Chromosome-scale assembly of the genome of Salix dunnii reveals a male-heterogametic sex determination system on chromosome 7

Gap-free X and Y chromosome assemblies of Salix arbutifolia reveal an evolutionary change from male to female heterogamety in willows, without a change in the position of the sex-determining locus

In the Vetrix clade of Salix, a genus of woody flowering plants, sex determination involves chromosome 15, but an XY system has changed to a ZW system. We studied the detailed genetic changes involved. We used genome sequencing, with chromosome conformation capture (Hi-C) and PacBio HiFi reads to assemble chromosome level gap-free X and Y of Salix arbutifolia, and distinguished the haplotypes in the 15X- and 15Y-linked regions, to study the evolutionary history of the sex-linked regions (SLRs). Our sequencing revealed heteromorphism of the X and Y haplotypes of the SLR, with the X-linked region being considerably larger than the corresponding Y region, mainly due to accumulated repetitive sequences and gene duplications. The phylogenies of single-copy orthogroups within the SLRs indicate that S. arbutifolia and Salix purpurea share an ancestral SLR within a repeat-rich region near the chromosome 15 centromere. During the change in heterogamety, the X-linked region changed to a W-linked one, while the Z was derived from the Y.

Exploring the cobia (Rachycentron canadum) genome: unveiling putative male heterogametic regions and identification of sex-specific markers

BACKGROUND

Cobia (Rachycentron canadum) is the only member of the Rachycentridae family and exhibits considerable sexual dimorphism in growth rate. Sex determination in teleosts has been a long-standing basic biological question, and the molecular mechanisms of sex determination/differentiation in cobia are completely unknown.

RESULTS

Here, we reported 2 high-quality, chromosome-level annotated male and female cobia genomes with assembly sizes of 586.51 Mb (contig/scaffold N50: 86.0 kb/24.3 Mb) and 583.88 Mb (79.9 kb/22.5 Mb), respectively. Synteny inference among perciform genomes revealed that cobia and the remora Echeneis naucrates were sister groups. Further, whole-genome resequencing of 31 males and 60 females, genome-wide association study, and sequencing depth analysis identified 3 short male-specific regions within a 10.7-kb continuous genomic region on male chromosome 18, which hinted at an undifferentiated sex chromosome system with a putative XX/XY mode of sex determination in cobia. Importantly, the only 2 genes within/between the male-specific regions, epoxide hydrolase 1 (ephx1, renamed cephx1y) and transcription factor 24 (tcf24, renamed ctcf24y), showed testis-specific/biased gene expression, whereas their counterparts cephx1x and ctf24x, located in female chromosome 18, were similarly expressed in both sexes. In addition, male-specific PCR targeting the cephx1y gene revealed that this genomic feature is conserved in cobia populations from Panama, Brazil, Australia, and Japan.

CONCLUSION

The first comprehensive genomic survey presented here is a valuable resource for future studies on cobia population structure and dynamics, conservation, and evolutionary history. Furthermore, it establishes evidence of putative male heterogametic regions with 2 genes playing a potential role in the sex determination of the species, and it provides further support for the rapid evolution of sex-determining mechanisms in teleost fish.

Genome-wide analysis of transposable elements and satellite DNA in Humulus scandens, a dioecious plant with XX/XY1Y2 chromosomes

Transposable elements (TEs) and satellite DNAs, two major categories of repetitive sequences, are expected to accumulate in non-recombining genome regions, including sex-linked regions, and contribute to sex chromosome evolution. The dioecious plant, Humulus scandens, can be used for studying the evolution of the XX/XY1Y2 sex chromosomes. In this study, we thoroughly examined the repetitive components of male and female H. scandens using next-generation sequencing data followed by bioinformatics analysis and florescence in situ hybridization (FISH). The H. scandens genome has a high overall repetitive sequence composition, 68.30% in the female and 66.78% in the male genome, with abundant long terminal repeat (LTR) retrotransposons (RTs), including more Ty3/Gypsy than Ty1/Copia elements, particularly two Ty3/Gypsy lineages, Tekay and Retand. Most LTR-RT lineages were found dispersed across the chromosomes, though CRM and Athila elements were predominately found within the centromeres and the pericentromeric regions. The Athila elements also showed clearly higher FISH signal intensities in the Y1 and Y2 chromosomes than in the X or autosomes. Three novel satellite DNAs were specifically distributed in the centromeric and/or telomeric regions, with markedly different distributions on the X, Y1, and Y2 chromosomes. Combined with FISH using satellite DNAs to stain chromosomes during meiotic diakinesis, we determined the synapsis pattern and distinguish pseudoautosomal regions (PARs). The results indicate that the XY1Y2 sex chromosomes of H. scandens might have originated from a centric fission event. This study improves our understanding of the repetitive sequence organization of H. scandens genome and provides a basis for further analysis of their chromosome evolution process.

