Competition at the Mouse t Complex: Rare Alleles Are Inherently Favored

We investigate the competition between alleles at a segregation distorter locus. The focus is on the invasion prospects of rare mutant distorter alleles in a population in which a wildtype and a resident distorter allele are present. The parameters are chosen to reflect the situation at the t complex of the house mouse, one of the best-studied examples of segregation distortion. By analyzing the invasion chances of rare alleles, we provide an analytical justification of earlier simulation results. We show that a new distorter allele can successfully invade even if it is inferior both at the gamete and at the individual level. In fact, newly arising distorter alleles have an inherent rareness advantage if their negative fitness consequences are restricted to homozygous condition. Likewise, rare mutant wildtype alleles may often invade even if their viability or fertility is reduced. As a consequence, the competition between alleles at a segregation distorter locus should lead to a high degree of polymorphism. We discuss the implications of this conclusion for the t complex of the house mouse and for the evolutionary stability of “honest” Mendelian segregation.     

General analysis of frequency-dependent sexual selection at a multi-allelic locus

A general model is analyzed in which arbitrarily frequency-dependent selection acts on one sex of a diploid population with several alleles at one locus, as a result of viability or mating-success differences. The existence of boundary and polymorphic equilibria is examined, and conditions for local stability, internal and external, are obtained. The status of Hardy-Weinberg approximations in studying stability and approach to equilibria is also considered. The general principles are then applied to two specific models: one where genotypes fall into two phenotypic classes; and one with a hierarchy of dominance where viability and sexual selection are opposed. In the latter case it is found that, of all the equilibria present, there is one and only one which could possibly be stable: the existence of a unique globally stable equilibrium might then be inferred.     

Protein markers for anther culturability in barley

Two-dimensional electrophoresis of proteins from a recombinant population of anther culture-derived doubled haploid lines identified 4 loci or linkage groups showing a deviation from an expected 11 segregation. It was hypothesized that these markers are linked to genes involved in the process of haploid plant production and that the deviation was due to a selection for alleles conferring higher anther culture response. To check this hypothesis, the anther culturability of 50 of the doubled haploid lines and their two inbred parents was assessed. It was found that 2 of the loci which had a distortion of segregation showed a significant effect on anther culture response, the most efficient allele being the most frequent in both loci. In addition, 2 more markers associated with anther culturability were found. One of the first mentioned 2 loci and one of the latter 2 were found to be linked to genes involved in both embryoid production and subsequent green plant regeneration. The remaining two were linked to genes involved only in green plant regeneration. Of the 4 favorable alleles 3 were inherited from one parent.     

Phylogeography of sex ratio and multiple mating in stalk-eyed flies from southeast Asia

The factors maintaining sex chromosome meiotic drive, or sex ratio (SR), in natural populations remain uncertain. Coevolution between segregation distortion and modifiers should produce transient SR distortion while selection can result in a stable polymorphism. We hypothesize that if SR is maintained by selection, then phylogenetically related populations should exhibit similar SR frequency and intensity. Furthermore, when drive is present, females should mate with multiple males more often both to insure fertility and to increase the probability of producing male progeny. In this paper we report on variation in SR frequency and multiple mating among seven populations and three species of stalk-eyed flies, genus Cyrtodiopsis, from southeast Asia. Using a phylogenetic hypothesis based on 1100 bp of mtDNA sequence we find that while sex chromosome meiotic drive is present in all populations of C. whitei and C. dalmanni, the frequency and intensity of drive only differs between populations or species with greater than 4.8% sequence divergence. The frequency of females mating with multiple males is higher in populations with SR. In addition, SR males mate less often, possibly to compensate for sperm depletion. Our results suggest that sex chromosome drive is maintained by balancing selection in populations of C. whitei and C. dalmanni. Nevertheless, coevolution between drive and suppressors deserves further study.     

