comparison of the genetic diversity within combined and individual populations confirmed that the Washington population was the primary cause of genetic differentiation between the populations. Each individual population exhibited linkage disequilibrium, with the magnitude of the disequilibrium being greatest in the Washington population and least in the UK population of R. leguminosarum bv. viciae. Linkage disequilibrium in the UK population was created between two clusters of 9 and 23 ETs, which, individually, were in linkage equilibrium. Strong linkage disequilibrium between the two major clusters of 8 and 12 ETs in the Washington population was caused by the low genetic diversity of the ETs within each cluster relative to the inter-cluster genetic distance. Because neither the magnitude of genetic diversity nor of linkage disequilibrium increased as hierarchical combinations of the six local populations were analysed, we conclude that the populations have not been isolated from each other for sufficient time, nor have they been exposed to enough selective pressure to develop unique multilocus genetic structure.     

Lethal genes in O5, chromosomes of Drosophila subobscura from Europe and America

In populations of D. subobscura , a species that is know for its high chromosomal polymorphism, the O₅ inversion has a rather erratic frequency distribution in the Palearctic region. An O₅ lethal chromosomal line obtained from a Balkan population near Zanjic (South Adriatic, Montenegro, Yugoslavia) was tested for lethal allelism with other O₅ lethal chromosomal lines derived from American (USA and Chile) colonizing populations, and from the French population of Taulé. No allelism was found between the Balkan lethal gene and those from America and France. Thus, the lethal genes of the O₅ inversions are not of the same origin and it is most probable that the American colonizations did not start from the Zanjic population. The general difference in the chromosomal inversion polymorphism corroborates this conclusion. The cytological analysis confirms the assumption that all O₅ chromosomes studied are identical with respect to breakage points.     

A comparison of five hybrid zones of the weta Hemideina thoracica (Orthoptera: Anostostomatidae): degree of cytogenetic differentiation fails to predict zone width

Abstract Tension zones are maintained by the interaction between selection against hybrids and dispersal of individuals. Investigating multiple hybrid zones within a single species provides the opportunity to examine differences in zone structure on a background of differences in extrinsic factors (e.g., age of the zone, ecology) or intrinsic factors (e.g., chromosomes). The New Zealand tree weta Hemideina thoracica comprises at least eight distinct chromosomal races with diploid numbers ranging from 2n = 11 (XO) to 2n = 23 (XO). Five independent hybrid zones were located that involve races differing from one another by a variety of chromosomal rearrangements. The predicted negative correlation between extent of karyotypic differentiation (measured in terms of both percent of genome and number of rearrangements) and zone width was not found. Conversely, the widest zones were those characterized by two chromosome rearrangements involving up to 35% of the genome. The narrowest zone occurred where the two races differ by a single chromosome rearrangement involving approximately 2% of the genome. The five estimates of chromosomal cline width ranged from 0.5 km to 47 km. A comparative investigation of cline width for both chromosomal and mitochondrial markers revealed a complex pattern of zone characteristics. Three of the five zones in this study showed cline concordance for the nuclear and cytoplasmic markers, and at two of the zones the clines were also coincident. Zones with the widest chromosomal clines had the widest mitochondrial DNA clines. It appears that, even within a single species, the extent of karyotypic differentiation between pairs of races is not a good predictor of the level of disadvantage suffered by hybrids.     

Genic differentiation and origin of Robertsonian populations of the house mouse (Mus musculus domesticus Rutty)

This paper examines the relation between chromosomal and nuclear-gene divergence in 28 wild populations of the house mouse semi-species, Mus musculus domesticus, in Western Europe and North Africa. Besides describing the karyotypes of 15 of these populations and comparing them to those of 13 populations for which such information was already known, it reports the results of an electrophoretic survey of proteins encoded by 34 nuclear loci in all 28 populations. Karyotypic variation in this taxon involves only centric (or Robertsonian) fusions which often differ in arm combination and number between chromosomal races. The electrophoretic analysis showed that the amount of genic variation within Robertsonian (Rb) populations was similar to that for all-acrocentric populations, i.e. bearing the standard karyotype. Moreover, divergence between the two types of populations was extremely low. These results imply that centric fusions in mice have not modified either the level or the nature of genic variability. The genetic similarity between Rb and all-acrocentric populations is not attributed to the persistence of gene flow, since multiple fusions cause marked reproductive isolation. Rather, we attribute this extreme similarity to the very recent origin of chromosomal races in Europe. Furthermore, genic diversity measures suggest that geographically separated Rb populations have in situ and independent origins. Thus, Rb translocations are probably not unique events, but originated repeatedly. Two models are presented to explain how the rapid fixation of a series of chromosomal rearrangements can occur in a population without lowering variability in the nuclear genes. The first model assumes that chromosomal mutation rates are between 10(-3) and 10(-4) and that populations underwent a series of transient bottlenecks in which the effective population size did not fall below 35. In the second model, genic variability is restored following severe bottlenecks, through gene flow and recombination.     