Evolution of the spinach sex-linked region within a rarely recombining pericentromeric region

Sex chromosomes have evolved independently in many different plant lineages. Here, we describe reference genomes for spinach (Spinacia oleracea) X and Y haplotypes by sequencing homozygous XX females and YY males. The long arm of 185-Mb chromosome 4 carries a 13-Mb X-linked region (XLR) and 24.1-Mb Y-linked region (YLR), of which 10 Mb is Y specific. We describe evidence that this reflects insertions of autosomal sequences creating a "Y duplication region" or "YDR" whose presence probably directly reduces genetic recombination in the immediately flanking regions, although both the X and Y sex-linked regions are within a large pericentromeric region of chromosome 4 that recombines rarely in meiosis of both sexes. Sequence divergence estimates using synonymous sites indicate that YDR genes started diverging from their likely autosomal progenitors about 3 MYA, around the time when the flanking YLR stopped recombining with the XLR. These flanking regions have a higher density of repetitive sequences in the YY than the XX assembly and include slightly more pseudogenes compared with the XLR, and the YLR has lost about 11% of the ancestral genes, suggesting some degeneration. Insertion of a male-determining factor would have caused Y linkage across the entire pericentromeric region, creating physically small, highly recombining, terminal pseudoautosomal regions. These findings provide a broader understanding of the origin of sex chromosomes in spinach.

The complete mitochondrial genome of the biodiesel plant Jatropha curcas L

Jatropha curcas (Linnaeus, 1753) is a plant species in the order Malpighiales and the family Euphorbiaceae and is native to the tropical regions of America, such as Mexico and Argentina. Currently, this plant species inhabits tropical and subtropical regions of the world. Jatropha has been widely used as a biofuel plant to produce high-quality diesel engine fuel. In this study, the complete mitochondrial genome sequence of J. curcas was assembled into 561,839 bp circular nucleotides with a GC content of 44.6%. The mitochondrial genome of J. curcas comprises 33 known protein-coding genes, 22 tRNA genes, three rRNA genes, one ncRNA gene, and 85 open reading frame genes. Phylogenetic analysis showed this species is closely related to the castor bean (Ricinus communis).

Structural variation of a sex-linked region confers monoecy and implicates GATA15 as a master regulator of sex in Salix purpurea

The Salicaceae, including Populus and Salix, are dioecious perennials that utilize different sex determination systems. This family provides a useful system to better understand the evolution of dioecy and sex chromosomes. Here, a rare monoecious genotype of Salix purpurea, 94003, was self- and cross-pollinated and progeny sex ratios were used to test hypotheses on possible mechanisms of sex determination. To delimit genomic regions associated with monoecious expression, the 94003 genome sequence was assembled and DNA- and RNA-Seq of progeny inflorescences was performed. Based on alignments of progeny shotgun DNA sequences to the haplotype-resolved monoecious 94003 genome assembly and reference male and female genomes, a 1.15 Mb sex-linked region on Chr15W was confirmed to be absent in monecious plants. Inheritance of this structural variation is responsible for the loss of a male-suppressing function in what would otherwise be genetic females (ZW), resulting in monoecy (ZW^H or WW^H ), or lethality, if homozygous (W^H W^H ). We present a refined, two-gene sex determination model for Salix purpurea, mediated by ARR17 and GATA15 that is different from the single-gene ARR17-mediated system in the related genus Populus.