Geographic distribution of alleles at the Ga2 locus for segregation distortion in barley

Summary A distorted segregation of esterase alleles at the complex loci, Est1, Est2 and Est4, was found in an F₂ population. This distortion is typical for cross combinations between the Ga2Ga2 and ga2ga2 genotypes responsible for segregation distortion, since the Ga2 locus is linked with the complex loci encoding the esterase isozymes. The segregation of esterase isozyme patterns in F₂ populations between 473 varieties of barley and a tester of ga2ga2 genotype was examined, and the genotypes inducing segregation distortion were detected. Varieties with a ga2ga2 genotype are widely distributed throughout the world, whereas Ga2Ga2 varieties are found only in eastern and southern regions of Asia, from Japan to North India, with a low frequency. In varieties collected from these regions, some associations were detected between alleles at the Ga2 locus and esterase isozyme patterns. Additionally, most of the Ga2 barley varieties are naked and possess a BtBtbt2bt2 genotype for a non-brittle rachis.     

Common features of segregation distortion in plants and animals

Segregation distortion is increasingly recognized as a potentially powerful evolutionary force. This runs counter to the perception that non-Mendelian genes are rare genetic curiosities, a view that seems to be supported by the near ubiquity of the Mendelian system of inheritance. There are several reasons why segregation distortion may be more important than is evidenced by known empirical examples. One possibility is that the types of segregation distorters we have found are only a subset of a broader range of non-Mendelian systems, many of which go undetected. In this paper, we review what is known about the sex-linked meiotic drive system in the plant, Silene latifolia, and present some data on the mechanism of segregation distortion. We outline the general features that segregation distorters in plants and animals have in common. In some cases, such as the paucity of systems that directly alter meiotic segregation, there are likely to be inherent constraints on the range of systems that can possibly occur. Other generalities, however, support the notion that many forms of meiotic drive are possible, and that the known examples of segregation distortion are likely to be only subset of those that can possibly occur. Non-Mendelian genes may therefore have greater evolutionary importance than their current abundance in nature would suggest.     

Competition at the mouse t complex: rare alleles are inherently favored

We investigate the competition between alleles at a segregation distorter locus. The focus is on the invasion prospects of rare mutant distorter alleles in a population in which a wildtype and a resident distorter allele are present. The parameters are chosen to reflect the situation at the t complex of the house mouse, one of the best-studied examples of segregation distortion. By analyzing the invasion chances of rare alleles, we provide an analytical justification of earlier simulation results. We show that a new distorter allele can successfully invade even if it is inferior both at the gamete and at the individual level. In fact, newly arising distorter alleles have an inherent rareness advantage if their negative fitness consequences are restricted to homozygous condition. Likewise, rare mutant wildtype alleles may often invade even if their viability or fertility is reduced. As a consequence, the competition between alleles at a segregation distorter locus should lead to a high degree of polymorphism. We discuss the implications of this conclusion for the t complex of the house mouse and for the evolutionary stability of "honest" Mendelian segregation.     

Sex-specific meiotic drive and selection at an imprinted locus

We present a one-locus model that breaks two symmetries of Mendelian genetics. Whereas symmetry of transmission is breached by allowing sex-specific segregation distortion, symmetry of expression is breached by allowing genomic imprinting. Simple conditions for the existence of at least one polymorphic stable equilibrium are provided. In general, population mean fitness is not maximized at polymorphic equilibria. However, mean fitness at a polymorphic equilibrium with segregation distortion may be higher than mean fitness at the corresponding equilibrium with Mendelian segregation if one (or both) of the heterozygote classes has higher fitness than both homozygote classes. In this case, mean fitness is maximized by complete, but opposite, drive in the two sexes. We undertook an extensive numerical analysis of the parameter space, finding, for the first time in this class of models, parameter sets yielding two stable polymorphic equilibria. Multiple equilibria exist both with and without genomic imprinting, although they occurred in a greater proportion of parameter sets with genomic imprinting.     

