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At least 19 recordsLinked to original sources
PubMed2026

Sexual selection purges mutation load, but not overall genetic diversity, decreasing vulnerability to extinction.

Theory suggests sexual selection will enhance population viability by purging deleterious alleles. However, direct genomic evidence for this fundamental idea is scarce and contradictory. We combined long-term experimental evolution with whole-genome resequencing to directly test how sexual selection affects mutation load, genomic divergence, and extinction risk in small populations (maximum Ne = 40) of Tribolium castaneum. After 156 generations, populations evolving under strong sexual selection carried substantially fewer deleterious alleles than populations under weak sexual selection, based on both individual-level estimates of missense and nonsense variants and population-level Rxy analyses, indicating more efficient purging of deleterious alleles. In contrast, nucleotide diversity and runs of homozygosity were similar across treatments, indicating that purging acted most strongly on deleterious variation, and that reduced mutation load in these small populations under strong sexual selection was not explained by demographic effects. Importantly, population-level mutation load estimates best explained extinction risk under inbreeding, directly linking sexual selection to purging and population viability. Genome scans of high and low sexual selection populations revealed peaks of divergence, which included genes involved in courtship, sex discrimination, and seminal fluid proteins. Our results provide direct genomic evidence that sexual selection can reduce mutation load without eroding standing genetic diversity and thus adaptive potential, while driving adaptive divergence in reproductive traits. This beneficial purging may help explain the widespread prevalence of sexual reproduction in nature despite inherent costs and have important ramifications as to how we manage populations of conservation concern.

Animals↗
PubMed2026

Ambrosia beetle invasions are structured by inbreeding, intraspecific hybridisation, and bridgeheads.

When invasive populations establish in regions far from their origin, they may accumulate deleterious mutations that limit population viability and later expansion. Invasions stemming from such bridgehead populations may experience further sequential bottlenecks. However, deleterious mutations can be masked or eliminated when populations outbreed with other lineages. Here, we analyse global invasions of a species complex of persistently inbreeding ambrosia beetles, using genomic data (N=247) from invasive populations in Africa, North America and Australia, and from native populations in Asia. We mostly focus on one species of this complex (Euwallacea fornicatus) which poses a severe threat to tree species worldwide and is rapidly expanding its global range. We uncover a single lineage of this species across California, South Africa, and Western Australia, involving an invasive bridgehead and containing almost no nuclear genetic variation. In South Africa we identify a second lineage that has repeatedly hybridised with the first lineage. Genetic patterns in the native range indicate that such opportunistic outbreeding may be common. Despite lacking nuclear variation, the first lineage contained two CO1 haplotypes that were also observed in every hybrid lineage, pointing to heteroplasmy and possible hybrid origins of this lineage. Native populations had fewer missense mutations than invasive populations, indicating that opportunistic outbreeding may help purge fixed deleterious mutations when local lineage diversity is high. These findings highlight the importance of outbreeding even when inbreeding is common, and they demonstrate the biosecurity threat posed by subsequent gene flow into invasive populations.

Journal Article↗
PubMed2026

Genetic structure and selection signatures of Beijing-You chicken populations provide insight into breed conservation.

Preserving genetic diversity and maintaining population viability are critical yet challenging goals that demand rigorous evaluation of conservation strategies. Beijing-You chicken, as the sole indigenous chicken breed originating from Beijing, China, is currently maintained as four independent populations under distinct conservation programs. How different conservation regimes have shaped its genomic architecture remains largely unknown, limiting evidence-based evaluation. Here, we generated whole-genome resequencing data from 240 individuals representing four Beijing-You chicken populations to assess population structure, genetic diversity, and signatures of selection over decades of conservation. All four populations formed distinct clusters, reflecting measurable differentiation after decades of separate conservation. The differences in genetic diversity were broadly consistent with the variation in effective population size estimates. Runs of homozygosity and linkage disequilibrium decay patterns further characterized each population, with extended values indicating reduced effective population size and increased inbreeding under long-term conservation. We applied the fixation index (FST) and pairwise diversity ratio (θπ) methods to identify selection signatures. A total of 171 genes were identified as candidates. These genes were enriched in pathways related to reproduction, growth regulation, and environmental adaptation. These findings highlight patterns of reduced diversity and skewed relatedness, which could arise from management-related factors such as breeding preferences or mating strategies. Still, they are also compatible with neutral processes, including drift and founder effects. Regardless of the underlying cause, integrating scientifically informed conservation strategies with routine genomic monitoring across generations is essential for sustaining genetic diversity in Beijing-You chicken and other indigenous breeds.

