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11 recordsLinked to original sources
PubMed2026

Genomic and evolutionary basis of parthenogenesis in a disease-vector tick species.

Haemaphysalis longicornis is an important tick species and pathogen vector characterized by the co-circulation of triploid parthenogenetic and diploid bisexual strains. However, the evolutionary basis of parthenogenesis in this species is unclear. Here we report reference-quality, haplotype-resolved genome assemblies of the parthenogenetic strain and two reference-quality genomes of the bisexual strains. Comparative genomic analysis revealed high collinearity between the parthenogenetic and bisexual genomes, with a stable chromosomal architecture maintained among the three haplotypes of the parthenogenetic strain. The parthenogenetic H. longicornis genome exhibited a major expansion in cell cycle-related gene families, including the inhibitor of apoptosis protein (IAP) family, but was characterized by a contraction in other gene families. Population resequencing of 179 individuals revealed two distinct subpopulations, with chromosome 7 harbouring high genetic differentiation and several candidate genes probably associated with parthenogenesis. Functional experiments showed that knockdown of the BIRC5 gene, a member of the IAP family, suppressed oviposition in both strains, with the parthenogenetic strain exhibiting milder adverse effects probably due to a stronger transcriptional response. Overall, our results reveal the genomic and evolutionary features associated with polyploid parthenogenesis in H. longicornis.

Animals↗
PubMed2026

The twofold cost of sex reconsidered: meiotic mechanisms protect anisogamous populations from invasion by thelytoky.

Most multicellular animals practice anisogamy (fertilization between eggs and sperm). When mothers produce sons and daughters at a 1:1 ratio, the "twofold cost of males" arises because males do not directly contribute to population growth. If thelytokous parthenogens producing only daughters invade a population, they should spread rapidly. Although thelytoky has repeatedly evolved across invertebrate and vertebrate taxa, it remains a minority. Why? The evolutionary transition from anisogamy to thelytoky requires eggs to initiate embryonic development without fertilization. However, in metazoan animals, meiotic metaphase (MM) arrest halts oogenesis midway and normally resumes only after stimulation by sperm penetration. Empirical and experimental evidences indicate that release of MM arrest without fertilization is extremely difficult, providing a strong mechanistic barrier against parthenogenesis. Even if MM arrest were released, oogenesis would proceed to produce either a haploid embryo or a diploid embryo through refusion with the second polar body (terminal fusion automixis). Outbred species typically accumulate more than one lethal equivalent of recessive deleterious alleles per genome as heterozygotes. Upon transition to haploid or automictic development, these recessive lethals normally masked in outbred diploids would be exposed simultaneously, causing embryonic death and creating the next barrier. Thus, thelytoky cannot be achieved simply by modification of the existing meiotic system; instead, other mechanisms, such as apomixis, that bypass meiosis are required. Mathematical models and simulations support this "meiotic constraint" hypothesis. Combined with recently proposed immediate benefits of anisogamy and traditional genetic benefits (e.g., Red Queen), it may largely explain the maintenance of costly anisogamy.

Animals↗
PubMed2025

The ecology, evolution, and physiology of Cardinium: a widespread heritable endosymbiont of invertebrates.

Candidatus Cardinium hertigii (Cardinium) are maternally transmitted obligate intracellular bacteria found in a wide range of invertebrate hosts, including arthropods and nematodes. Infection with Cardinium has substantial consequences for host biology, with many strains manipulating host reproduction to favor symbiont transmission by (i) feminizing male hosts, (ii) altering host sex allocation, (iii) inducing parthenogenesis, or (iv) causing cytoplasmic incompatibility. Other Cardinium strains can confer benefits to their host or alter host behavior. Cardinium-modified host phenotypes can result in selective sweeps of cytological elements through host populations and potentially reinforce host speciation. Cardinium has potential for applications in controlling arthropod pest species and arthropod-vectored disease transmission, although much remains to be explored regarding Cardinium physiology and host interactions. In this review, we provide an overview of Cardinium evolution and host distribution. We describe the various host phenotypes associated with Cardinium and how biological and environmental factors influence these symbioses. We also provide an overview of Cardinium metabolism, physiology, and potential mechanisms for interactions with hosts based on recent studies using genomics and transcriptomics. Finally, we discuss new methodologies and directions for Cardinium research, including improving our understanding of Cardinium physiology, response to environmental stress, and potential for controlling arthropod pest populations.