The male-heterogametic sex determination system on chromosome 15 of Salix triandra and Salix arbutifolia reveals ancestral male heterogamety and subsequent turnover events in the genus Salix

Dioecious Salix evolved more than 45 million years ago, but have homomorphic sex chromosomes, suggesting that turnover event(s) prevented major differentiation. Sex chromosome turnover events have been inferred in the sister genus Populus. The genus Salix includes two main clades, Salix and Vetrix, with several previously studied Vetrix clade species having female-heterogametic (ZW) or male-heterogametic (XY) sex-determining systems (SDSs) on chromosome 15, while three Salix clade species have XY SDSs on chromosome 7. We here studied two basal taxa of the Vetrix clade, S. arbutifolia and S. triandra using S. purpurea as the reference genome. Analyses of whole genome resequencing data for genome-wide associations (GWAS) with the sexes and genetic differentiation between the sexes (FST values) showed that both species have male heterogamety with a sex-determining locus on chromosome 15, suggesting an early turnover event within the Vetrix clade, perhaps promoted by sexually antagonistic or (and) sex-ratio selection. Changepoint analysis based on FST values identified small sex-linked regions of ~3.33 Mb and ~2.80 Mb in S. arbutifolia and S. triandra, respectively. The SDS of S. arbutifolia was consistent with recent results that used its own genome as reference. Ancestral state reconstruction of SDS suggests that at least two turnover events occurred in Salix.

De Novo Assembly and Annotation of 11 Diverse Shrub Willow (Salix) Genomes Reveals Novel Gene Organization in Sex-Linked Regions

Poplar and willow species in the Salicaceae are dioecious, yet have been shown to use different sex determination systems located on different chromosomes. Willows in the subgenus Vetrix are interesting for comparative studies of sex determination systems, yet genomic resources for these species are still quite limited. Only a few annotated reference genome assemblies are available, despite many species in use in breeding programs. Here we present de novo assemblies and annotations of 11 shrub willow genomes from six species. Copy number variation of candidate sex determination genes within each genome was characterized and revealed remarkable differences in putative master regulator gene duplication and deletion. We also analyzed copy number and expression of candidate genes involved in floral secondary metabolism, and identified substantial variation across genotypes, which can be used for parental selection in breeding programs. Lastly, we report on a genotype that produces only female descendants and identified gene presence/absence variation in the mitochondrial genome that may be responsible for this unusual inheritance.

The whole-genome assembly of an endangered Salicaceae species: Chosenia arbutifolia (Pall.) A. Skv

BACKGROUND

As a fast-growing tree species, Chosenia arbutifolia has a unique but controversial taxonomic status in the family Salicaceae. Despite its importance as an industrial material, in ecological protection, and in landscaping, C. arbutifolia is seriously endangered in Northeast China because of artificial destruction and its low reproductive capability.

RESULTS

To clarify its phylogenetic relationships with other Salicaceae species, we assembled a high-quality chromosome-level genome of C. arbutifolia using PacBio High-Fidelity reads and Hi-C sequencing data, with a total size of 338.93 Mb and contig N50 of 1.68 Mb. Repetitive sequences, which accounted for 42.34% of the assembly length, were identified. In total, 33,229 protein-coding genes and 11,474 small noncoding RNAs were predicted. Phylogenetic analysis suggested that C. arbutifolia and poplars diverged approximately 15.3 million years ago, and a large interchromosomal recombination between C. arbutifolia and other Salicaceae species was discovered.

CONCLUSIONS

Our study provides insights into the genome architecture and systematic evolution of C. arbutifolia, as well as comprehensive information for germplasm protection and future functional genomic studies.

Repeated turnovers keep sex chromosomes young in willows

Background

Salicaceae species have diverse sex determination systems and frequent sex chromosome turnovers. However, compared with poplars, the diversity of sex determination in willows is poorly understood, and little is known about the evolutionary forces driving their turnover. Here, we characterized the sex determination in two Salix species, S. chaenomeloides and S. arbutifolia, which have an XY system on chromosome 7 and 15, respectively.

