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Results for “Self-Fertilization”

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PubMed1979

Isoelectric focusing of some enzymes from Echinococcus granulosus (horse and sheep strains) and E. multilocularis.

Extracts of the horse and sheep strains of Echinococcus granulosus and E. multilocularis were compared on the basis of their isoenzyme patterns for 10 enzymes by means of isoelectric focusing in polyacrylamide gels. The enzymes examined were: acid phosphatase, lactate dehydrogenase, malate dehydrogenase, malic enzyme, phosphoglucoseisomerase, isocitrate dehydrogenase, adenylate kinase, aldolase and alpha-glycerophosphate dehydrogenase. Interspecific and intraspecific differences are apparent in the isoenzyme profiles of all the enzymes except adenylate kinase; the pattern and activity of adenylate kinase are identical for both strains of E. granulosus but this enzyme clearly distinguishes these forms from E. multilocularis. The absence of electromorphic variation in any of the enzymes from either form of E. granulosus may be a result of the self-fertilizing hermaphraditism of these organisms.

Adenylate Kinase↗
PubMed1992

An RFLP map of the Plasmodium falciparum genome, recombination rates and favored linkage groups in a genetic cross.

We report a genetic linkage map of the Plasmodium falciparum genome, using the inheritance patterns of nearly 90 RFLP markers in a genetic cross. Markers were assigned to polymorphic loci on all 14 nuclear chromosomes. Genetic recombination between parental markers was detected in each of the progeny, indicating that progeny from cross-fertilization events were favored over progeny from self-fertilization of either parent alone. Inheritance patterns among the markers suggested that certain parental linkage groups on chromosomes 2, 3, 12 and 13 were favored in the cross. Recombination frequencies on five chromosomes indicated an approximate map unit size of 15-30 kb per centiMorgan for P. falciparum.

Animals↗
PubMed1992

The S locus of flowering plants: when self-rejection is self-interest.

In certain families of flowering plants, a self-incompatibility (SI) locus prevents self-fertilization, by a specific interaction between the S-gene product produced in the pistil and the S-gene products borne on or expressed by the male gametophyte, the pollen grain. The female S-locus gene products for two families showing different types of SI have been putatively identified as major pistil glycoproteins (the S-locus-specific glycoproteins of the Brassicaceae and the S-RNases of the Solanaceae). However, they are distinct in sequence and mode of action. The nature of the S-locus gene product borne by the pollen is still uncertain in both systems.

Brassica↗
PubMed1992

Population genetics and dynamics of Plasmodium falciparum: an ecological view.

Molecular characterization of the Plasmodium falciparum genome has led to identification of polymorphic loci and the mechanisms generating genetic diversity in this parasite. This information has resulted in the development of molecular methods to type parasite diversity in the field. Consequently, we are now in a position to describe the population genetics and dynamics of P. falciparum. The limited number of field studies that have been conducted to date have revealed an extraordinary degree of genetic diversity in natural parasite populations. Heterozygous recombination which occurs during meiosis appears to be one mechanism for generating genetic diversity. The rate at which such recombination occurs in natural parasite populations defines the genetic structure of the parasite population and can influence the ability of the parasite to respond to selection pressure. The high frequency of single genotype infections and the female-biased gametocyte sex ratios found in hyperendemic malaria areas suggest that self-fertilization occurs frequently. Population-wide surveys of allele frequencies in endemic areas have, however, shown no evidence of linkage disequilibrium and are consistent with a panmictic population structure. We argue that these studies have only sampled symptomatic infections, within which rare or recombinant genotypes may be disproportionately represented. They also take no account of the spatial structure of P. falciparum populations. Systematic investigations of the amount of heterozygosity in small areas as part of population-wide surveys are required to define the genetic structure of P. falciparum populations. Population dynamic studies which consider genetic heterogeneity of P. falciparum have shown fluctuations of different serotypes in space and time. The host immune response appears to play an important role in generating these dynamics. Integrated field and laboratory studies, which consider the interaction between population genetics and dynamics, will be necessary to describe the population biology of P. falciparum.

Animals↗
PubMed1991

Effects of a change in the level of inbreeding on the genetic load.