Symbiosis↗
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

Mutation accumulation in a hybrid parthenogenetic vertebrate.

Asexual lineages are thought to experience elevated extinction rates compared with sexual species, yet direct evidence for the underlying genetic causes remains scarce. Muller's ratchet predicts that the absence of recombination in asexual organisms facilitates the accumulation of deleterious mutations, thereby reducing long-term fitness. Here, we test this hypothesis in the hybrid-origin, parthenogenetic whiptail lizard Aspidoscelis tesselatus by integrating short-read RNAseq and long-read IsoSeq data from both the asexual lineage and its parental sexual species. We reconstructed phased transcripts for A. tesselatus to quantify mutation accumulation relative to the parental sexual species. Comparative analyses revealed elevated ω ratios in both parental genomic complements (subgenomes) of the parthenogenetic lineage, consistent with accelerated accumulation of nonsynonymous mutations. Structural variant analyses identified multiple indels in expressed transcripts predicted to disrupt protein domains. Functional annotation indicated that genes affected by both single-nucleotide variants and indels were enriched for roles in chromatin organization, apoptosis regulation, and transcriptional control. While both parental subgenomes showed similar evolutionary patterns, the maternal complement exhibited more structural and missense mutations than the paternal complement. Together, these results provide evidence that mutations accumulate in asexual A. tesselatus in genes involved in core cellular functions, supporting theoretical predictions that Muller's ratchet contributes to mutation accumulation in asexual lineages.

Animals↗
PubMed2025

The release of sexual conflict after sex loss is associated with evolutionary changes in gene expression.

Sexual conflict can arise because males and females, while sharing most of their genome, can have different phenotypic optima. Sexually dimorphic gene expression may help reduce conflict, but the expression of many genes may remain sub-optimal owing to unresolved tensions between the sexes. Asexual lineages lack such conflict, making them relevant models for understanding the extent to which sexual conflict influences gene expression. We investigate the evolution of sexual conflict subsequent to sex loss by contrasting the gene expression patterns of sexual and asexual lineages in the pea aphid Acyrthosiphon pisum. Although asexual lineages of this aphid produce a small number of males in autumn, their mating opportunities are limited because of geographic isolation between sexual and asexual lineages. Therefore, gene expression in parthenogenetic females of asexual lineages is no longer constrained by that of other morphs. We found that the expression of genes in males from asexual lineages tended towards the parthenogenetic female optimum, in agreement with theoretical predictions. Surprisingly, males and parthenogenetic females of asexual lineages overexpressed genes normally found in the ovaries and testes of sexual morphs. These changes in gene expression in asexual lineages may arise from the relaxation of selection or the dysregulation of gene networks otherwise used in sexual lineages.

Animals↗
PubMed2026

Chromosome-Level Reference Genome of the Desert Night Lizard Xantusia vigilis.

We present a reference-quality genome assembly for the desert night lizard (Xantusia vigilis). The night lizards (Xantusiidae) are a family of small-bodied lizards found in North America (Xantusia), Central America (Lepidophyma), and Cuba (Cricosaura). The night lizard family has an independent evolutionary history of at least 80 million years from its sister taxa within Scincoidea. The Xantusiids have several unique ecological, behavioral and evolutionary characteristics. For instance, the family contains the only squamate species that form diploid, unisexual, parthenogenic lineages. In addition, most night lizards are viviparous and form stable kin groups that are maintained over multiple years, an unusual life history strategy among lizards. Combining PacBio long-read sequencing, Hi-C, and RNAseq data we developed a reference-quality genome for the desert night lizard, X. vigilis. We assembled a complete mitochondrion and ~ 2.2 Gb nuclear genome, with 20 scaffolds that correlate in size to the X. vigilis karyotype. In addition, we found that X. vigilis chromosome 1 aligns with gene content of both of macrochromosome 1 and microchromosome 9 from a genome assembly of a species in the sister family Cordylidae (Hemicordylus capensis).

Xantusia↗