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PubMed2002

Recombination-dependent mtDNA partitioning: in vivo role of Mhr1p to promote pairing of homologous DNA.

Yeast mhr1-1 was isolated as a defective mutation in mitochondrial DNA (mtDNA) recombination. About half of mhr1-1 cells lose mtDNA during growth at a higher temperature. Here, we show that mhr1-1 exhibits a defect in the partitioning of nascent mtDNA into buds and is a base-substitution mutation in MHR1 encoding a mitochondrial matrix protein. We found that the Mhr1 protein (Mhr1p) has activity to pair single-stranded DNA and homologous double-stranded DNA to form heteroduplex joints in vitro, and that mhr1-1 causes the loss of this activity, indicating its role in homologous mtDNA recombination. While the majority of the mtDNA in the mother cells consists of head-to-tail concatemers, more than half of the mtDNA in the buds exists as genome-sized monomers. The mhr1-1 deltacce1 double mutant cells do not maintain any mtDNA, indicating the strict dependence of mtDNA maintenance on recombination functions. These results suggest a mechanism for mtDNA inheritance similar to that operating in the replication and packaging of phage DNA.

Amino Acid Substitution↗
PubMed2026

Digest: Speciation involves both barriers and bridges in the tropical Andes.

Mountains are important centers of biodiversity and studies of speciation. In particular, the north-south linear orientation of the Andes allows examination of how geographic isolation, elevation, and latitude influence divergence in closely related species such as Myioborus warblers (Parulidae), the focal taxa of this study. Traditional models of Andean speciation have emphasized allopatric divergence due to geographic barriers, but thanks to the limited sampling of hybrid zones and lack of genome-wide datasets in tropical taxa, we may have underestimated of the role of gene flow in shaping patterns of divergence in the region. Using ddRAD-seq genomic data, Céspedes Arias et al. (2026) demonstrated that geographic isolation and hybridization both contribute to the generation and maintenance of Andean warbler lineages. Their results revealed how complex patterns of genetic divergence, introgression, and isolation-by-distance contribute to the historical buildup of biodiversity in the Andes, challenging strictly isolationist models of mountain evolution.

Animals↗
PubMed1985

A direct assessment of the role of genetic drift in determining allele frequency variation in populations of Euphydryas editha.

Estimates of allele frequencies at six polymorphic loci were collected over eight generations in two populations of Euphydryas editha. We have estimated, in addition, the effective population size for each generation for both populations with results from mark-recapture and other field data. The variation in allele frequencies generated by random genetic drift was then studied using computer simulations and our direct estimates of effective population size. Substantial differences between observed values and computer-generated expected values assuming drift alone were found for three loci (Got, Hk, Pgi) in one population. These observations are consistent with natural selection in a variable environment.

Alleles↗
PubMed1988

Adaptation at specific loci. V. Metabolically adjacent enzyme loci may have very distinct experiences of selective pressures.

The polymorphic phosphoglucomutase (PGM) and glucose-6-phosphate dehydrogenase (G6PD) loci have been studied in parallel to experimental work on the phosphoglucose isomerase (PGI) polymorphism in Colias butterflies. PGI, PGM and G6PD are also autosomal in Colias. PGM and G6PD are loosely linked (and represent the first identified autosomal linkage group in Colias); they assort independently from PGI. Recombination occurs in both sexes. Neither PGM nor G6PD shows large, consistent differences in flight capacity through the day among its genotypes, as PGI does. PGM shows some change of allele frequencies, and match to Hardy-Weinberg expectation, with air temperature in middle and latter parts of the season, but not early in the season. G6PD may show some heterozygote excess over Hardy-Weinberg expectation early in the day, but more testing is needed. No evidence for differential survivorship was seen at PGM or G6PD, in contrast to PGI. At the PGM and G6PD loci, male heterozygotes are advantaged in mating with females, but without the evidence of female choice which occurs for PGI. These effects are not correlated among the three loci. There is no assortative mating at G6PD (nor at PGI). There is minor positive assortative mating of PGM heterozygotes, but it is too weak to account for the PGM-genotype-specific male mating advantage. No trends of multilocus genotype frequencies involving PGI are seen. Certain PGM-G6PD two-locus genotypes are over-represented, and others under-represented, in wild adult samples, particularly among males and uniformly among successfully mating males. Our results emphasize that enzyme loci sharing a substrate need not have common experience of the existence or strength of natural selection, and suggest initial food-resource processing and allocation as a possible context for fitness-related effects of the PGM and G6PD polymorphisms.