Results

Based on the assemblies of their sex determination regions, we found that the sex determination mechanism of willows may have underlying similarities with poplars, both involving intact and/or partial homologs of a type A cytokinin response regulator (RR) gene. Comparative analyses suggested that at least two sex turnover events have occurred in Salix, one preserving the ancestral pattern of male heterogamety, and the other changing heterogametic sex from XY to ZW, which could be partly explained by the “deleterious mutation load” and “sexually antagonistic selection” theoretical models. We hypothesize that these repeated turnovers keep sex chromosomes of willow species in a perpetually young state, leading to limited degeneration.

Conclusions

Our findings further improve the evolutionary trajectory of sex chromosomes in Salicaceae species, explore the evolutionary forces driving the repeated turnovers of their sex chromosomes, and provide a valuable reference for the study of sex chromosomes in other species.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02769-w.

Does polyploidy inhibit sex chromosome evolution in angiosperms?

Dioecy is rare in flowering plants (5-6% of species), but is often controlled genetically by sex-linked regions (SLRs). It has so far been unclear whether, polyploidy affects sex chromosome evolution, as it does in animals, though polyploidy is quite common in angiosperms, including in dioecious species. Plants could be different, as, unlike many animal systems, degenerated sex chromosomes, are uncommon in plants. Here we consider sex determination in plants and plant-specific factors, and propose that constraints created at the origin of polyploids limit successful polyploidization of species with SLRs. We consider the most likely case of a polyploid of a dioecious diploid with an established SLR, and discuss the outcome in autopolyploids and allopolyploids. The most stable system possibly has an SLR on just one chromosome, with a strongly dominant genetic factor in the heterogametic sex (e.g., xxxY male in a tetraploid). If recombination occurs with its homolog, this will prevent Y chromosome degeneration. Polyploidy may also allow for reversibility of multiplied Z or X chromosomes into autosomes. Otherwise, low dosage of Y-linked SLRs compared to their multiple homologous x copies may cause loss of reliable sex-determination at higher ploidy levels. We discuss some questions that can be studied using genome sequencing, chromosome level-assemblies, gene expression studies and analysis of loci under selection.

Identification of sex determination locus in sea cucumber Apostichopus japonicus using genome-wide association study

Background

Sex determination mechanisms are complicated and diverse across taxonomic categories. Sea cucumber Apostichopus japonicus is a benthic echinoderm, which is the closest group of invertebrates to chordate, and important economic and ecologically aquaculture species in China. A. japonicus is dioecious, and no phenotypic differences between males and females can be detected before sexual maturation. Identification of sex determination locus will broaden knowledge about sex-determination mechanism in echinoderms, which allows for the identification of sex-linked markers and increases the efficiency of sea cucumber breeding industry.

Results

Here, we integrated assembly of a novel chromosome-level genome and resequencing of female and male populations to investigate the sex determination mechanisms of A. japonicus. We built a chromosome-level genome assembly AJH1.0 using Hi-C technology. The assembly AJH1.0 consists of 23 chromosomes ranging from 22.4 to 60.4 Mb. To identify the sex-determination locus of A. japonicus, we conducted genome-wide association study (GWAS) and analyses of distribution characteristics of sex-specific SNPs and fixation index F_ST. The GWAS analysis showed that multiple sex-associated loci were located on several chromosomes, including chromosome 4 (24.8%), followed by chromosome 9 (10.7%), chromosome 17 (10.4%), and chromosome 18 (14.1%). Furthermore, analyzing the homozygous and heterozygous genotypes of plenty of sex-specific SNPs in females and males confirmed that A. japonicus might have a XX/XY sex determination system. As a physical region of 10 Mb on chromosome 4 included the highest number of sex-specific SNPs and higher F_ST values, this region was considered as the candidate sex determination region (SDR) in A. japonicus.

Conclusions

In the present study, we integrated genome-wide association study and analyses of sex-specific variations to investigate sex determination mechanisms. This will bring novel insights into gene regulation during primitive gonadogenesis and differentiation and identification of master sex determination gene in sea cucumber. In the sea cucumber industry, investigation of molecular mechanisms of sex determination will be helpful for artificial fertilization and precise breeding.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08632-3.

