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At least 19 recordsLinked to original sources
PubMed2026

T2T genomes of Caenorhabditis nigoni and Caenorhabditis briggsae reveals extensive loss of satellite DNA associated with self-fertilization.

The two closely related Caenorhabditis nematode species, C. nigoni and C. 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 of these species revealed genome shrinkage associated with selfing and proposed that at least some gene loss can be adaptive. However, the incomplete C. nigoni reference genome limited most comparative analyses to genic regions. Here, we leveraged long-read sequencing to generate a telomere-to-telomere (T2T) assembly for the C. nigoni strain JU1422 and the C. briggsae strain AF16. This new 139Mb C. nigoni genome resolved 57 gaps and 149 unassigned scaffolds from the previous genome assembly. Comparison with the 107Mb T2T C. briggsae genome reveals that the major driver of genome content differences are deletions to satellite DNA arrays, reflecting a loss of 9.6Mb. Interestingly, many of the differences are on the C. nigoni X chromosome, which is >13Mb larger than in the previous assembly. The transition to selfing was thus accompanied by a 37% reduction in the size of the sex chromosome compared to 16-21% shrinkage of the autosomes. We also document a surprising degree of plasticity in the ribosomal DNA, with the X chromosome harboring a second 45S rDNA array that is absent in C. briggsae . Our analysis reveals that obligatory outcrossing may play a major role in the maintenance of satellite DNA arrays.

Journal Article↗
PubMedDate not supplied

[Development of pollen tubes in self-fertile and self-sterile lines of sugar beets in isolation].

Studies in pollen germination on stigmas of isolated plants showed a great difference in this process in different forms. A rapid and steady growth of pollen tubes, their accumulation near the micropyle and penetration in it are observed in self-fertile plants. Pollen tubes grow slowly, often forming thickenings and swellings in self-sterile forms. Some of them penetrate deeper, reaching sometimes the seedbud but turn back without penetrating into it.

Plants, Edible↗
PubMed1976

Expectations for inbreeding depression on self-fertilization of tetraploids.

The contribution to the inbreeding depression from a digenic tetrasomic locus upon self-fertilization involves three genotypic interaction effects which may be thought of as a generalization of the dominance deviation for a diploid locus. It is shown how this contribution may be expressed in terms of these genotypic interaction effects, the gene frequencies and the number of generations of selfing.

Alleles↗
PubMed1976

Isolation of homozygous mutants after induced self-fertilization in Tetrahymena.

Genomic exclusion, an unusual cytogenetic sequence during mating in Tetrahymena pyriformis, results in the production of homozygous germinal nuclei by the diploidization of haploid nuclei following meiosis. A method is presented for selecting cells that have made new somatic nuclei from these homozygous germinal nuclei, a step necessary for phenotypic expression of new mutations; this variation of the normal set of events is termed short-circuit genomic exclusion. The utility of thisapproach for obtaining induced mutations is demonstrated by the isolation and analysis of a strain homozygous for a recessive mutation conferring resistance to 2-fluoroadenosine. Occurring in about 5% of the unmutagenized pairs in specific crosses, short-circuit genomic exclusion should be of general use for the isolation of dominant or recessive induced mutations in this protozoan.

Cell Nucleus↗
PubMed1975

The mating system and microevolution.

Studies of natural and experimental plant populations have revealed that genotypic frequency distributions are highly structured in predominantly selfing species. This high degree of genetic organization is manifested in intense correlations in allelic state over loci and also in striking micro-geographical heterogeneity. Both aspects of this structure are facilitated by self-fertilization. Development of non-random associations of alleles within populations is facilitated because inbreeding reduces heterozygosity and thus also the randomizing effect of recombination. Spatial differentiation is facilitated because self-fertilization retards gene flow from population to population. The effect of organizing the entire populational genotype into a sort of giant supergene is to increase the frequency in the population of genotypes which confers high fitness and hence to increase adaptation to the local environment. However the recombinational potential remains substantial. As a result considerable free genetic variability remains in the population and it is available for long-term response to natural selection. Thus the organization of genetic variability within populations provides for high immediate fitness and also for flexibility to meet longer term evolutionary needs. At the same time selfing is a barrier to migration and it promotes the development and maintenance of different multilocus organizations in adjacent populations occupying unlike habitats. In total, therfore, a pattern of genetic differentiation develops in space which is an almost exact overlay of the environmental heterogeneity. The plant genetic and plant breeding literature contains extensive evidence that the mating system in plants can be modified simply and drastically by selection and that different populations within the same species often practice very different amounts of inbreeding. Considering the ease with which the mating system can be altered, and the benefits of adjusting genetic variability through regulation of the mating system, it is not surprising that a high proportion of flowering plants self-fertilize to some extent and that at least one-third of species have adopted predominant selfing as a strategy in ecogenetic adaptation.

Adaptation, Biological↗
PubMed1977

Effect of polyploidy on phosphoglucose isomerase diversity in Festuca microstachys.

Studies of the inheritance of electrophoretic banding patterns in Festuca microstachys support the hypothesis that three closely related loci, one located in each of the three ancestral genomes, code the multiple phosphoglucose isomerase (glucosephosphate isomerase; D-glucose-6-phosphate ketol-isomerase, EC 5.3.1.9) variants found in this hexaploid species. The close relationship among the three loci is indicated by the observation that hybrid enzymes of intercistronic origin form when the loci in different genomes carry alleles coding homodimers with unlike migration rates. Homozygous individuals fixed for different alleles in different genomes produce hybrid enzymes and, when self-fertilized, they breed true for isozyme patterns normally found only in the heterozygotes of diploid species. Biochemical diversity due to this "fixed heterozygosity" is high in F. microstachys; although this species is more than 99% self-fertilized the proportion of individuals with at least one heterodimer exceeded 61% in all of the 16 natural populations studied and it exceeded 92% in 11 of the populations. This great biochemical diversity may contribute to the ability of F. microstachys to survive in the wide range of habitats in which it is found over western North America.

Alleles↗
PubMed2025

Genomic signature and evolutionary history of completely cleistogamous lineages in the non-photosynthetic orchid Gastrodia.

Despite a long-standing interest since Darwin's time, the genomic implications of obligate self-fertilization remain elusive. Complete cleistogamy-the obligate production of closed, self-pollinating flowers-represents an extreme reproductive strategy. Here, we present the genomic profiles and evolutionary history of two lineages of the mycoheterotrophic orchid Gastrodia, both of which independently acquired complete cleistogamy, based on detailed sampling and a combination of simple sequence repeat (SSR), multiplexed ISSR genotyping by sequencing (MIG-seq) and RNA-seq data. Our analysis reveals clear species delimitation, with no evidence of introgression between the completely cleistogamous species and their co-occurring allogamous sisters. Intriguingly, all analyses indicate that both the completely cleistogamous Gastrodia species and their allogamous sisters exhibit genetic profiles typical of self-pollinating plants. This pattern suggests that their ancestors, probably bearing allogamous flowers, had already evolved mechanisms to mitigate the deleterious effects of selfing, potentially facilitating the emergence of complete cleistogamy through benefits such as reproductive assurance, enhanced colonization ability and species reinforcement. Meanwhile, further analyses suggest that complete cleistogamy evolved very recently (possibly within the last 1000-2000 years) in these two Gastrodia lineages. Combined with the scant evidence of complete cleistogamy outside Gastrodia, our findings imply a limited and ephemeral role for complete cleistogamy in plant speciation.

Biological Evolution↗
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↗
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↗
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↗
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↗