Animals↗
PubMed1990

Interacting genes that affect microtubule function in Drosophila melanogaster: two classes of mutation revert the failure to complement between haync2 and mutations in tubulin genes.

The recessive male sterile mutation haync2 of Drosophila melanogaster fails to complement certain beta 2-tubulin and alpha-tubulin mutations, suggesting that the haywire product plays a role in microtubule function, perhaps as a structural component of microtubules. The genetic interaction appears to require the presence of the aberrant product encoded by haync2, which may act as a structural poison. Based on this observation, we have isolated ten new mutations that revert the failure to complement between haync2 and B2tn. The revertants tested behaved as intragenic mutations of hay in recombination tests, and fell into two phenotypic classes, suggesting two functional domains of the hay gene product. Some revertants were hemizygous viable and less severe than haync2 in their recessive phenotype. These mutations might revert the poison by restoring the aberrant product encoded by the haync2 allele to more wild-type function. Most of the revertants were recessive lethal mutations, indicating that the hay gene product is essential for viability. These more extreme mutations could revert the poison by destroying the ability of the aberrant haywirenc2 product to interact structurally with microtubules. Flies heterozygous for the original haync2 allele and an extreme revertant show defects in both the structure and the function of the male meiotic spindle.

Animals↗
PubMed1990

A hybrid dysgenesis syndrome in Drosophila virilis.

A new example of "hybrid dysgenesis" has been demonstrated in the F1 progeny of crosses between two different strains of Drosophila virilis. The dysgenic traits were observed only in hybrids obtained when wild-type females (of the Batumi strain 9 from Georgia, USSR) were crossed to males from a marker strain (the long-established laboratory strain, strain 160, carrying recessive markers on all its autosomes). The phenomena observed include high frequencies of male and female sterility, male recombination, chromosomal nondisjunction, transmission ratio distortion and the appearance of numerous visible mutations at different loci in the progeny of dysgenic crosses. The sterility demonstrated in the present study is similar to that of P-M dysgenesis in Drosophila melanogaster and apparently results from underdevelopment of the gonads in both sexes, this phenomenon being sensitive to developmental temperature. However, in contrast to the P-M and I-R dysgenic systems in D. melanogaster, in D. virilis the highest level of sterility (95-98%) occurs at 23-25 degrees. Several of the mutations isolated from the progeny of dysgenic crosses (e.g., singed) proved to be unstable and reverted to wild type. We hypothesize that a mobile element ("Ulysses") which we have recently isolated from a dysgenically induced white eye mutation may be responsible for the phenomena observed.

Animals↗
PubMed1990

Factors affecting the distribution of cytoplasmic incompatibility in Drosophila simulans.

In Drosophila simulans a Wolbachia-like microorganism is responsible for reduced egg-hatch when infected males mate with uninfected females. Both incompatibility types have previously been found in North America, Europe and Africa. Some California populations have remained polymorphic for over two years, and the infection is apparently spreading in central California. Egg hatch proportions for wild-caught females from polymorphic populations show that the incompatibility system acts in nature, but egg mortality rates are apparently lower than observed in laboratory populations. Although infected females maintained under various laboratory conditions never produce uninfected offspring, some wild-caught infected females produce both infected and uninfected progeny. This helps explain the persistence of a low frequency of uninfected flies in predominantly infected populations and may also explain the other polymorphisms observed. Fitness comparisons of infected and uninfected stocks, including both larval and adult fitness components, indicate that fecundity may be the component most affected. Infected females suffer a fecundity reduction of 10-20% in the laboratory, but the reduction seems to be smaller in nature. A theoretical analysis provides some insight into the population biology of the infection.

Aging↗
PubMed1991

Influence of gene flow and breeding tactics on gene diversity within populations.