Beijing-You chicken↗
PubMed2026

Genomic erosion in the assessment of species' extinction risk and recovery potential.

Many species are undergoing rapid population declines and environmental deterioration, leading to genomic erosion. Here we define genomic erosion as the loss of genetic diversity, accumulation of deleterious mutations, maladaptation, and introgression, all of which can undermine individual fitness and long-term population viability. Critically, this process continues even after demographic recovery due to a time-lagged impact of genetic drift, which is known as drift debt. Current conservation assessments, such as the International Union for Conservation of Nature Red List, focus on short-term extinction risk and do not capture the long-term consequences of genomic erosion. Likewise, the longer-term assessments of the International Union for Conservation of Nature Green Status may overestimate population recovery by failing to account for the enduring effects of genomic erosion. As genome sequencing becomes increasingly accessible, there is a growing opportunity to quantify genomic erosion and integrate it into conservation planning. Here, we use genomic simulations to illustrate how different genomic metrics are sensitive to the drift debt. We test how ancestral effective population size (Ne) and bottleneck history influence the tempo and severity of genomic erosion. Furthermore, we demonstrate how these dynamics shape genetic load and additive genetic variation, which are key indicators of long-term evolutionary potential. Finally, we present a proof-of-concept for a Genomic Green Status framework that aligns genomic metrics with conservation impact assessments, laying the foundation for genomics-informed strategies to support species recovery.

Extinction, Biological↗
PubMed1976

The effect of cooling and warming rates on the survival of a variety of bacteria.

Cooling and warming rates affect bacterial survival profoundly with all bacteria tested (Azotobacter chroococcum, Klebsiella aerogenes, Salmonella typhimurium, Pseudomonas aeruginosa, Streptococcus faecalis) behaving similarly. Most bacteria were sensitive to salt on freezing and thawing. Viabilities of population frozen and thawed in saline were always less (by at least 20%) than comparable ones frozen in water alone. Under these conditions, S. faecalis was resistant to the presence of sodium chloride; viabilities of populations frozen in the presence or absence of sodium chloride were always less than 5% different. The implications of these results in light of the organisms' shape, Gram stain, and respiration are discussed.

Azotobacter↗
PubMed1976

[Viability of beta-thalassemia hetero- and homozygotes in several populations of Central Asia].

Viability of hetero- and homozygotes for beta-thalassemia was studied in two isolated populations born after 1950 in conditions of malaria absence. Pregnancy outcomes and probability to get 16 years old were compared in marriages, in which one or both parents were heterozygous for beta-thalassemia, and in marriages, with both parents being normal. The ratio of children with normal genotype and children heterozygous for beta-thalassemia in families, where one parent was heterozygous for beta-thalassemia. Preliminary conclusions are made on the basis of the data obtained. Viability of children in families where one parent is heterozygous for beta-thalassemia is found to decrease in the absence of selective factors (malaria) as compared with normal children. The highest viability decrease was observed in the progeny of those families where both parents were heterozygous beta-thalassemia. Elimination mainly takes place within the first year of life.

Female↗
PubMed1978

Inbreeding effects: evidence for a genetic system which regulates viability in Drosophila melanogaster populations.