The phylogeny of Salix revealed by whole genome re-sequencing suggests different sex-determination systems in major groups of the genus

BACKGROUND AND AIMS

The largest genus of Salicaceae sensu lato, Salix, has been shown to consist of two main clades: clade Salix, in which species have XY sex-determination systems (SDSs) on chromosome 7, and clade Vetrix including species with ZW SDSs on chromosome 15. Here, we test the utility of whole genome re-sequencing (WGR) for phylogenomic reconstructions of willows to infer changes between different SDSs.

METHODS

We used more than 1 TB of WGR data from 70 Salix taxa to ascertain single nucleotide polymorphisms on the autosomes, the sex-linked regions (SLRs) and the chloroplast genomes, for phylogenetic and species tree analyses. To avoid bias, we chose reference genomes from both groups, Salix dunnii from clade Salix and S. purpurea from clade Vetrix.

KEY RESULTS

Two main largely congruent groups were recovered: the paraphyletic Salix grade and the Vetrix clade. The autosome dataset trees resolved four subclades (C1-C4) in Vetrix. C1 and C2 comprise species from the Hengduan Mountains and adjacent areas and from Eurasia, respectively. Section Longifoliae (C3) grouped within the Vetrix clade but fell into the Salix clade in trees based on the chloroplast dataset analysis. Salix triandra from Eurasia (C4) was revealed as sister to the remaining species of clade Vetrix. In Salix, the polyploid group C5 is paraphyletic to clade Vetrix and subclade C6 is consistent with Argus's subgenus Protitea. Chloroplast datasets separated both Vetrix and Salix as monophyletic, and yielded C5 embedded within Salix. Using only diploid species, both the SLR and autosomal datasets yielded trees with Vetrix and Salix as well-supported clades.

CONCLUSION

WGR data are useful for phylogenomic analyses of willows. The different SDSs may contribute to the isolation of the two major groups, but the reproductive barrier between them needs to be studied.

A Selection of Reliable Reference Genes for Gene Expression Analysis in the Female and Male Flowers of Salix suchowensis

Salix is a dioecious plant. Research on the molecular regulation mechanism of male and female inflorescence differentiation and development is necessary to analyze sex differentiation in the willow and the underlying mechanisms of unisexual flower development. However, at present, there are no reference genes suitable for stable expression in the process of willow inflorescence development. In this study, Salix suchowensis was used as the research material, nine candidate reference genes (α-TUB1, α-TUB2, ACT, H2A, DnaJ, CDC2, GAPDH, TIP41, β-TUB) were selected, and qRT-PCR technology was used to detect the expression of each candidate reference gene in female and male flowers at different developmental stages and using five algorithms (geNorm, Normfinder, Delta Ct, BestKeeper, and RefFinder) to comprehensively evaluate the stability of candidate reference genes. The results showed that ACT and DnaJ were stably expressed in all samples and could be used as reference genes. In addition, the reliability of the screening results was further verified via an expression pattern analysis of the CFS gene that encodes flower specific transcription factor in different samples. The stable reference genes selected in this study provide the basis for future research on the expression analysis of functional genes related to the development of male and female flowers of S. suchowensis.

Phylogenomics and Biogeography of Populus Based on Comprehensive Sampling Reveal Deep-Level Relationships and Multiple Intercontinental Dispersals

Populus not only has significant economic and ecological values, but also serves as a model tree that is widely used in the basic research of tree growth, physiology, and genetics. However, high levels of morphological variation and extensive interspecific hybridization of Populus pose an obstacle for taxonomy, and also to the understanding of phylogenetic interspecific relationships and biogeographical history. In this study, a total of 103 accessions representing almost all the wild species of Populus were collected and whole-genome re-sequenced to examine the phylogenetic relationships and biogeography history. On the basis of 12,916,788 nuclear single nucleotide polymorphisms (SNPs), we reconstructed backbone phylogenies using concatenate and coalescent methods, we highly disentangled the species relationships of Populus, and several problematic taxa were treated as species complexes. Furthermore, the phylogeny of the chloroplast genome showed extensive discordance with the trees from the nuclear genome data, and due to extensive chloroplast capture and hybridization of Populus species, plastomes could not accurately evaluate interspecies relationships. Ancient gene flow between clades and some hybridization events were also identified by ABBA-BABA analysis. The reconstruction of chronogram and ancestral distributions suggested that North America was the original region of this genus, and subsequent long dispersal and migration across land bridges were contributed to the modern range of Populus. The diversification of Populus mainly occurred in East Asia in recent 15 Ma, possibly promoted by the uplift of the Tibetan Plateau. This study provided comprehensive evidence on the phylogeny of Populus and proposed a four-subgeneric classification and a new status, subgenus Abaso. Meanwhile, ancestral distribution reconstruction with nuclear data advanced the understanding of the biogeographic history of Populus.