Expressions describing the accumulation of gene correlations within and among lineages and individuals of a population are derived. The model permits different migration rates by males and females and accounts for various breeding tactics within lineages. The resultant equations enable calculation of the probabilistic quantities for the fixation indices, rates of loss of genetic variation, accumulation of inbreeding, and coefficients of relationship for the population at any generation. All fixation indices were found to attain asymptotic values rapidly despite the consistent loss of genetic variation and accumulation of inbreeding within the population. The time required to attain asymptotic values, however, was prolonged when gene flow among lineages was relatively low (less than 20%). The degree of genetic differentiation among breeding groups, inbreeding coefficients, and gene correlations within lineages were found to be primarily functions of breeding tactics within groups rather than gene flow among groups. Thus, the asymptotic value of S. Wright's island model is not appropriate for describing genetic differences among groups within populations. An alternative solution is provided that under limited conditions will reduce to the original island model. The evolution of polygynous breeding tactics appears to be more favorable for promoting intragroup gene correlations than modification of migration rates. Inbreeding and variance effective sizes are derived for populations that are structured by different migration and breeding tactics. Processes that reduce the inbreeding effective population size result in a concomitant increase in variance effective population size.

Animals↗
PubMed1994

The molecular through ecological genetics of abnormal abdomen in Drosophila mercatorum. VI. The non-neutrality of the Y chromosome rDNA polymorphism.

An association between quantitative variation of rDNA on the Y chromosome and male expression of the juvenilized, adult cuticle of the abnormal abdomen syndrome has been found for Drosophila mercatorum. Many pleiotropic effects of this syndrome have been described previously for females, but little was known about possible pleiotropic effects in males. The effects on males open up new avenues for the action of natural selection operating on the system. In females, the syndrome causes an increase in egg-to-adult development time, precocious sexual maturation, increased fecundity and decreased longevity. In addition to the cuticle phenotype, in males abnormal abdomen causes delayed sexual maturation, increased longevity, and decreased mating success, yet no change in egg-to-adult development time. Thus the syndrome has opposing fitness effects in the two sexes, which may help explain the genetic polymorphism observed in this system. Although investigated intensively, associations between naturally occurring Y-linked polymorphism and fitness phenotypes have not been found in Drosophila melanogaster.

Animals↗
PubMed1994

Does diploidy increase the rate of adaptation?

Explanations of the evolution of diploidy have focused on the advantages gained from masking deleterious alleles. Recent theory has shown, however, that masking does not always provide an advantage to diploidy and would never favor diploidy in predominantly asexual organisms. We explore a neglected alternative theory which posits that, by doubling the genome size, diploids double the rate at which favorable mutations arise. Consequently, the rate of adaptation in diploids is presumed to be faster than in haploids. The rate of adaptation, however, depends not only on the rate of appearance of new favorable mutations but also on the rate at which these mutations are incorporated (which depends on the population size and on the dominance of favorable mutations). We show that, in both asexuals and sexuals, doubling the mutation rate via diploidy often does not accelerate the rate of adaptation. Indeed, under many conditions, diploidy slows adaptation.

Adaptation, Physiological↗
PubMed1994

Gene and allelic genealogies at a gametophytic self-incompatibility locus.

The properties of gene and allelic genealogies at a gametophytic self-incompatibility locus in plants have been investigated analytically and checked against extensive numerical simulations. It is found that, as with overdominant loci, there are two genealogical processes with markedly different time scales. First, functionally distinct allelic lines diverge on an extremely long time scale which is inversely related to the mutation rate to new alleles. These alleles show a genealogical structure which is similar, after an appropriate rescaling of time, to that described by the coalescent process for genes at a neutral locus. Second, gene copies sampled within the same functional allelic line show genealogical relationships similar to neutral gene genealogies but on a much shorter time scale, which is on the same order of magnitude as the harmonic mean of the number of gene copies within an allelic line. These results are discussed in relation to data showing trans-specific polymorphisms for alleles at the gametophytic self-incompatibility locus in the Solanaceae. It is shown that population sizes on the order of 4 x 10(5) and a mutation rate per locus per generation as high as 10(-6) could account for estimated allelic divergence times in this family.

