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Results for “Relaxed purifying selection”

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PubMed2025

Positive selection and relaxed purifying selection contribute to rapid evolution of sex-biased genes in green seaweed Ulva.

BACKGROUND: The evolution of differences in gamete size and number between sexes is a cornerstone of sexual selection theories. The green macroalga Ulva, with incipient anisogamy and parthenogenetic gametes, provides a unique system to investigate theoretical predictions regarding the evolutionary pressures that drive the transition from isogamy to anisogamy, particularly in relation to gamete size differentiation and sexual selection. Its minimal gamete dimorphism and facultative parthenogenesis enable a rare window into early evolutionary steps toward anisogamy. RESULTS: By analyzing the expression profiles of sex-biased genes (SBGs) during gametogenesis, we found that SBGs evolve faster than unbiased genes, driven by higher rates of non-synonymous substitution (dN), indicating that SBGs are under stronger selective pressures. Mating type minus-biased genes (mt-BGs) exhibit higher dN/dS values than mating type plus-biased genes (mt+BGs), suggesting stronger selective pressures on mt-BGs, although this difference was not statistically significant (P = 0.08). Using branch-site and RELAX models, we found positive selection and relaxed purifying selection acting on a significant proportion of SBGs, particularly those associated with flagella function. CONCLUSIONS: This study highlights the selective pressures shaping anisogamy and provides insights into the molecular mechanisms underlying its evolution. The faster evolution of SBGs, particularly mt-BGs, and the positive selection on genes associated with motility, such as those related to flagella function, suggest the importance of enhanced gamete motility in the transition to anisogamy. These findings contribute to our understanding of sexual selection and the evolutionary forces that drive the differentiation of gamete size and number between sexes.

Selection, Genetic↗

PubMed2026

Comparative mitogenomics of Ocnus glacialis reveals lineage-specific evolutionary rates and complex gene rearrangements in Dendrochirotida.

The order Dendrochirotida (Class Holothuroidea) is a species-rich echinoderm group, yet its internal evolutionary history remains poorly resolved due to limited mitogenomic resources. In this study, we characterized the first complete mitochondrial genome of Ocnus glacialis and conducted comparative analyses to elucidate its phylogenetic position and molecular evolutionary patterns. The circular mitogenome of O. glacialis is 16,776 bp in length, containing the canonical set of 37 genes. Among the analyzed dendrochirotids, O. glacialis exhibited the highest A + T content (70.88%) and a near-zero AT-skew, a compositional profile often linked to lineage-specific evolution in specialized environments. Selection pressure analyses, including branch-model tests, revealed that these compositional features are associated with relaxed purifying selection and an accelerated rate of sequence evolution. Branch-site analyses further identified specific codon sites in cytb, nad2, nad4l, nad5, and nad6 under positive or relaxed constraints. Structurally, O. glacialis displayed the most complex gene rearrangement pattern among the studied species, characterized by multiple tandem duplication-random loss (TDRL) events and extensive intergenic sequences. Furthermore, divergence time estimation suggests that these structural and compositional shifts occurred in tandem with the lineage's diversification. We propose that these mitogenomic signatures reflect a synergistic outcome of habitat transition toward Arctic cold-water and deep-sea environments, coupled with demographic factors such as reduced effective population sizes inherent to its benthic life history. By resolving taxonomic uncertainties, this study provides a robust temporal and molecular framework for understanding the evolutionary history and ecological diversification of the Ocnus lineage.

Animals↗

PubMed2026

Transposable Element Dynamics Drive the Genomic Evolution and Phenotypic Diversification of Allotetraploid Common Carp.