Decoding sex: Elucidating sex determination and how high-quality genome assemblies are untangling the evolutionary dynamics of sex chromosomes

Sexual reproduction is a diverse and widespread process. In gonochoristic species, the differentiation of sexes occurs through diverse mechanisms, influenced by environmental and genetic factors. In most vertebrates, a master-switch gene is responsible for triggering a sex determination network. However, only a few genes have acquired master-switch functions, and this process is associated with the evolution of sex-chromosomes, which have a significant influence in evolution. Additionally, their highly repetitive regions impose challenges for high-quality sequencing, even using high-throughput, state-of-the-art techniques. Here, we review the mechanisms involved in sex determination and their role in the evolution of species, particularly vertebrates, focusing on sex chromosomes and the challenges involved in sequencing these genomic elements. We also address the improvements provided by the growth of sequencing projects, by generating a massive number of near-gapless, telomere-to-telomere, chromosome-level, phased assemblies, increasing the number and quality of sex-chromosome sequences available for further studies.

Genome-wide association mapping uncovers sex-associated copy number variation markers and female hemizygous regions on the W chromosome in Salix viminalis

Background

Sex chromosomes are in some species largely undifferentiated (homomorphic) with restricted sex determination regions. Homomorphic but different sex chromosomes are found in the closely related genera Populus and Salix indicating flexible sex determination systems, ideal for studies of processes involved in sex chromosome evolution. We have performed genome-wide association studies of sex and analysed sex chromosomes in a population of 265 wild collected Salix viminalis accessions and studied the sex determining locus.

Results

A total of 19,592 markers were used in association analyses using both Fisher’s exact tests and a single-marker mixed linear model, which resulted in 48 and 41 sex-associated (SA) markers respectively. Across all 48 SA markers, females were much more often heterozygous than males, which is expected if females were the heterogametic sex. The majority of the SA markers were, based on positions in the S. purpurea genome, located on chromosome 15, previously demonstrated to be the sex chromosome. Interestingly, when mapping the genotyping-by-sequencing sequence tag harbouring the two SA markers with the highest significance to the S. viminalis genomic scaffolds, five regions of very high similarity were found: three on a scaffold that represents a part of chromosome 15, one on a scaffold that represents a part of chromosome 9 and one on a scaffold not anchored to the genome. Based on segregation differences of the alleles at the two marker positions and on differences in PCR amplification between females and males we conclude that females had multiple copies of this DNA fragment (chromosome 9 and 15), whereas males only had one (chromosome 9). We therefore postulate that the female specific sequences have been copied from chromosome 9 and inserted on chromosome 15, subsequently developing into a hemizygous W chromosome linked region.

Conclusions

Our results support that sex determination in S. viminalis is controlled by one locus on chromosome 15. The segregation patterns observed at the SA markers furthermore confirm that S. viminalis females are the heterogametic sex. We also identified a translocation from chromosome 9 to the W chromosome.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-08021-2.

