Chromosome Evolution Research Articles

Delineating the W sex chromosome in the clam shrimp, Eulimnadia texana

Introduction: Sex chromosomes have evolved independently across various lineages, often showing convergent degradation of the sex-limited chromosome. While extensively studied in model organisms with ancient sex chromosomal systems, the evolution of early-stage sex chromosomes remains poorly understood. Eulimnadia texana, a freshwater crustacean with a unique androdioecious breeding system (ZZ, ZW, and viable WW genotypes), provides a rare opportunity to study early sex chromosome evolution. This study examines E. texana's W chromosome for evidence of a small localized non-recombining region, characterized by a transposable element (TE) "hotspot", low gene density, and low GC content. Methods: Sex-linked markers were mapped onto the W chromosome (Scaffold 1). TEs in the WW genome were identified using RepeatModeler and RepeatMasker. Statistical analyses compared TE distribution between the genome and Scaffold 1, which was then divided into 20 equal-sized "bins" for finer-scale statistical analyses. Gene density and GC content were analyzed across these bins. Results: While no significant TE accumulation was found across the entire W chromosome compared to the remaining genome, a specific region (6.6-8.8 Mb, fourth bin) showed significantly higher TE accumulation. This region also exhibited low gene density and low GC content, indicative of reduced recombination. Conclusion: Our findings suggest that E. texana's W chromosome contains a smaller region of crossover suppression, supporting the hypothesis that it is in a proto-sex chromosome in early evolutionary development. This study provides valuable insights into early sex chromosome evolution and establishes E. texana as an ideal model for further investigation of evolutionary processes driving proto-sex chromosome differentiation.

The burst of satellite DNA in Leptidea wood white butterflies and their putative role in karyotype evolution.

Satellite DNAs (satDNAs) are abundant components of eukaryotic genomes, playing pivotal roles in chromosomal organization, genome stability and evolution. Here, we combined cytogenetic and genomic methods to characterize the satDNAs in the genomes of Leptidea butterflies. Leptidea is characterized by the presence of a high heterochromatin content, large genomes and extensive chromosomal reshuffling as well as the occurrence of cryptic species. We show that, in contrast to other Lepidoptera, satDNAs constitute a considerable proportion of Leptidea genomes, ranging between 4.11%-11.05%. This amplification of satDNAs, together with the hyperactivity of transposable elements, contributes to the substantial genome expansion in Leptidea. Using chromosomal mapping, we show that, particularly LepSat01-100 and LepSat03-167 satDNAs, are preferentially localized in heterochromatin exhibiting variable distribution that could influence in the highly diverse karyotypes within the genus. The satDNAs also exhibit W-chromosome accumulation, suggesting their involvement in sex chromosome evolution. Our results provide insights into the dynamics of satDNAs in Lepidoptera genomes and highlight their role in genome expansion and chromosomal organization, which could influence the speciation process. The high proportion of repetitive DNAs in the genomes of Leptidea underscores the complex evolutionary dynamics revealing the interplay between repetitive DNAs and genomic architecture in the genus.

Whole-chromosome oligo-painting in licorice unveils interspecific chromosomal evolutionary relationships and possible origin of triploid genome species.

Licorice is one of the most extensively studied medicinal plants in the world, whose roots and rhizomes have long been used as both a sweetener and an essential component in numerous herbal preparations. However, the genus Glycyrrhiza has a complex composition, and the interspecies chromosomal relationships, origin, and evolution are still largely unclear. Here, we develop a set of whole-chromosome painting probes that allowed identification of all eight chromosomes of licorice on same metaphase chromosomes. Comparative chromosome painting analyses in seven different Glycyrrhiza species revealed that the genus Glycyrrhiza maintained extraordinarily conserved chromosomal synteny after about 3-12 million years of divergence. No cytologically visible inter-chromosomal rearrangements were identified in any species. By comparative chromosomal karyotype analyses, we revealed interspecific chromosome evolutionary relationships and dramatic variable chromosomal karyotype after independent divergence and demonstrated that G. prostrate was the most closely related to the ancestral type among the seven Glycyrrhiza species. Furthermore, we also discovered a G. glandulosa seed with distinct triploid-genome for the first time in China, suggesting the existence of a polyploid evolutionary pathway in the genus Glycyrrhiza, which challenges the previous notion that only diploids of licorice existed in nature. This study expands our knowledge of the chromosome evolution of licorice and will lay an important foundation for the genome origin and evolution studies in the genus Glycyrrhiza.