Alleles↗
PubMed1996

The genetical and environmental determination of phally polymorphism in the freshwater snail Bulinus truncatus.

In some species of self-fertile pulmonate snails, two sexual morphs co-occur in natural populations: regular individuals and aphallic individuals that cannot transmit sperm to other snails. Purely aphallic populations therefore reproduce obligatorily by selfing. Understanding the evolution of aphally and selfing in these snails requires a precise knowledge of phally determination. In this paper, we investigate the genetic and environmental determination of aphally in Bulinus truncatus by a survey of the family (offspring) aphally ratio of 233 individuals originating from seven natural populations and a study of the reaction norm of the family aphally ratio to temperature using 60 individuals from 10 selfed lineages of one population. Our results indicate a high genetic variability for the determination of aphally between populations and within some populations, associated with a high level of genetic determination. Our second experiment indicates a significant temperature and lineage effect though no interaction between these two effects. We discuss our results in the framework of threshold models developed for dimorphic traits with polygenic inheritance. We propose that the sexual morph of an individual at a given temperature is determined by a temperature threshold value depending on both the individual genotype and probabilistic processes.

Animals↗
PubMed1998

The male-sterility polymorphism of Silene vulgaris: analysis of genetic dat from two populations and comparison with Thymus vulgaris.

Results are given of genetic studies of male sterility using plants from two natural populations from Sussex, England. Both populations have substantial frequencies of females, approximately 0.25 in population 1 and 0.60 in population 3. As in the few other gynodioecious populations studied in detail, many genetic factors are present. In population 1, there are at least two, and more likely three, different cytoplasmic types, one of which appears to produce male sterility in progeny from any hermaphrodite pollen donor; in other words restorer alleles for this cytoplasm are rare or absent from the population. The other two populations can be carried in hermaphrodites that have the dominant restorers. In population 1, there are also probably three restorer loci with complementary recessive male-sterility alleles, as well as a locus with duplicate action, which cannot produce male sterility unless the plant is also homozygous for the recessive allele at another locus. The results from population 3 are quite similar, though there was no evidence in this population for an unrestored sterility cytoplasm. A similar joint nucleocytoplasmic model with multiple restorers fits data from Thymus vulgaris.

Genes, Plant↗
PubMed1998

Genetic components of variation in Nemophila menziesii undergoing inbreeding: morphology and flowering time.

The standard approaches to estimation of quantitative genetic parameters and prediction of response to selection on quantitative traits are based on theory derived for populations undergoing random mating. Many studies demonstrate, however, that mating systems in natural populations often involve inbreeding in various degrees (i.e. , self matings and matings between relatives). Here we apply theory developed for estimating quantitative genetic parameters for partially inbreeding populations to a population of Nemophila menziesii recently obtained from nature and experimentally inbred. Two measures of overall plant size and two of floral size expressed highly significant inbreeding depression. Of three dominance components of phenotypic variance that are defined under partial inbreeding, one was found to contribute significantly to phenotypic variance in flower size and flowering time, while the remaining two components contributed only negligibly to variation in each of the five traits considered. Computer simulations investigating selection response under the more complete genetic model for populations undergoing mixed mating indicate that, for parameter values estimated in this study, selection response can be substantially slowed relative to predictions for a random mating population. Moreover, inbreeding depression alone does not generally account for the reduction in selection response.

Asteraceae↗
PubMed1999

Beneficial mutations, hitchhiking and the evolution of mutation rates in sexual populations.

Natural selection acts in three ways on heritable variation for mutation rates. A modifier allele that increases the mutation rate is (i) disfavored due to association with deleterious mutations, but is also favored due to (ii) association with beneficial mutations and (iii) the reduced costs of lower fidelity replication. When a unique beneficial mutation arises and sweeps to fixation, genetic hitchhiking may cause a substantial change in the frequency of a modifier of mutation rate. In previous studies of the evolution of mutation rates in sexual populations, this effect has been underestimated. This article models the long-term effect of a series of such hitchhiking events and determines the resulting strength of indirect selection on the modifier. This is compared to the indirect selection due to deleterious mutations, when both types of mutations are randomly scattered over a given genetic map. Relative to an asexual population, increased levels of recombination reduce the effects of beneficial mutations more rapidly than those of deleterious mutations. However, the role of beneficial mutations in determining the evolutionarily stable mutation rate may still be significant if the function describing the cost of high-fidelity replication has a shallow gradient.