Genome Assembly and Sex-Determining Region of Male and Female Populus × sibirica

The genus Populus is presented by dioecious species, and it became a promising object to study the genetics of sex in plants. In this work, genomes of male and female Populus × sibirica individuals were sequenced for the first time. To achieve high-quality genome assemblies, we used Oxford Nanopore Technologies and Illumina platforms. A protocol for the isolation of long and pure DNA from young poplar leaves was developed, which enabled us to obtain 31 Gb (N50 = 21 kb) for the male poplar and 23 Gb (N50 = 24 kb) for the female one using the MinION sequencer. Genome assembly was performed with different tools, and Canu provided the most complete and accurate assemblies with a length of 818 Mb (N50 = 1.5 Mb) for the male poplar and 816 Mb (N50 = 0.5 Mb) for the female one. After polishing with Racon and Medaka (Nanopore reads) and then with POLCA (Illumina reads), assembly completeness was 98.45% (87.48% duplicated) for the male and 98.20% (76.77% duplicated) for the female according to BUSCO (benchmarking universal single-copy orthologs). A high proportion of duplicated BUSCO and the increased genome size (about 300 Mb above the expected) pointed at the separation of haplotypes in a large part of male and female genomes of P. × sibirica. Due to this, we were able to identify two haplotypes of the sex-determining region (SDR) in both assemblies; and one of these four SDR haplotypes, in the male genome, contained partial repeats of the ARR17 gene (Y haplotype), while the rest three did not (X haplotypes). The analysis of the male P. × sibirica SDR suggested that the Y haplotype originated from P. nigra, while the X haplotype is close to P. trichocarpa and P. balsamifera species. Moreover, we revealed a Populus-specific repeat that could be involved in translocation of the ARR17 gene or its part to the SDR of P. × sibirica and other Populus species. The obtained results expand our knowledge on SDR features in the genus Populus and poplar phylogeny.

Telomere-to-telomere assembly of a fish Y chromosome reveals the origin of a young sex chromosome pair

BACKGROUND

The origin of sex chromosomes requires the establishment of recombination suppression between the proto-sex chromosomes. In many fish species, the sex chromosome pair is homomorphic with a recent origin, providing species for studying how and why recombination suppression evolved in the initial stages of sex chromosome differentiation, but this requires accurate sequence assembly of the X and Y (or Z and W) chromosomes, which may be difficult if they are recently diverged.

RESULTS

Here we produce a haplotype-resolved genome assembly of zig-zag eel (Mastacembelus armatus), an aquaculture fish, at the chromosomal scale. The diploid assembly is nearly gap-free, and in most chromosomes, we resolve the centromeric and subtelomeric heterochromatic sequences. In particular, the Y chromosome, including its highly repetitive short arm, has zero gaps. Using resequencing data, we identify a ~7 Mb fully sex-linked region (SLR), spanning the sex chromosome centromere and almost entirely embedded in the pericentromeric heterochromatin. The SLRs on the X and Y chromosomes are almost identical in sequence and gene content, but both are repetitive and heterochromatic, consistent with zero or low recombination. We further identify an HMG-domain containing gene HMGN6 in the SLR as a candidate sex-determining gene that is expressed at the onset of testis development.

CONCLUSIONS

Our study supports the idea that preexisting regions of low recombination, such as pericentromeric regions, can give rise to SLR in the absence of structural variations between the proto-sex chromosomes.

The Evolutionary History, Diversity, and Ecology of Willows (Salix L.) in the European Alps

The genus Salix (willows), with 33 species, represents the most diverse genus of woody plants in the European Alps. Many species dominate subalpine and alpine types of vegetation. Despite a long history of research on willows, the evolutionary and ecological factors for this species richness are poorly known. Here we will review recent progress in research on phylogenetic relationships, evolution, ecology, and speciation in alpine willows. Phylogenomic reconstructions suggest multiple colonization of the Alps, probably from the late Miocene onward, and reject hypotheses of a single radiation. Relatives occur in the Arctic and in temperate Eurasia. Most species are widespread in the European mountain systems or in the European lowlands. Within the Alps, species differ ecologically according to different elevational zones and habitat preferences. Homoploid hybridization is a frequent process in willows and happens mostly after climatic fluctuations and secondary contact. Breakdown of the ecological crossing barriers of species is followed by introgressive hybridization. Polyploidy is an important speciation mechanism, as 40% of species are polyploid, including the four endemic species of the Alps. Phylogenomic data suggest an allopolyploid origin for all taxa analyzed so far. Further studies are needed to specifically analyze biogeographical history, character evolution, and genome evolution of polyploids.