An important question in evolutionary biology is how polyploidization generates raw material for phenotypic diversification. Transposable elements (TEs) represent an underestimated source of genetic variation in eukaryotic genomes. By integrating 516 whole-genome resequencing datasets and 236 transcriptomes from common carp (Cyprinus carpio), a representative allotetraploid fish, we constructed the first population-scale landscape of TE insertions in teleosts. TE insertions are widespread in the carp genome and preferentially associated with stress-responsive genes, with DNA transposons as major contributors. Relaxed purifying selection and TE burst events coexist, generating abundant variation for subsequent subspecies differentiation. Compared with a closely related diploid species, carp exhibits more exonic TE insertions and shorter TE-gene distances, and multiple TE superfamilies expanded during tetraploidization. Genome-wide association analyses uncovered intragenic TE variants underlying domesticated traits missed by SNPs, including DNA transposon deletions associated with scale reduction and altered body shape. Notably, lighter-colored individuals harbor homozygous deletions of LTR and DNA transposons within mdfic2, whose knockout in zebrafish reduces pigmentation. Most trait-associated variants reflect lineage-specific loss of ancient TE insertions rather than recent transposition. Overall, these findings highlight the distinct role of TEs in polyploid genome evolution and phenotypic diversification, providing new insights into TE dynamics in vertebrates.

allotetraploidization↗

PubMed2026

A single-nucleus transcriptome atlas of soybean anthers.

Anther development is crucial for plant sexual reproduction. However, a high-resolution, cell-type-specific transcriptomic atlas of this process is lacking for the legume crop soybean (Glycine max). Here, we construct a comprehensive transcriptional atlas of developing soybean anthers using single-nucleus RNA sequencing (snRNA-seq). We identify and characterize nine distinct cell types spanning both somatic and reproductive lineages. Our analysis reveals robust transcriptional continuity across anther developmental stages and dynamic reprogramming during key transitions. Notably, the shift from diploid meiocytes to haploid unicellular microspores is marked by the induction of previously inactive genes, despite an overall reduction in transcript abundance. Subsequently, within bicellular microspores, generative and vegetative cell lineages exhibit sharply divergent transcriptional programs: generative cells specialize in mRNA export and turnover, whereas vegetative cells up-regulate translational machinery. Evolutionary analysis further indicates that generative-cell-specific genes are subject to more relaxed purifying selection compared to those specific to vegetative cells. Functional validation using mutants generated by CRISPR/Cas9-mediated genome editing and EMS mutagenesis reveals the essential roles of OSD1A and PKSA in pollen development and fertility. This high-resolution atlas provides fundamental insights into the transcriptional regulation of soybean anther development and serves as a valuable resource for manipulating male fertility to advance hybrid breeding programs. The data are available at https://databases.genedenovo.com/pollen.

Glycine max↗

PubMed2026

Disruption of mitonuclear coadaptation and compensatory evolution after an extreme dietary shift in carnivorous butterflies.

Mitochondrial function depends on tight coordination between mitochondrial and nuclear genomes, which requires long-term coevolution to maintain mitonuclear coadaptation. While mitonuclear incompatibility is typically studied in the context of hybridization, other evolutionary scenarios that may disrupt coadaptation between the two genomes remain less explored. Here, we propose that extreme ecological niche shifts may disrupt mitonuclear coadaptation, which we test in carnivorous Miletinae butterflies with an extreme dietary transition. By generating high-quality genome assemblies, we found that Miletinae exhibit extensive chromosomal rearrangements. Comparative phylogenomic analyses revealed a striking asymmetric mitonuclear evolutionary response: Miletinae exhibit elevated mitochondrial nucleotide substitution rates compared to phytophagous relatives, whereas nuclear rates remain stable. This shift reverses the typical lepidopteran pattern where nuclear rates exceed mitochondrial rates. Interestingly, this mitochondrial acceleration is driven primarily by relaxed purifying selection rather than positive selection. To sustain mitochondrial function, the nuclear genome of Miletinae underwent pervasive, multilayered compensatory evolution. We detected strong signatures of positive selection and accelerated evolution in nuclear genes directly interacting with mitochondrial components across oxidative phosphorylation (OXPHOS) complexes, the mitochondrial translation, and replication and transcription machinery. Furthermore, this nuclear compensatory response extends to systems governing mitochondrial homeostasis, including protein quality control and RNA degradation and stabilization. Our results support a model in which extreme ecological transitions can disrupt ancestral mitonuclear coadaptation and promote the emergence of a new coadapted state through systemic nuclear compensation. This study broadens the conceptual framework of mitonuclear coevolution and highlights its role in facilitating evolutionary persistence after major ecological shifts.