Mating studies on individual couples of Drosophila melanogaster allowed the structure of inbred populations in terms of egg hatchability and egg-to-adult survival to be elucidated. The comparison between inbred (from brother-sister matings) and control (randomly crossed) populations distinguished between couples sensitive and insensitive to inbreeding. Mendelian ratios were observed for this "phenotypic trait" in the progeny of single couples. These ratios and a double mating experiment indicated that a gene or gene complex implicated in morphogenetic events blocked development during embryonic and larvo-pupal stages. The expression of this lethal gene in homozygous embryos depended on the genetic makeup of both parental flies, i.e., on male and female factors which act as regulatory components. Homozygous embryos which survived the first critical phase (from fertilization to hatching) continued to develop normally until the larvo-pupal period, when they died. Lethality was also modulated by the cytoplasmic composition of the parental eggs, as shown by taxonomic analysis and the double mating experiment. The results account for lethality throughout development due to inbreeding. They lead to an estimated genetic load from laying to adult stage of one "lethal equivalent".

Animals↗
PubMed2026

Genomic consequences of admixture in an experimentally founded sand lizard population.

Conservation interventions are increasingly required for species threatened by population declines and isolation due to anthropogenic pressures. Small, isolated populations are particularly vulnerable to the loss of genetic diversity, increased inbreeding, and the accumulation of deleterious mutations. Translocations or supplementation of allopatric individuals for genetic rescue may be the only way to increase genetic diversity and increase population persistence via increased adaptive potential. Here, we use an experimentally admixed population of sand lizards on a small island in Sweden as a valuable model of genetic rescue. This population was established approximately 20 years ago (5-6 generations), resulting in increased fecundity and hatchling viability. This population was founded from crossings between individuals from an inbred population from the nearby mainland and individuals sourced from populations in southern Sweden. Low-coverage whole-genome sequencing revealed elevated genetic diversity and reduced realized genetic load in this admixed population relative to the source populations. Ancestry analyses indicated a greater contribution of southern Swedish genetic variation, potentially reflecting the contribution of beneficial adaptive variation from this region that may underlie the positive population effects. This system provides valuable empirical insights into the long-term genomic consequences of genetic rescue in this model vertebrate population.

Journal Article↗
PubMed2025

Reproductive Isolation due to Divergent Ecological Selection Is Accompanied by Vast Genomic Instability in Experimentally Evolved Yeast Populations.

Populations evolving independently in divergent environments accumulate genetic differences and potentially evolve reproductive isolation as a by-product of divergence. The speed and mechanisms underlying this process are difficult to investigate because we rarely get the opportunity to witness them in natural settings, and histories of selection and gene flow between populations are often unknown. Here, we experimentally evolved yeast for 1000 generations of evolution in both divergent and parallel environments. At regular time points during experimental evolution, we made crosses between parallel- and divergent-evolving populations to measure postzygotic reproductive isolation (gamete viability). We used whole genome population sequencing to determine the mutational load, the number and types of structural variation, and other genomic features of the parent, F1 and F2 intraspecific hybrids. We found evidence for large-scale phenotypic and genome-wide differentiation in response to divergent laboratory selection. Divergent-selected populations produced hybrids with reduced gamete viability-a classic signature of postzygotic reproductive isolation in the form of hybrid breakdown. Parallel-selected populations, on the other hand, remained more reproductively compatible (with exceptions). We found that F2 hybrid genomes contained vast genomic instability, that is, new structural variants (especially insertions, deletions and interchromosomal translocations) that were not observed in parent and F1 genomes, which is likely a result of chromosome missegregation and recombination errors in hybrid meiosis. Our results provide phenotypic and genomic evidence that partial reproductive isolation evolved due to adaptation to divergent environments, consistent with predictions of ecological speciation theory.

Reproductive Isolation↗
PubMed1977

[Characteristics of the morphology and ultrastructure of the typhoid bacteria under limited and excess glucose conditions].

The authors present the results of the study of morphology and physico-biochemical indices of periodic and continuous population of typhoid bacilli under conditions of glucose limit and excess in the medium. Changes in the parameters of cell distribution by length proved to reflect the physiologico-biochemical processes in the population. The results of the study of the ultrastructure of typhoid bacilli of the "aerobic" and "anaerobic" population in continuous cultivation are presented. A possibility of application of morphological tests for the assessment of stability, homogeneity, and viability of the population is discussed.