Segregation characteristics of multiple chromosomes ofAzotobacter vinelandii

The genome ofAzotobacler vinelandii has been taggedin vivo with transposons. The cells have then been allowed to divide and the pattern of segregation of the genomes has been studied. The results suggest the presence of multiple (possibly identical) copies of the genome inA. vinelandii. Only a fraction of the total number of genomes seem to have been tagged with transposon and an equilibrium between alleles of the same gene with and without the transposon was evident during random segregation.     

Evolution of the segregation ratio: modification of gene conversion and meiotic drive

We compare the evolutionary pressures that direct the modification of gene conversion and meiotic drive at loci subject to purifying and overdominant viability selection. Gene conversion differs from meiotic drive in that modifers do not affect their own segregation ratios, even when linked to the viability locus. Segregation distortion generates gametic level disequilibria between alleles at the viability locus and modifiers of gene conversion and meiotic drive: enhancers of segregation distortion become positively associated with driven alleles. Suppression of gene conversion evolves if the driven allele is marginally disadvantageous (overdominant viability selection), and higher rates evolve if the driven alleles are relatively advantageous (purifying viability selection). Gametic disequilibria permit enhancers of meiotic drive that are linked to the driven locus to promote their own segregation. We attribute the failure of genetic modifiers of gene conversion and meiotic drive to maximinize mean fitness to the generation of such associations.     

On the evolutionary significance of Mendel's ratios

Length of time in polymorphism is investigated as a possible evolutionary criterion for Mendel's laws in the case of two alleles at one locus and finite population. Deterministic models with constant and random segregation schemes are investigated. In deterministic models the optimum segregation system depends on the zygotic selection and is Mendelian only in symmetric models. In finite population models the initial gene frequencies interact with the segregation to determine mean time to fixation. In deterministic models with random distortion reduction of the variance of the distortion is more likely to produce polymorphism.     

The recombination landscape in Arabidopsis thaliana F2 populations

Recombination during meiosis shapes the complement of alleles segregating in the progeny of hybrids, and has important consequences for phenotypic variation. We examined allele frequencies, as well as crossover (XO) locations and frequencies in over 7000 plants from 17 F(2) populations derived from crosses between 18 Arabidopsis thaliana accessions. We observed segregation distortion between parental alleles in over half of our populations. The potential causes of distortion include variation in seed dormancy and lethal epistatic interactions. Such a high occurrence of distortion was only detected here because of the large sample size of each population and the number of populations characterized. Most plants carry only one or two XOs per chromosome pair, and therefore inherit very large, non-recombined genomic fragments from each parent. Recombination frequencies vary between populations but consistently increase adjacent to the centromeres. Importantly, recombination rates do not correlate with whole-genome sequence differences between parental accessions, suggesting that sequence diversity within A. thaliana does not normally reach levels that are high enough to exert a major influence on the formation of XOs. A global knowledge of the patterns of recombination in F(2) populations is crucial to better understand the segregation of phenotypic traits in hybrids, in the laboratory or in the wild.     

Fitness effects of X chromosome drive in the stalk-eyed fly, Cyrtodiopsis dalmanni

Sex-ratio (SR) males produce predominantly female progeny because most Y chromosome sperm are rendered nonfunctional. The resulting transmission advantage of XSR chromosomes should eventually cause population extinction unless segregation distortion is masked by suppressors or balanced by selection. By screening male stalk-eyed flies, Cyrtodiopsis dalmanni, for brood sex ratio we found unique SR alleles at three X-linked microsatellite loci and used them to determine if SR persists as a balanced polymorphism. We found that XSR/XST females produced more offspring than other genotypes and that SR males had lower sperm precedence and exhibited lower fertility when mating eight females in 24 h. Adult survival was independent of SR genotype but positively correlated with eye span. We infer that the SR polymorphism is likely maintained by a combination of weak overdominance for female fecundity and frequency dependent selection acting on male fertility. Our discovery of two SR haplotypes in the same population in a 10-year period further suggests that this SR polymorphism may be evolving rapidly.     