Expanding the Triangle of U: Comparative analysis of the Hirschfeldia incana genome provides insights into chromosomal evolution, phylogenomics and high photosynthesis-related traits.

The Brassiceae tribe encompasses many economically important crops and exhibits high intraspecific and interspecific phenotypic variation. After a shared whole-genome triplication (WGT) event (Br-α, ~15.9 million years ago), differential lineage diversification and genomic changes contributed to an array of divergence in morphology, biochemistry, and physiology underlying photosynthesis-related traits. Here, the C3 species Hirschfeldia incana is studied as it displays high photosynthetic rates under high-light conditions. Our aim was to elucidate the evolution that gave rise to the genome of H. incana and its high-photosynthesis traits. We reconstructed a chromosome-level genome assembly for H. incana (Nijmegen, v2.0) using nanopore and chromosome conformation capture (Hi-C) technologies, with 409Mb in size and an N50 of 52Mb (a 10× improvement over the previously published scaffold-level v1.0 assembly). The updated assembly and annotation was subsequently employed to investigate the WGT history of H. incana in a comparative phylogenomic framework from the Brassiceae ancestral genomic blocks and related diploidized crops. Hirschfeldia incana (x=7) shares extensive genome collinearity with Raphanus sativus (x=9). These two species share some commonalities with Brassica rapa and B. oleracea (A genome, x=10 and C genome, x=9, respectively) and other similarities with B. nigra (B genome, x=8). Phylogenetic analysis revealed that H. incana and R. sativus form a monophyletic clade in between the Brassica A/C and B genomes. We postulate that H. incana and R. sativus genomes are results of hybridization or introgression of the Brassica A/C and B genome types. Our results might explain the discrepancy observed in published studies regarding phylogenetic placement of H. incana and R. sativus in relation to the "Triangle of U" species. Expression analysis of WGT retained gene copies revealed sub-genome expression divergence, likely due to neo- or sub-functionalization. Finally, we highlighted genes associated with physio-biochemical-anatomical adaptive changes observed in H. incana which likely facilitate its high-photosynthesis traits under high light. The improved H. incana genome assembly, annotation and results presented in this work will be a valuable resource for future research to unravel the genetic basis of its ability to maintain a high photosynthetic efficiency in high-light conditions and thereby improve photosynthesis for enhanced agricultural production.

High prevalence of chromosome 17 in breast cancer micronuclei: a means to get rid of tumor suppressors?

Micronuclei (MN), defined as small extra-nuclear chromatin bodies enclosed by a nuclear envelope, serve as noticeable markers of chromosomal instability (CIN). The MN have been used for breast cancer (BC) screening, diagnosis, and prognosis. However, more recently they have gained attention as seats for active chromosomal rearrangements. BC subtypes exhibit differential CIN levels and aggressiveness. This study aimed to investigate MN chromosomal contents across BC subtypes, exploring its potential role in aggressiveness and pathogenesis. Immunostaining of BC cells was performed with anti-centromeric antibody followed by confocal microscopy. Further, fluorescence in situ hybridization (FISH) was done to check the presence of specific chromosomes in the MN. The real time PCR was also done from the RNA isolated from MN to check the expression of TP53 gene. BC cell lines (CLs) showed the presence of both centromere-positive ( +) and -negative ( -) MN, with significant variation in frequency among hormone and human epidermal growth factor receptor positive and triple-negative (TN) BC cells. FISH targeting chromosomes 1, 3, 8, 11, and 17 detected centromeric signals for all the above chromosomes in MN with a relatively higher prevalence of chromosome 17 in all the CLs. Out of all the CLs, TNBC cells demonstrated the highest frequency of centromere + and chromosome 17 + MN. TP53 expression could also be demonstrated inside the MN by FISH and real time PCR. Patient sample imprints also confirmed the presence of chromosome 17 in MN with polysomy of the same in corresponding nuclei. The high prevalence of chromosome 17 in BC MN may connote the importance of its rearrangements in the pathogenesis of BC. Further, the higher prevalence of chromosome 17 and 1 signals in TNBC MN point towards the significance of pathogenetic events involving the genes located in these chromosomes in evolution of this more aggressive phenotype.