Aerobiosis↗
PubMed1978

Effect of adriamycin on the reproductive integrity of cultured leukemia L1210 and P388 cells.

Proliferating cultured P388 cells exhibited a greater degree of sensitivity to adriamycin than did proliferating cultured L1210 cells, although both leukemia cell populations had approximately the same doubling time. The rate of reduction in viability when cultured L1210 cell populations were exposed to adriamycin (0.0625-2.0 microgram/ml, concentrations that are comparable to tissue drug levels during therapy) was concentration-dependent. Therefore, the results indicated a possible therapeutic advantage to be gained by an increase in drug concentrations (within the limits of acceptable host toxicity) at the target cell site.

Animals↗
PubMed1976

Influence of the rate of ethanol production and accumulation on the viability of Saccharomyces cerevisiae in "rapid fermentation".

Whereas "rapid fermentation" of diluted clover honey (25 degrees Brix) fortified with yeast nutrients using 8 X 10(8) brewers' yeast cells per ml resulted in an ethanol content of 9.5% (wt/vol; 12% vol/vol) in 3 h at 30 C, death rate of the yeast cells during this period was essentially logarithmic. Whereas 6 h was required to reach the same ethanol content at 15 C, the yeast cells retained their viability. Using a lower cell population (6 X 10(7) cells/ml), a level at which the fermentation was no longer "rapid," the yeast cells also retained their viability at 30 C. Ethanol added to the medium was much less lethal than the same or less quantities of ethanol produced by the cell in "rapid fermentation." It was considered possible that ethanol was produced so rapidly at 30 C that it could not diffuse out of the cell as rapidly as it was formed. The hypothesis was postulated that ethanol accumulating in the cell was contributing to the high death rate at 30 C. It was found that the intracellular ethanol concentration reached a level of approximately 2 X 10(11) ethanol molecules/cell in the first 30 min of fermentation at 30 C. At 15 C, with the same cell count, intracellular ethanol concentration reached a level of approximately 4 X 10(10) ethanol molecules/cell and viability remained high. Also, at 30 C with a lower cell population (6 X 10(7) cells/ml), under which conditions fermentation was no longer "rapid," intracellular ethanol concentration reached a similar level (4 X 10(10) molecules ethanol/cell) and the cells retained their viability. Alcohol dehydrogenase (ADH) lost its activity in brewers' yeast under conditions of "rapid fermentation" at 30 C but retained its activity in cells under similar conditions at 15 C. ADH activity was also retained in fermentations at 30 C with cell populations of 6 X 10(7)/ml. It would appear that an intracellular level of about 5 X 10(10) ethanol molecules/cell is normal and that this level does not damage either cell viability or ADH activity. Higher intracellular ethanol concentrations, such as 2 X 10(11) molecules ethanol/cell (a fourfold increase in intracellular ethanol concentration), are accompanied by inactivation of ADH and loss of cell viability.

Alcohol Oxidoreductases↗
PubMed1978

Cultural and environmental factors affecting the longevity of Escherichia coli in Histosols.

The survival of Escherichia coli in organic soils (Histosols) was examined. The death rate of this organism in Pahokee muck was less than that observed in Pompano fine sand. The number of viable E. coli cells found in the muck was approximately threefold greater than that found in the sand following 8 days of incubation. The initial population of the coliform affected the death rate. The rate of loss of viability varied 100-fold when the population size decreased from 2.5 x 10(7) to 3.4 x 10(4). Other factors affecting the viability of E. coli in muck were aerobic versus anaerobic growth of the organism and moist versus flooded conditions in the soil. The greatest survival of the coliform was noted with anaerobically grown cells amended to flooded soil. That the observed decrease in E. coli viability in soil was the result of biotic factors was demonstrated with amendment of sterile soil with E. coli. When 1.1 x 10(5) bacteria per g of soil were added to sterile muck, a population of 3.0 x 10(7) organisms per g of soil developed over a 10-day period. The role of the protozoa in eradication of the coliform from the muck was indicated by a sixfold increase in the protozoan population in natural soil amended with E. coli. Higher organic matter content in a Histosol compared with a mineral soil resulted in an increased survival of the fecal coliforms. Biotic factors are instrumental in the decline in coliform populations, but the potential for growth of the coliform in the organic soil could extend the survival of the organism.