Animals↗

PubMed2026

Pan-genome characterization of the maize 4CL gene family and its dynamic responses to abiotic stress.

1.Pan-genome analysis across 26 maize inbred lines identified 13 Zm4CL genes (nine core and four near-core) classified into three evolutionary clades.2.Structural variations (SVs) are significantly associated with the expression and altered conserved protein domains of key Zm4CL genes.3.Zm4CL genes exhibit distinct tissue-specific expression patterns and dynamic enzymatic and transcriptional responses to stresses, particularly cold and drought.4-Coumarate:CoA ligase (4CL) is a key enzyme in the phenylpropanoid pathway and plays important roles in plant growth, development, and responses to environmental stresses. However, a comprehensive pan-genome analysis of the 4CL gene family in maize is still lacking. In this study, 13 Zm4CL genes were identified from a maize pan-genome comprising 26 diverse inbred lines, including nine core genes and four near-core genes. Phylogenetic analysis classified these genes into three evolutionary clades, while Ka/Ks analysis indicated that most members have been maintained under purifying selection, although several genes exhibited greater evolutionary divergence and relatively relaxed evolutionary constraints. Structural variation (SV) analysis revealed significant associations between SVs and the expression of Zm4CL2 and Zm4CL3, while sequence comparisons suggested that SVs were also associated with alterations in conserved protein domains in some genotypes. Transcriptome analyses revealed distinct tissue-specific expression patterns and diverse transcriptional responses to abiotic and biotic stresses. Enzyme activity assays showed that cold stress significantly increased 4CL activity at 12 h, whereas heat, salt, and alkali stresses caused an initial decrease followed by recovery, while drought had no significant effect. Time-course RT-qPCR further validated dynamic expression changes of representative Zm4CL genes under cold and drought stresses. Overall, this study provides a comprehensive pan-genome framework for understanding the evolutionary conservation, regulatory diversification, and stress-responsive characteristics of the maize Zm4CL gene family, providing valuable resources for future functional studies and the genetic improvement of stress tolerance in maize.

Zea mays↗

PubMed2026

Comparative analysis of olfactory receptor repertoires reveals evolutionary dynamics and high-altitude adaptation in Schizopygopsis younghusbandi based on the chromosome-level genomes.

The olfactory receptor (OR) gene represent a significant multigene family in vertebrates, forming the core molecular basis of olfactory perception and playing a crucial role in the environmental adaptation of species. High-altitude ecosystems represent extreme habitats characterized by specific abiotic stresses, including low oxygen levels, low temperatures, and intense ultraviolet radiation. These environments also exhibit low aquatic biodiversity and a limited variety of odor molecules, factors that have influenced the adaptive evolution of the sensory systems in endemic species. However, the genetic mechanisms underlying olfactory adaptation in high-altitude freshwater fish remained inadequately understood. In this study, we performed comparative genomics analyses to reveal the evolutionary processes underlying the adaptive and functional evolution of OR genes in S. younghusbandi, a cyprinid fish endemic to the Qinghai-Xizang Plateau. The results indicated that, compared to their low-altitude relatives, S. younghusbandi possessed a significantly smaller number of OR genes, with only 98 genes, which revealed the contraction of the gene family. Phylogenetic analysis revealed that the OR genes of cyprinid fish could be categorized into two major lineages: type I and type II. The η and δ families, which perceive water-soluble odors, in S. younghusbandi underwent significant and specific expansion, while the ε family was completely absent. This pattern reflected adaptive changes in olfactory recognition to accommodate the simplified odor spectrum of high-altitude water bodies. Chromosomal localization analysis demonstrated that OR genes were clustered, and collinearity analysis confirmed the presence of conserved genomic fragments among species. Selection pressure analysis revealed that the Ka/Ks values of all homologous gene pairs were less than 1, indicating that the OR genes of S. younghusbandi underwent strong purifying selection as a group to preserve core olfactory function. A few genes exhibited relaxed selection characteristics, which may have facilitated the fine-tuning of adaptability to high-altitude environments. In conclusion, this study elucidated the evolutionary dynamics and adaptive characteristics of the OR gene in S. younghusbandi, offering a new perspective on the molecular mechanisms underlying olfactory adaptation at high altitudes and enriching the research on sensory evolution in vertebrates.