Sex chromosome evolution: hallmarks and question marks.

Sex chromosomes are widespread in species with separate sexes. They have evolved many times independently, and display a truly remarkable diversity. New sequencing technologies and methodological developments have allowed the field of molecular evolution to explore this diversity in a large number of model and non-model organisms, broadening our vision on the mechanisms involved in their evolution. Diverse studies have allowed us to better capture the common evolutionary routes that shape sex chromosomes, however we still mostly fail to explain why sex chromosomes are so diverse. We review over half a century of theoretical and empirical work on sex chromosome evolution and highlight pending questions on their origins, turnovers, rearrangements, degeneration, dosage compensation, gene content, and rates of evolution. We also report recent theoretical progress on our understanding of the ultimate reasons for sex chromosomes' existence.

Comparative analysis of chromosome-level genomes provides insights into chromosomal evolution in Chiroptera.

Chiroptera (bats) presents a fascinating model due to its remarkable variation in chromosome numbers, which range from 14 to 62. This astonishing diversity makes bats an excellent subject for studying chromosome evolution. The black-bearded tomb bat (Taphozous melanopogon) occupies a pivotal phylogenetic position within Chiroptera, emphasizing its crucial role in the systematic examination of bat chromosome evolution. In this study, we present the first chromosome-level genome of T. melanopogon within the family Emballonuridae. Together with previously published genomes, we construct a strongly supported phylogenetic tree of bats, which supports that Emballonuridae forms a basal group within Yangochiroptera. Furthermore, we reconstruct ancestral karyotypes at key nodes along the bat phylogeny and conduct a synteny analysis among the genomes of 12 bat species. Our findings identified evolutionary breakpoint regions (EBRs) that are of particular interest. Notably, some bat genomes exhibit an enrichment of genes related to host defense against microbial pathogens within EBRs. Remarkably, one species possesses multiple copies of some β-defensin genes, while six other species have experienced the loss of some β-defensin genes due to EBRs. Furthermore, some olfactory receptor genes are located in EBRs of 12 species, 4 of which have a significant enrichment in sensory perception of smell. Together, our comparative genomic analysis underscores the potential link between chromosome rearrangements and the adaptation of bats to defend against microbial pathogens.

Frequencies of house fly proto-Y chromosomes across populations are predicted by temperature heterogeneity within populations.

Sex chromosomes often differ between closely related species and can even be polymorphic within populations. Species with multifactorial sex determination segregate for multiple different sex determining loci within populations, making them uniquely informative of the selection pressures that drive the evolution of sex chromosomes. The house fly (Musca domestica) is a model species for studying multifactorial sex determination because male determining genes have been identified on all six of the chromosomes, which means that any chromosome can be a "proto-Y". Natural populations of house fly also segregate for a recently derived female-determining locus, meaning house flies also have a proto-W chromosome. The different proto-Y chromosomes are distributed along latitudinal clines on multiple continents, their distributions can be explained by seasonality in temperature, and they have temperature-dependent effects on physiological and behavioral traits. It is not clear, however, how the clinal distributions interact with the effect of seasonality on the frequencies of house fly proto-Y and proto-W chromosomes across populations. To address this question, we measured the frequencies of house fly proto-Y and proto-W chromosomes across nine populations in the United States of America. We confirmed the clinal distribution along the eastern coast of North America, but it is limited to the eastern coast. In contrast, annual mean daily temperature range predicts proto-Y chromosome frequencies across the entire continent. Our results therefore suggest that temperature heterogeneity can explain the distributions of house fly proto-Y chromosomes in a way that does not depend on the cline.

Long-term hematopoietic dysfunction in patients with large-scale mitochondrial DNA deletion syndromes.