"The effects of inbreeding may not be as noticeable in the first generation as the invigoration immediately apparent after crossing". This statement, published in 1919, has received little attention, and has apparently never been tested empirically, although the reduction of the genetic load of populations by inbreeding is well known in theoretical terms. Because inbreeding increases homozygosity, and hence the effectiveness of selection against recessive or partially recessive detrimental alleles, changes in levels of inbreeding can lead to a reduction in the frequencies of such mutant alleles. This results in equilibration at higher population mean fitness and is referred to as 'purging' populations of their genetic load. Severe inbreeding can also reduce genetic load due to overdominant alleles, provided selection coefficients are not symmetrical at all loci, because alleles giving lower fitness will be reduced in frequency at equilibrium. With either fitness model, however, reduction in genetic load takes time, and the initial effect of an increase in inbreeding is reduced fitness due to homozygosity. There are few data relating to the extent to which fitness is reduced during inbreeding in a set of lines and to how long the reduction lasts before increasing again to the initial level, or higher. Inbreeding experiments involving sib mating in mice and Drosophila subobscura, and successive bottlenecks in house flies have yielded some evidence consistent with the purging hypothesis. Here, we report results of an experiment demonstrating a prolonged time-course of recovery of mean fitness under self-fertilization of a naturally outcrossing plant, and also compare our results with expectations derived by computer calculations. Our results show that the genetic load present in an outcrossing population can be explained only with a high mutation rate to partially recessive deleterious alleles, and that inbreeding purges the population of mutant alleles.

Animals↗
PubMed1992

Analysis of mating systems in the schistosome-vector hermaphrodite snail Bulinus globosus by DNA fingerprinting.

Bulinus globosus, one of the intermediate hosts for Schistosoma, is a hermaphrodite freshwater snail. Multilocus DNA fingerprinting was applied in order to investigate the mating system of this facultatively self-fertilizing species. By analysing DNA fingerprints of eight broods, we have shown that selfed and outcrossed individuals could be unambiguously distinguished by comparison with their parent fingerprints. First, selfed offspring patterns shared a higher number of bands with their mother than did outcrossed ones. Secondly, at least three paternal bands could be detected per outcrossed offspring pattern.

Animals↗
PubMed1978

Mass isolation and fertility testing of temperature-sensitive mutants in Tetrahymena.

A set of 239 heat-sensitive (38 degrees) and 16 cold-sensitive (18 degrees) conditional mutants of Tetrahymena has been generated by combining techniques to manipulate large numbers of clones with a method for the selection of self-fertilized cells after mutagen treatment. A simple technique is presented for determining the fertility of individual clones; 179 of the clones in this set (71%) are fertile. The fertile conditional mutants have already been shown to have lesions in a number of diverse functions, including nucleic acid metabolism, mobility, cell cycle, and cortical pattern.

Animals↗
PubMed1992

Comparative effects of pollen and seed migration on the cytonuclear structure of plant populations. II. Paternal cytoplasmic inheritance.

We continue our study of the effects of pollen and seed migration on the cytonuclear structure of mixed-mating plant populations by analyzing two deterministic continent-island models under the critical assumption of paternal cytoplasmic inheritance. The major results of this study that contrast with our previous conclusions based on maternal cytoplasmic inheritance are (i) pollen gene flow can significantly affect the cytonuclear structure of the island population, and in particular can help to generate cytonuclear disequilibria that greatly exceed the magnitude of those that would be produced by seed migration or mixed mating alone; (ii) with simultaneous pollen and seed migration, nonzero cytonuclear disequilibria will be maintained not only when there is disequilibrium in the immigrant pollen or seeds, but also through a variety of intermigrant admixture effects when the two pools of immigrants differ appropriately in their cytonuclear compositions; (iii) either immigrant pollen or immigrant seeds can generate disequilibria de novo in populations with initially random cytonuclear associations, but pollen migration alone generally produces lower levels of disequilibrium than does comparable seed migration, especially at high levels of self-fertilization when the overall fraction of immigrant pollen is low; (iv) the equilibrium state of the island population will be influenced by the rate of pollen gene flow whenever there is either allelic disequilibrium in the immigrant pollen or simultaneous seed migration coupled with different cytoplasmic or nuclear allele frequencies in immigrant pollen and seeds or nonzero allelic disequilibrium in either immigrant pool. The estimation of pollen migration should therefore be facilitated with paternal cytoplasmic inheritance relative to the case of maternal cytoplasmic inheritance. These basic conclusions hold whether the population is censused as seeds or as adults, but with simultaneous pollen and seed migration, the relationship between census time and the ability to detect nonrandom cytonuclear associations is complex. When migration is through pollen alone, however, the cytonuclear structure of the island population is independent of the life stage censused.

Alleles↗
PubMed1992

Uniparental cytogamy: a novel method for bringing micronuclear mutations of Tetrahymena into homozygous macronuclear expression with precocious sexual maturity.

A new method of inducing self-fertilization, uniparental cytogamy, yields homozygous germinal and somatic genotypes in the ciliate Tetrahymena thermophila. Progeny are highly fertile and show a marked tendency for precocious sexual maturity. This method is highly effective in protocols designed to generate and express nonlethal dominant or recessive mutations.