Low courtship song variation in south and southeastern Brazilian populations of Drosophila meridionalis (Diptera, Drosophilidae)

Drosophila meridionalis is a cactus-breeding species with a wide distribution in South America. Most populations of this species are geographically isolated, what provides a promising scenario for studying evolution. Former studies of this species revealed a remarkable karyotypic variation among its populations. Up to six distinct metaphase chromosomes were described, showing that this species is polymorphic at least at the chromosomal level. In order to elucidate the taxonomic status of populations showing different metaphase chromosomes, we analyzed the courtship song of five populations of D. meridionalis in South and Southeastern Brazil. In addition, we analyzed the metaphase chromosomes of each population. Our results show that, despite the two karyotype observed, most courtship song parameters did not vary among the populations. Altogether, our results suggest that D. meridionalis from South and Southeastern Brazil represents one species with an inter-population chromosomal variability.     

Two developmentally temporal quantitative trait loci underlie convergent evolution of increased branchial bone length in sticklebacks

In convergent evolution, similar phenotypes evolve repeatedly in independent populations, often reflecting adaptation to similar environments. Understanding whether convergent evolution proceeds via similar or different genetic and developmental mechanisms offers insight towards the repeatability and predictability of evolution. Oceanic populations of threespine stickleback fish, Gasterosteus aculeatus, have repeatedly colonized countless freshwater lakes and streams, where new diets lead to morphological adaptations related to feeding. Here, we show that heritable increases in branchial bone length have convergently evolved in two independently derived freshwater stickleback populations. In both populations, an increased bone growth rate in juveniles underlies the convergent adult phenotype, and one population also has a longer cartilage template. Using F₂ crosses from these two freshwater populations, we show that two quantitative trait loci (QTL) control branchial bone length at distinct points in development. In both populations, a QTL on chromosome 21 controls bone length throughout juvenile development, and a QTL on chromosome 4 controls bone length only in adults. In addition to these similar developmental profiles, these QTL show similar chromosomal locations in both populations. Our results suggest that sticklebacks have convergently evolved longer branchial bones using similar genetic and developmental programmes in two independently derived populations.     

Numerical and structural variations of the X chromosomes and no. 2 autosomes in mandarin vole, Microtus mandarinus (Rodentia)

Here we describe our studies on Microtus mandarinus faeceus of Jiangyan in Jiangsu province of China. By karyotype and G-banding analysis we have found variation in chromosome number and polymorphisms of the X chromosome and the second pair of autosomes of the subspecies. Chromosome number of the subspecies is 2n=47-50. The subspecies has three kinds of chromosomal sex: XX, XO and XY, among which one of the X chromosomes is subtelocentric (X(ST)) and the other is metacentric (X(M)). After comparing karyotypes of different subspecies, we found the specific cytogenetic characteristics of Microtus mandarinus, that is they have three kinds of chromosomal sex: XX, XO and XY; X chromosomes are heteromorphic; the chromosome number of female individuals are one less than male individuals; chromosome number of XX individuals are equal to that of XO ones. We hypothesize that Robertsonian translocation is the main reason of the polymorphism of the second pair of autosomes and variety of chromosome number, and it also causes the chromosome number evolution in different subspecies of Microtus mandarinus.     

Reconstructing genetic ancestry blocks in admixed individuals

A chromosome in an individual of recently admixed ancestry resembles a mosaic of chromosomal segments, or ancestry blocks, each derived from a particular ancestral population. We consider the problem of inferring ancestry along the chromosomes in an admixed individual and thereby delineating the ancestry blocks. Using a simple population model, we infer gene-flow history in each individual. Compared with existing methods, which are based on a hidden Markov model, the Markov-hidden Markov model (MHMM) we propose has the advantage of accounting for the background linkage disequilibrium (LD) that exists in ancestral populations. When there are more than two ancestral groups, we allow each ancestral population to admix at a different time in history. We use simulations to illustrate the accuracy of the inferred ancestry as well as the importance of modeling the background LD; not accounting for background LD between markers may mislead us to false inferences about mixed ancestry in an indigenous population. The MHMM makes it possible to identify genomic blocks of a particular ancestry by use of any high-density single-nucleotide-polymorphism panel. One application of our method is to perform admixture mapping without genotyping special ancestry-informative-marker panels.