Pearson syndrome (PS) and Kearns-Sayre syndrome (KSS) are single large-scale mitochondrial DNA deletion (SLSMD) syndromes. PS is characterized by severe, transient childhood cytopenia, whereas KSS typically manifests later in life without hematologic abnormalities. Despite distinct clinical presentations, both share a common mitochondrial DNA deletion. Recent observations suggest a potential link between PS progression and myeloid malignancy development, indicating that bone marrow failure (BMF) may be a key aspect of PS pathology and potentially universal across SLSMDs. This study explores longitudinal hematological manifestations of SLSMD syndromes, focusing on bone marrow (BM) dysfunction. Sixteen patients with SLSMDs (13 PS and 3 KSS) were followed, of whom 75% experienced cytopenia, necessitating blood transfusions in 56%. Despite achieving transfusion independence at a median age of 24 months, persistent hematological abnormalities were noted. Comprehensive longitudinal BM studies were conducted in 62% of subjects and consistently revealed signs of marrow dysfunction, even without concurrent cytopenia. Median BM cellularity at a median age of four years and eight months was 50%, with histological signs of dyserythropoiesis, abnormal megakaryocytes, and signs suggesting myelodysplasia. Reduced CD34+ counts and BM colony-forming unit capacity were noted, alongside chromosome 7 aberrations in 16% of patients on cytogenetic studies. Our findings establish BM dysfunction as a persistent hallmark of SLSMD syndromes, posing a risk of clonal evolution and acquisition of chromosome 7 aberrations. This aligns with recent literature, emphasizing enduring BMF in SLSMD syndromes and advocating for tailored hematological monitoring guidelines for this unique patient cohort.

Genome-Wide Association Studies (GWAS) and Transcriptome Analysis Reveal Male Heterogametic Sex-Determining Regions and Candidate Genes in Northern Snakeheads (Channa argus).

The Northern snakehead (Channa argus) is a significant economic aquaculture species in China. Exhibiting sexual dimorphism in the growth rate between females and males, mono-sex breeding holds substantial value for aquaculture. This study employed GWAS and transcriptome analysis were applied to identify sex determination genomic regions and develop sex-specific markers. A total of 270 single-nucleotide polymorphisms (SNPs) and 31 insertion-deletions (InDels) were identified as being sexually dimorphic through GWAS and fixation index (Fst) scanning. Based on GWAS results, two sex-specific InDel markers were developed, effectively distinguishing genetic sex for XX females, XY males, and YY super-males via (polymerase chain reaction) PCR amplification. A major genomic segment of approximately 115 kb on chromosome 3 (Chr 03) was identified as the sex-determination region. A comparative transcriptome analysis of gonads for three sexes identified 158 overlapping differentially expressed genes (DEGs). Additionally, three sex-related candidate genes were identified near the sex determination region, including id2, sox11, and rnf144a. Further studies are required to elucidate the functions of these genes. Overall, two sex-specific InDel markers support a male heterogametic XX/XY sex-determination system in Northern snakeheads and three candidate genes offer new insights into sex determination and the evolution of sex chromosomes in teleost fish.

Three genome assemblies of Opsariichthys bidens from Yangzte River, Pearl River and Qiantang River basins

Opsariichthys bidens is an endemic minnow, mainly distributed in China as an emerging aquaculture species. In this study, O. bidens collected from Yangzte River (YR), Pearl River (PR) and Qiantang River (QR) basins were used for karyotypic study and genome sequencing. Using PacBio long reads and Hi-C data, we assembled high-quality O. bidens genomes of 852.41 Mb (YR), 843.11 Mb (PR) and 840.94 Mb (QR) with scaffold N50 lengths of 21.01 Mb, 23.62 Mb and 24.75 Mb, respectively, of which 90.39%, 95.67% and 99.01% were anchored to 37, 38 and 39 chromosomes, respectively. 26,556 (YR), 25,036 (PR) and 26,283 (QR) protein-coding genes were predicted, respectively. The karyotype of O. bidens from YR, PR and QR were 2 N = 74 (6 m + 6sm + 4st + 58t), 2 N = 76 (4 m + 6sm + 4st + 62t) and 2 N = 78 (4 m + 4sm + 4st + 66t), respectively. Collinearity analysis and telomere predictions indicated that the observed chromosomal evolution was driven by Robertsonian translocation. These genome assemblies facilitate cryptic species determination, evolutionary study and genetic breeding of genus Opsariichthys.