Animals↗
PubMed1977

Age-correlated changes in expression of micronuclear damage and repair in Paramecium tetraurelia.

In Paramecium, age is defined as the number of mitotic divisions which have elapsed since the previous cross-fertilization (conjugation) or self-fertilization (autogamy). As the mitotic interval between fertilizations increases, the percentage of nonviable progeny clones increases. In the current study, resolution of conflicting previous reports on the pattern of increase of death and reduced viability in progeny from aging parent cells is found. Some exautogamous clones exhibit a high mortality at young clonal ages, others show no mortality throughout their life span, but most (73%) show an abrupt increase in the percent death and reduced viability in progeny from cells 50-80 fissions old. Ultraviolet-irradiation-induced micronuclear mutations, repairable by photoreactivation, increased with increased clonal age when monitored by percent death and reduced viability of exautogamous progeny of irradiated cells. Loss of dark repair is considered a contributor to the increased expression of micronuclear mutations with increased clonal age.

Cell Division↗
PubMedDate not supplied

The inheritance of albinism in a freshwater snail, Physa heterostropha.

Complementation tests revealed that albinism in four laboratory strains of Physa (Physella) heterostropha pomilia resulted from two recessive, nonallelic genes. F2 dihybrid progeny displayed the 9:7 ratio classically associated with reciprocal recessive epistasis between unlinked loci. This offers a contrast to the situation in the better known planorbid snails and provides a valuable tool for the study of reproductive biology in these facultatively self-fertilizing hermaphrodites.

Animals↗
PubMed1979

The evolution of genetic diversity.

The existence within natural populations of large amounts of genetic variation in molecules and morphology presents an evolutionary problem. The 'neutralist' solution to this problem, that the variation is usually unimportant to the organism displaying it, has now lost much of its strength. Interpretations that assume widespread heterozygous advantage also face serious difficulties. A resolution is possible in terms of frequency-dependent selection by predators, parasites and competitors. The evidence for pervasive frequency-dependent selection is now very strong. It appears to follow naturally from the behaviour of predators, from the evolutionary lability of parasites, from the ecology of competition and, at the molecular level, from the phenomena of enzyme kinetics. Such selection can explain the maintenance not only of conventional polymorphism but also of continuous variation in both molecular and morphological characters. It can account for the occurrence of diversity within groups of haploid and self-fertilizing organisms, and for the evolution of differences between individuals in their systems of genetic control.

Alleles↗
PubMed1979

The evolutionary genetics of sexual systems in flowering plants.

Population genetic studies of the evolution of breeding systems in flowering plants are reviewed. The selective advantage of a gene's increasing the selfing rate is stressed. In the evolution of outbreeding mechanisms, some strong disadvantage to selfing must therefore be acting; it is suggested that this disadvantage is inbreeding depression. Populations with no absolute barrier to selfing, and with intermediate levels of self-fertilization, appear to be the most likely starting state for the evolution of outbreeding mechanisms. There is some evidence for inbreeding depression in such populations. The evolution of distyly and dioecy are considered in some detail. An explanation for the existence of supergenes controlling these systems is proposed. The breakdown of distyly and tristyly are also considered. The evolution of recombination rates in selfing and outcrossing species is examined briefly.

Biological Evolution↗
PubMed2025

Inbreeding load in finite populations from dominant and overdominant mutations.

Inbreeding depression is a widespread phenomenon that reflects the burden of deleterious effects hidden in heterozygosis in non-inbred populations but exposed in homozygosis in inbred individuals, known as inbreeding load (B). This load can be due to partially or fully recessive deleterious mutations (dominance model) or to heterozygote advantage (overdominance model, where both homozygotes are deleterious relative to the heterozygote). There are many studies addressing the changes in inbreeding load in finite populations assuming the dominance model. However, the contribution of overdominance to inbreeding depression has been focused on infinite-size populations. We carried out computer simulations to investigate the joint impact of dominant and pure overdominant mutations on inbreeding load, both for self-fertilizing populations and for panmictic populations suffering from a drastic bottleneck. We found that the overdominant inbreeding load can be substantially reduced by drift even for symmetrical overdominance, at least when considering mutations of small effect. For panmictic bottlenecked populations, the reduction in inbreeding load under dominance and overdominance loci cannot be easily distinguished. However, while purging depletes inbreeding load from dominant loci, slowing inbreeding depression and leading to partial fitness recovery, for overdominant loci fitness declines monotonically.