Alleles↗
PubMed1999

Identification of three putative signal transduction genes involved in R gene-specified disease resistance in Arabidopsis.

The RPS5 disease resistance gene of Arabidopsis mediates recognition of Pseudomonas syringae strains that possess the avirulence gene avrPphB. By screening for loss of RPS5-specified resistance, we identified five pbs (avrPphB susceptible) mutants that represent three different genes. Mutations in PBS1 completely blocked RPS5-mediated resistance, but had little to no effect on resistance specified by other disease resistance genes, suggesting that PBS1 facilitates recognition of the avrPphB protein. The pbs2 mutation dramatically reduced resistance mediated by the RPS5 and RPM1 resistance genes, but had no detectable effect on resistance mediated by RPS4 and had an intermediate effect on RPS2-mediated resistance. The pbs2 mutation also had varying effects on resistance mediated by seven different RPP (recognition of Peronospora parasitica) genes. These data indicate that the PBS2 protein functions in a pathway that is important only to a subset of disease-resistance genes. The pbs3 mutation partially suppressed all four P. syringae-resistance genes (RPS5, RPM1, RPS2, and RPS4), and it had weak-to-intermediate effects on the RPP genes. In addition, the pbs3 mutant allowed higher bacterial growth in response to a virulent strain of P. syringae, indicating that the PBS3 gene product functions in a pathway involved in restricting the spread of both virulent and avirulent pathogens. The pbs mutations are recessive and have been mapped to chromosomes I (pbs2) and V (pbs1 and pbs3).

Arabidopsis↗
PubMed2001

Sex determination in the androdioecious plant Datisca glomerata and its dioecious sister species D. cannabina.

Datisca glomerata is an androdioecious plant species containing male and hermaphroditic individuals. Molecular markers and crossing data suggest that, in both D. glomerata and its dioecious sister species D. cannabina, sex is determined by a single nuclear locus, at which maleness is dominant. Supporting this conclusion, an amplified fragment length polymorphism (AFLP) is heterozygous in males and homozygous recessive in hermaphrodites in three populations of the androdioecious species. Additionally, hermaphrodite x male crosses produced 1:1 sex ratios, while hermaphrodite x hermaphrodite crosses produced almost entirely hermaphroditic offspring. No perfectly sex-linked marker was found in the dioecious species, but all markers associated with sex mapped to a single linkage group and were heterozygous in the male parent. There was no sex-ratio heterogeneity among crosses within D. cannabina collections, but males from one collection produced highly biased sex ratios (94% females), suggesting that there may be sex-linked meiotic drive or a cytoplasmic sex-ratio factor. Interspecific crosses produced only male and female offspring, but no hermaphrodites, suggesting that hermaphroditism is recessive to femaleness. This comparative approach suggests that the hermaphrodite form arose in a dioecious population from a recessive mutation that allowed females to produce pollen.

Chimera↗
PubMed2002

Levels of DNA polymorphism vary with mating system in the nematode genus caenorhabditis.

Self-fertilizing species often harbor less genetic variation than cross-fertilizing species, and at least four different models have been proposed to explain this trend. To investigate further the relationship between mating system and genetic variation, levels of DNA sequence polymorphism were compared among three closely related species in the genus Caenorhabditis: two self-fertilizing species, Caenorhabditis elegans and C. briggsae, and one cross-fertilizing species, C. remanei. As expected, estimates of silent site nucleotide diversity were lower in the two self-fertilizing species. For the mitochondrial genome, diversity in the selfing species averaged 42% of diversity in C. remanei. Interestingly, the reduction in genetic variation was much greater for the nuclear than for the mitochondrial genome. For two nuclear genes, diversity in the selfing species averaged 6 and 13% of diversity in C. remanei. We argue that either population bottlenecks or the repeated action of natural selection, coupled with high levels of selfing, are likely to explain the observed reductions in species-wide genetic diversity.

Animals↗