Chromosome-scale genome of the polyphagous pest Anastrepha ludens (Diptera: Tephritidae) provides insights on sex chromosome evolution in Anastrepha.

The Mexican fruit fly, Anastrepha ludens, is a polyphagous true fruit fly (Diptera: Tephritidae) considered one of the most serious insect pests in Central and North America to various economically relevant fruits. Despite its agricultural relevance, a high-quality genome assembly has not been reported. Here, we described the generation of a chromosome-level genome for the A. ludens using a combination of PacBio high fidelity long-reads and chromatin conformation capture sequencing data. The final assembly consisted of 140 scaffolds (821 Mb, N50 = 131 Mb), containing 99.27% complete conserved orthologs (BUSCO) for Diptera. We identified the sex chromosomes using three strategies: 1) visual inspection of Hi-C contact map and coverage analysis using the HiFi reads, 2) synteny with Drosophila melanogaster, and 3) the difference in the average read depth of autosomal versus sex chromosomal scaffolds. The X chromosome was found in one major scaffold (100 Mb) and eight smaller contigs (1.8 Mb), and the Y chromosome was recovered in one large scaffold (6.1 Mb) and 35 smaller contigs (4.3 Mb). Sex chromosomes and autosomes showed considerable differences of transposable elements and gene content. Moreover, evolutionary rates of orthologs of A. ludens and Anastrepha obliqua revealed a faster evolution of X-linked, compared to autosome-linked, genes, consistent with the faster-X effect, leading us to new insights on the evolution of sex chromosomes in this diverse group of flies. This genome assembly provides a valuable resource for future evolutionary, genetic, and genomic translational research supporting the management of this important agricultural pest.

Cancer Genomes Sometimes Take the Longest Evolutionary Road.

Baker and colleagues developed a new algorithm called "Gain Route Identification and Timing In Cancer" (GRITIC) to uncover the path of chromosomal evolution in a tumor, particularly in the context of whole-genome duplication. Their approach found that tumors with genome doubling frequently take an indirect path from one copy number state to another. In addition, the timing of genome doubling within a tumor's evolution impacts its consequences on downstream chromosomal instability. See related article by Baker et al., p. 1810.

Limited population structure but signals of recent selection in introduced African Fig Fly (Zaprionus indianus) in North America.

Invasive species have devastating consequences for human health, food security, and the environment. Many invasive species adapt to new ecological niches following invasion, but little is known about the early steps of adaptation. Here we examine population genomics of a recently introduced drosophilid in North America, the African Fig Fly, Zaprionus indianus. This species is likely intolerant of subfreezing temperatures and recolonizes temperate environments yearly. We generated a new chromosome-level genome assembly for Z. indianus. Using resequencing of over 200 North American individuals collected over four years in temperate Virginia, plus a single collection from subtropical Florida, we tested for signatures of recolonization, population structure, and adaptation within invasive populations. We show founding populations are sometimes small and contain close genetic relatives, yet temporal population structure and differentiation of populations is mostly absent across recurrent recolonization events. Although we find limited signals of genome-wide spatial or temporal population structure, we identify haplotypes on the X chromosome that are repeatedly differentiated between Virginia and Florida populations. These haplotypes show signatures of natural selection and are not found in African populations. We also find evidence for several large structural polymorphisms segregating within North America populations and show X chromosome evolution in invasive populations is strikingly different from the autosomes. These results show that despite limited population structure, populations may rapidly evolve genetic differences early in an invasion. Further uncovering how these genomic regions influence invasive potential and success in new environments will advance our understanding of how organisms evolve in changing environments.

Bursts of Rapid Diversification, Dispersals Out of Southern Africa, and Two Origins of Dioecy Punctuate the Evolution of Asparagus.