Schizopygopsis younghusbandi↗

PubMed2026

Evidence of genome-wide relaxed selection on mildly deleterious mutations in an ancient subterranean catfish.

About one hundred subterranean catfish species have been described, resulting from repeated colonization of cave environments by multiple surface lineages. Most cave-dwelling species are found in the Americas, in particular in South America, but a few species also live in Central and North America. Despite the availability of high-quality genome assemblies for two cave species, the Mexican blind catfish Prietella phreatophila and the Colombian blind catfish Trichomycterus rosablanca, genomic approaches to investigate genetic changes associated with subterranean life or to estimate cave colonization times remain largely unexplored. To fill this gap, we additionally sequenced the genomes of four blind and depigmented subterranean catfishes from Peru (three Trichomycterus and one Astroblepus), as well as the genomes of four close surface relatives. We first extracted a large set of light-related genes, such as phototransduction and crystallin genes, and found contrasting decays of these sequences in different cave species, from 1% of pseudogenes in T. rosablanca to 48% in P. phreatophila. Two independent molecular dating methods gave congruent ages, indicating that these catfishes colonized subterranean habitats at different times, ranging from Early Pliocene to Late Pleistocene, supporting the hypothesis that surface catfishes repeatedly and rapidly adapted to subterranean habitats. The oldest cave species, P. phreatophila, appears to have been thriving in the dark for over 3.5 million years. Moreover, a genome-wide analysis of protein-coding genes suggests weaker purifying selection on mildly deleterious mutations in this cavefish than in other catfish lineages, likely reflecting a long-term small effective population size.

cavefishes↗

PubMed2024

Evolutionary consequences of domestication on the selective effects of new amino acid changing mutations in canids.

The domestication of wild canids led to dogs no longer living in the wild but instead residing alongside humans. Extreme changes in behavior and diet associated with domestication may have led to the relaxation of the selective pressure on traits that may be less important in the domesticated context. Thus, here we hypothesize that strongly deleterious mutations may have become less deleterious in domesticated populations. We test this hypothesis by estimating the distribution of fitness effects (DFE) for new amino acid changing mutations using whole-genome sequence data from 24 gray wolves and 61 breed dogs. We find that the DFE is strikingly similar across canids, with 26-28% of new amino acid changing mutations being neutral/nearly neutral (|s| < 1e-5), and 41-48% under strong purifying selection (|s| > 1e-2). Our results are robust to different model assumptions suggesting that the DFE is stable across short evolutionary timescales, even in the face of putative drastic changes in the selective pressure caused by artificial selection during domestication and breed formation. On par with previous works describing DFE evolution, our data indicate that the DFE of amino acid changing mutations depends more strongly on genome structure and organismal characteristics, and less so on shifting selective pressures or environmental factors. Given the constant DFE and previous data showing that genetic variants that differentiate wolf and dog populations are enriched in regulatory elements, we speculate that domestication may have had a larger impact on regulatory variation than on amino acid changing mutations.

Journal Article↗