Inbreeding↗
PubMed2026

T2T genomes of Caenorhabditis nigoni and Caenorhabditis briggsae reveal divergence in satellite DNA abundance.

The two closely related nematode species, Caenorhabditis nigoni and Caenorhabditis briggsae, are commonly used to study the evolution of reproductive modes in animals, with the self-fertile C. briggsae and outcrossing C. nigoni sharing a common ancestor ∼3.5 million years ago. Earlier genomic analyses revealed that selfing Caenorhabditis species have smaller genomes and proposed that at least some gene loss in C. briggsae is adaptive. However, the incomplete C. nigoni reference genome has limited most comparative analyses to genic regions. Here, we leverage long-read sequencing to generate and annotate telomere-to-telomere (T2T) assemblies for the C. nigoni strain JU1422 and the C. briggsae strain AF16. This new 139 Mb C. nigoni genome resolves 57 gaps and 149 unassigned scaffolds from the previous genome assembly. A major driver of the size difference with the 107 Mb T2T C. briggsae genome is the abundance of satellite DNA, which accounts for 12.8 Mb (9.2%) in C. nigoni and only 3.2 Mb (3.0%) in C. briggsae Notably, the C. nigoni X Chromosome is 13.4 Mb larger than in the previous assembly, making it 60% larger than the C. briggsae X Chromosome compared with 18%-26% difference for the autosomes. We also document a surprising degree of plasticity in the ribosomal DNA, with the C. nigoni X Chromosome harboring a second 45S rDNA array that is absent in C. briggsae The hitherto undocumented divergence in the abundance of repetitive DNA elements makes the new genomes an invaluable resource for genomic analysis.

Journal Article↗
PubMed2025

Alternative splice acceptor site in MSH4 gene is responsible for male sterility conferred by ms5 in soybean.

In soybean breeding, using the recessive male-sterile ms5 gene, derived from fast neutron mutagenesis, for recurrent selection is advantageous because of the d2 locus, which controls cotyledon color in mature seeds and can be used as a phenotypic selection marker for ms5 male sterility. However, occasional self-fertilization occurs because of the elimination of d2 linkage and instability of male sterility. Elucidating the mechanism and the gene responsible for ms5 male sterility may resolve these problems. Using fine mapping with 15 simple sequence repeat (SSR) markers, we narrowed down the candidate ms5 locus to a 54-kbp region. Bulked-DNA analysis using next-generation sequencing revealed a deletion as a candidate variation in the region. This 15-bp deletion and a nucleotide substitution were identified in intron 1 of MutS homolog (GmMSH4), which modulates chromosomal recombination in meiosis. The ms5 transcript contained a novel exon with a premature termination codon. This exon originated from an alternative splice acceptor site caused by the deletion and nucleotide substitution, disrupting gene function. Co-segregation of male sterility with five independent mutations in GmMSH4 was confirmed using progeny of mutant lines. Mutations in GmMSH4 led to biased DNA partitioning during meiosis, resulting in collapsed or enlarged pollen and suggesting that ms5 male sterility is caused by the failure of pollen formation during meiosis due to the loss of function of GmMSH4. These findings could help explain the mechanism of instability of ms5 male sterility and improve the efficiency of recurrent selection using DNA markers in soybean breeding.

Glycine max↗
PubMed1979

Osmotic shock prevents nuclear exchange and produces whole-genome homozygotes in conjugating Tetrahymena.

Exposure of conjugating Tetrahymena to a hyperosmotic shock blocks the exchange of gametic nuclei and produces self-fertilized exconjugants that are homozygous for their whole genome. Cells are sensitive to this induction during a brief period after meiosis. The high efficiency of the treatment and the fertility of the progeny make this a useful method for the isolation of induced recessive mutations and enhances the value of Tetrahymena as an animal-cell model system in which genetic dissection is practical. The sharp peak of sensitivity is useful in the study of those cellular mechanisms responsible for the independent handling of several functionally distinct nuclei during conjugation.

Animals↗
PubMed1992

Mating behavior in the pulmonate small melampus: can regeneration restore function?

Previous anatomical observations have established that the penial complex of adult Melampus bidentatus is specifically reinnervated and that the penial complex itself can regenerate. This review describes experiments to determine whether a reinnervated penial complex and a regenerated penial complex can function in mating. The snails are not self-fertile, so the ability to transfer sperm, evaluated by production of fertile eggs, was the test of successful regeneration. Snails with nerve transections produced fertile eggs within the time-span expected for axonal regeneration to the target organ. Penial complex ablation interfered with sperm transfer for two egg-laying cycles but fertile eggs were laid within a time-span consistent with regeneration of a penial complex.

Animals↗