Aerobiosis↗
PubMed1979

Some models of genetic selection.

This paper begins with a description of the classical theory of viability selection in which probabilities that individuals of various genotypes survive are in proportions that do not change with time and are independent of population structure. Salient features of viability selection with one and two loci are reviewed. This theory is intimately connected with the usual theory of mass selection in quantitative genetics. It is well known that the mean of the relative viabilities does not necessarily increase if there is viability selection at more than one locus. It also turns out that if there is selection for fecundity with one locus, the mean fecundity may steadily decrease or oscillate rather than increase. This and the fact that a Hardy-Weinberg structure may no longer exist at any stage of life may have a bearing on predicting progress from artificial selection on reproductive characters. Classical viability selection theory does not completely describe natural selection. Other possibilities are discussed. Among these is the density and frequency dependent selection induced when the population lives in a limited habitat. Implications in quantitative genetics are discussed.

Alleles↗
PubMed1978

On the residual water content of dried but viable cells.

We have examined the residual water in dried cysts of the brine shrimp, Artemia salina, by gas-bombardment techniques at reduced pressures and temperatures. This treatment reduced, but did not remove all the residual water, the lower limit being about 0.0069 gH2O/g dried weight. The significance of such small amounts of water to cell hydration is assessed. The treatment did not appreciably reduce the viability of this cyst population.

Animals↗
PubMed1976

Factors affecting the efficiency of purine analogues as selective agents for mutants of mammalian cells induced by ionising radiation.

In the Chinese hamster cell line V79-4, the frequencies of the cells selected for their resistance to purine analogues do not always reflect the true frequencies of resistant mutants. The frequency of cells resistant to 8-azaguanine varied widely, especially when different sources of serum were used in the selective medium. Even with the more efficient analogue, 6-thioguanine, small colonies arose in the selective medium at a frequency which was strongly dependent upon analogue concentration and viable cell seeding density. These colonies were shown to have a phenotype which was indistinguishable from wild type. Hence with irradiated cells, where the viability of the cell population is reduced to an extent varying with the dose and the interval allowed for mutant expression, the counting of all colonies arising in selective medium can lead to spuriously variable, and sometimes very high, "mutation frequencies". Although the frequency of wild type colonies selected in thioguanine was diminished by the use of high concentrations of the analogue, a loss of induced mutants also occurred at these concentrations. Further, the V79-4 line contained two distinct types of mutant with different levels of hypoxanthine-guanine phosphoribosyl transferase (HGPRT) activity, and only of these types (HGPRT-negative mutants) increased in frequency with radiation dose. These results can account for many of the anomalies encountered in previous studies with purine analogues as selective agents, and show that some care has to be taken to characterize the mutants selected by resistance to purine analogues before meaningful dose-response relationships can be established.

Aneuploidy↗
PubMed1977

Stimulation of thymus- and bone marrow-derived lymphocytes by tumor cells in culture.

In vitro lymphocyte stimulation by mitomycin-blocked tumor cells can be used to measure tumor-specific immune responses. In order to determine the responding cell type(s) in this reaction, lymph node and spleen cell populations were specifically depleted of thymus- or bone marrow-derived cells by the use of the appropriate antisera and complement or by immunoadsorption of the Fc receptor-bearing cells to antibody-coated sheep red blood cell monolayers. The compositions of both the original and the modified lymphocyte populations were determined by (a) viability counting following treatment with antisera and complement, (b) direct and indirect immunofluorescence, (c) antibody-coated erythrocyte rosette formation, and (d) response to thymus- and bone marrow-derived cell mitogens. In the lymph node cell populations, only the thymus-derived cells were stimulated by the tumor cells. However, both bone marrow- and thymus-derived cells from tumor-immune spleens underwent stimulation when exposed to tumor cells in culture.

Animals↗