Multiallelic selection polymorphism

The existence and stability of an internal (i.e., completely polymorphic) equilibrium for viability selection at a single multiallelic locus is investigated. Generations are discrete and nonoverlapping; the population is panmictic, monoecious, and diploid. Various necessary and sufficient conditions for the existence of an internal equilibrium are established and applied to the loss of alleles. Some necessary conditions for the existence of an asymptotically stable internal equilibrium are also established. All these conditions are simpler and yield general biological conclusions more easily than the classical necessary and sufficient conditions.     

Evolution of mammalian X chromosome-linked imprinting

We analyse the evolution of X chromosome-linked imprinting by modifying our previous model of imprinting of autosomal genes that influence the trade-off between maternal fecundity and offspring viability through alterations in maternal investment (Mills and Moore, 2004). Unlike previous genetic models, we analyse X-linked imprinting in the context of populations at equilibrium for either autosomal or X-linked biallelically expressed alleles at loci that influence the fecundity/viability trade-off. We show that selection under parental conflict over maternal investment in offspring can parsimoniously explain the occurrence of sex-specific gene expression patterns, without a requirement to postulate direct selection for sexual dimorphism mediated through imprinting. We note that sex chromosome imprinting causes a small distortion of the post-weaning sex ratio, providing a possible selection pressure against the evolution of X-linked imprints. We discuss our conclusions in the context of recent reports of imprinting of mouse X-linked Xlr genes.     

Disentangling the effect of genes, the environment and chance on sex ratio variation in a wild bird population

Sex ratio theory proposes that the equal sex ratio typically observed in birds and mammals is the result of natural selection. However, in species with chromosomal sex determination, the same 1 : 1 sex ratio is expected under random Mendelian segregation. Here, we present an analysis of 14 years of sex ratio data for a population of song sparrows (Melospiza melodia) on Mandarte Island, at the nestling stage and at independence from parental care. We test for the presence of variance in sex ratio over and above the binomial variance expected under Mendelian segregation, and thereby quantify the potential for selection to shape sex ratio. Furthermore, if sex ratio variation is to be shaped by selection, we expect some of this extra-binomial variation to have a genetic basis. Despite ample statistical power, we find no evidence for the existence of either genetic or environmentally induced variation in sex ratio, in the nest or at independence. Instead, the sex ratio variation observed matches that expected under random Mendelian segregation. Using one of the best datasets of its kind, we conclude that female song sparrows do not, and perhaps cannot, adjust the sex of their offspring. We discuss the implications of this finding and make suggestions for future research.     

Polymorphism in multiallelic migration–selection models with dominance

The evolution of the gene frequencies at a single multiallelic locus under the joint action of migration and viability selection with dominance is investigated. The monoecious, diploid population is subdivided into finitely many panmictic colonies that exchange adult migrants independently of genotype. Underdominance and overdominance are excluded. If the degree of dominance is deme independent for every pair of alleles, then under the Levene model, the qualitative evolution of the gene frequencies (i.e., the existence and stability of the equilibria) is the same as without dominance. In particular: (i) the number of demes is a generic upper bound on the number of alleles present at equilibrium; (ii) there exists exactly one stable equilibrium, and it is globally attracting; and (iii) if there exists an internal equilibrium, it is globally asymptotically stable. Analytic examples demonstrate that if either the Levene model does not apply or the degree of dominance is deme dependent, then the above results can fail. A complete global analysis of weak migration and weak selection on a recessive allele in two demes is presented.     