The genus Asparagus arose ∼9 to 15 million years ago (Ma), and transitions from hermaphroditism to dioecy (separate sexes) occurred ∼3 to 4 Ma. Roughly 27% of extant Asparagus species are dioecious, while the remaining are bisexual with monoclinous flowers. As such, Asparagus is an ideal model taxon for studying the early stages of dioecy and sex chromosome evolution in plants. Until now, however, understanding of diversification and shifts from hermaphroditism to dioecy in Asparagus has been hampered by the lack of robust species tree estimates for the genus. In this study, a genus-wide phylogenomic analysis including 1,726 nuclear loci and comprehensive species sampling supports two independent origins of dioecy in Asparagus-first in a widely distributed Eurasian clade and then in a clade restricted to the Mediterranean Basin. Modeling of ancestral biogeography indicates that both dioecy origins were associated with range expansion out of southern Africa. Our findings also reveal several bursts of diversification across the phylogeny, including an initial radiation in southern Africa that gave rise to 12 major clades in the genus, and more recent radiations that have resulted in paraphyly and polyphyly among closely related species, as expected given active speciation processes. Lastly, we report that the geographic origin of domesticated garden asparagus (Asparagus officinalis L.) was likely in western Asia near the Mediterranean Sea. The presented phylogenomic framework for Asparagus is foundational for ongoing genomic investigations of diversification and functional trait evolution in the genus and contributes to its utility for understanding the origin and early evolution of dioecy and sex chromosomes.

Chromosome-level genome assembly of the morabine grasshopper Vandiemenella viatica19

Morabine grasshoppers in the Vandiemenella viatica species group, which show karyotype diversity, have been studied for their ecological distribution and speciation in relation to their genetic and chromosomal diversity. They are good models for studying sex chromosome evolution as “old” and newly emerged sex chromosomes co-exist within the group. Here we present a reference genome for the viatica19 chromosomal race, that possesses the ancestral karyotype within the group. Using PacBio HiFi and Hi-C sequencing, we generated a chromosome-level assembly of 4.09 Gb in span, scaffold N50 of 429 Mb, and complete BUSCO score of 98.1%, containing 10 pseudo-chromosomes. We provide Illumina datasets of males and females, used to identify the X chromosome. The assembly contains 19,034 predicted protein-coding genes, and a total of 75.21% of repetitive DNA sequences. By leveraging HiFi reads, we mapped the genome-wide distribution of methylated bases (5mC and 6 mA). This comprehensive assembly offers a robust reference for morabine grasshoppers and supports further research into speciation and sex chromosome diversification within the group and its related species.

Two near-chromosomal-level genomes of globally-distributed Macroascomycete based on single-molecule fluorescence and Hi-C methods

Discinaceae holds significant importance within the Pezizales, representing a prominent group of macroascomycetes distributed globally. However, there is a dearth of genomic studies focusing on this family, resulting in gaps in our understanding of its evolution, development, and ecology. Here we utilized state-of-the-art genome assembly methodologies, incorporating third-generation single-molecule fluorescence and Hi-C-assisted methods, to elucidate the genomic landscapes of Gyromitra esculenta and Paragyromitra xinjiangensis. The genome sizes of two species were determined to be 47.10 Mb and 48.20 Mb, with 23 and 22 scaffolds, respectively. 10,438 and 11,469 coding proteins were identified, with functional annotations encompassing over 96.47% and 94.40%, respectively. Assessment of completeness using BUSCO revealed that 98.71% and 98.89% of the conserved proteins were identified. The application of comparative genomic technology has helped in identifying traits associated with of heterothallic life cycle traits and elucidating unique patterns of chromosomal evolution. Additionally, we identified potential saprotrophic nutritional modes and systematic phylogenetic relationships between the two species. Therefore, this study provides crucial genomic insights into the evolution, nutritional type, and ecological roles of species within the Pezizales.

Satellitome analysis on the pale-breasted thrush Turdus leucomelas (Passeriformes; Turdidae) uncovers the putative co-evolution of sex chromosomes and satellite DNAs

Do all birds' sex chromosomes follow the same canonical one-way direction of evolution? We combined cytogenetic and genomic approaches to analyze the process of the W chromosomal differentiation in two selected Passeriform species, named the Pale-breasted Thrush Turdus leucomelas and the Rufous-bellied thrush T. rufiventris. We characterized the full catalog of satellite DNAs (satellitome) of T. leucomelas, and the 10 TleSatDNA classes obtained together with 16 microsatellite motifs were in situ mapped in both species. Additionally, using Comparative Genomic Hybridization (CGH) assays, we investigated their intragenomic variations. The W chromosomes of both species did not accumulate higher amounts of both heterochromatin and repetitive sequences. However, while T. leucomelas showed a heterochromatin-poor W chromosome with a very complex evolutionary history, T. rufiventris showed a small and partially heterochromatic W chromosome that represents a differentiated version of its original autosomal complement (Z chromosome). The combined approach of CGH and sequential satDNA mapping suggest the occurrence of a former W-autosomal translocation event in T. leucomelas, which had an impact on the W chromosome in terms of sequence gains and losses. At the same time, an autosome, which is present in both males and females in a polymorphic state, lost sequences and integrated previously W-specific ones. This putative W-autosomal translocation, however, did not result in the emergence of a multiple-sex chromosome system. Instead, the generation of a neo-W chromosome suggests an unexpected evolutionary trajectory that deviates from the standard canonical model of sex chromosome evolution.

Centromere drive may propel the evolution of chromosome and genome size in plants.

Genome size is influenced by natural selection and genetic drift acting on variations from polyploidy and repetitive DNA sequences. We hypothesized that centromere drive, where centromeres compete for inclusion in the functional gamete during meiosis, may also affect genome and chromosome size. This competition occurs in asymmetric meiosis, where only one of the four meiotic products becomes a gamete. If centromere drive influences chromosome size evolution, it may also impact post-polyploid diploidization, where a polyploid genome is restructured to function more like a diploid through chromosomal rearrangements, including fusions. We tested if plant lineages with asymmetric meiosis exhibit faster chromosome size evolution compared to those with only symmetric meiosis, which lack centromere drive as all four meiotic products become gametes. We also examined if positive selection on centromeric histone H3 (CENH3), a protein that can suppress centromere drive, is more frequent in these asymmetric lineages. We analyzed plant groups with different meiotic modes: asymmetric in gymnosperms and angiosperms, and symmetric in bryophytes, lycophytes, and ferns. We selected species based on available CENH3 gene sequences and chromosome size data. Using Ornstein-Uhlenbeck evolutionary models and phylogenetic regressions, we assessed the rates of chromosome size evolution and the frequency of positive selection on CENH3 in these clades. Our analyses showed that clades with asymmetric meiosis have a higher frequency of positive selection on CENH3 and increased rates of chromosome size evolution compared to symmetric clades. Our findings support the hypothesis that centromere drive accelerates chromosome and genome size evolution, potenatially also influencing the process of post-polyploid diploidization. We propose a model which in a single famework helps explain the stability of chromosome size in symmetric lineages (bryophytes, lycophytes, and ferns) and its variability in asymmetric lineages (gymnosperms and angiosperms), providing a foundation for future research in plant genome evolution.

Development and validation of sex-linked molecular markers for rapid and accurate identification of male and female Hippophae tibetana plants

Hippophae tibetana, one of the highest-altitude woody plants endemic to the Qinghai-Tibet Plateau, primarily thrives on riverbanks formed by glacial meltwater. As a dioecious species, it demonstrates significant ecological and economic value in extreme alpine environments. However, the lack of sex identification techniques outside of the flowering period severely limits research on sex ratio, differentiation, and breeding. There is an urgent need to develop effective sex-linked molecular markers that are independent of developmental stages, but current research in this area remains limited. This study developed a set of accurate sex-linked molecular markers for the rapid identification of male and female individuals of H. tibetana. Through whole-genome resequencing of 32 sexually differentiated H. tibetana samples, this study offers strong evidence supporting chromosome 2 as the sex chromosome and successfully identified key loci related to sex determination on this chromosome. Utilizing these loci, we, for the first time, developed three reliable pairs of sex-specific molecular markers, which exhibited high accuracy during validation across various geographic populations, offering an effective tool for the sex identification of H. tibetana. Additionally, this study lays the groundwork for further research into the mechanisms of sex determination and the evolution of sex chromosomes in H. tibetana.