Genetic diversity and individual identification of reintroduced otters (<Emphasis Type="Italic">Lutra lutra</Emphasis>) in north-eastern Spain by DNA genotyping of spraints

During the last century, otter populations in the Mediterranean area of the Iberian Peninsula were dramatically reduced and disappeared in many localities. A reintroduction programme was established in north-eastern Spain (Muga and Fluvià basins and the “Aiguamolls de l’Empordà” wetlands), by releasing 42 otters from four different Iberian populations, between 1995 and 2002. In order to evaluate the success of the reintroduction programme, we investigated microsatellite variation in the native populations of released otters as well as in the population present in the release area in 2004. We used non-invasively collected samples as the DNA source to avoid disturbing the animals in the wild. Laboratory procedures included the screening of samples and a multiple-tubes approach to detect and correct genotyping errors. Our results show that founders have been replaced by descendants in the release area and the population is in Hardy-Weinberg equilibrium, with a 1:1 sex ratio. Western Iberian otters—representing two-thirds of the founder group and released earlier—have greatly contributed to the genetic composition of the current population. The genetic patterns of otters found in a basin north of the studied area suggest a common origin with the released population. We also detected in a few otters the presence of alleles not identified in the original founder group, and we shall discuss the possible origin of these alleles. This paper is dedicated to the memory of Dr. Xavier Domingo-Roura.     

Gene duplication,allelic diversity,selection processes and adaptive value of MHC class II <Emphasis Type="Italic">DRB</Emphasis> genes of the bank vole, <Emphasis Type="Italic">Clethrionomys glareolus</Emphasis>

The generation and maintenance of allelic polymorphism in genes of the major histocompatibility complex (MHC) is a central issue in evolutionary genetics. Recently, the focus has changed from ex situ to in situ populations to understand the mechanisms that determine adaptive MHC polymorphism under natural selection. Birth-and-death evolution and gene conversion events are considered to generate sequence diversity in MHC genes, which subsequently is maintained by balancing selection through parasites. The ongoing arms race between the host and parasites leads to an adaptive selection pressure upon the MHC, evident in high rates of non-synonymous vs synonymous substitution rates. We characterised the MHC class II DRB exon 2 of free living bank voles, Clethrionomys glareolus by single-strand conformation polymorphism and direct sequencing. Unlike other arvicolid species, the DRB locus of the bank vole is at least quadruplicated. No evidence for gene conversion events in the Clgl-DRB sequences was observed. We found not only high allelic polymorphism with 26 alleles in 36 individuals but also high rates of silent polymorphism. Exceptional for MHC class II genes is a purifying selection pressure upon the majority of MHC-DRB sequences. Further, we analysed the association between certain DRB alleles and the parasite burden with gastrointestinal trichostrongyle nematodes Heligmosomum mixtum and Heligmosomoides glareoli and found significant quality differences between specific alleles with respect to infection intensity. Our findings suggest a snapshot in an evolutionary process of ongoing birth-and-death evolution. One allele cluster has lost its function and is already silenced, another is loosing its adaptive value in terms of gastrointestinal nematode resistance, while a third group of alleles indicates all signs of classical functional MHC alleles.     

Sexual segregation in ungulates: individual behaviour and the missing link

Previous "explanations" of sexual segregation in ungulates establish no more than a prerequisite for habitat segregation because they do not include a model of competitive habitat selection. Here we provide one based on the ideal free distributions of mutually competing, optimally foraging, individual deer. We parameterised our model using field data collected from a population of fallow deer (Dama dama) in a Mediterranean forest. The predictions of the inter-sex competition model were in full agreement with observational data, but those of single sex distributions (conventional theory) were not. The "conventional" hypothesis, that segregation arises simply from sex differences, predicted no more than moderate (20–40%) levels of segregation, even in optimal conditions. By introducing inter-sex resource competition, the predicted segregation can generally more than double and full segregation becomes possible in some circumstances. The modelling showed segregation to be density-dependent, varying in complicated ways with season and animal density. Sensitivity analysis showed the vulnerability of the "conventional" understanding of environmental variation and uncertainty. Using our competition model we show that as diet difference increases, direct competition between the sexes declines, so that as males increasingly differ from females, segregation declines and the two sexes are more likely to be found mixed (as long as the chosen food is available to both in the same area). Conversely, small differences among male and female deer are amplified by both food depletion and inter-sex competition to give substantial levels of segregation. The theoretical framework on which our model is built strongly suggests that sexual dimorphism in the context of scramble competition may be the fundamental cause of sexual habitat-segregation among ungulates.