Repeated turnovers keep sex chromosomes young in willows

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.

Evolution of a ZW sex chromosome system in willows

Transitions in the heterogamety of sex chromosomes (e.g., XY to ZW or vice versa) fundamentally alter the genetic basis of sex determination, however the details of these changes have been studied in only a few cases. In an XY to ZW transition, the X is likely to give rise to the W because they both carry feminizing genes and the X is expected to harbour less genetic load than the Y. Here, using a new reference genome for Salix exigua, we trace the X, Y, Z, and W sex determination regions during the homologous transition from an XY system to a ZW system in willow (Salix). We show that both the W and the Z arose from the Y chromosome. We find that the new Z chromosome shares multiple homologous putative masculinizing factors with the ancestral Y, whereas the new W lost these masculinizing factors and gained feminizing factors. The origination of both the W and Z from the Y was permitted by an unexpectedly low genetic load on the Y and this indicates that the origins of sex chromosomes during homologous transitions may be more flexible than previously considered.

Functional analysis of Salix purpurea genes support roles for ARR17 and GATA15 as master regulators of sex determination

The Salicaceae family is of growing interest in the study of dioecy in plants because the sex determination region (SDR) has been shown to be highly dynamic, with differing locations and heterogametic systems between species. Without the ability to transform and regenerate Salix in tissue culture, previous studies investigating the mechanisms regulating sex in the genus Salix have been limited to genome resequencing and differential gene expression, which are mostly descriptive in nature, and functional validation of candidate sex determination genes has not yet been conducted. Here, we used Arabidopsis to functionally characterize a suite of previously identified candidate genes involved in sex determination and sex dimorphism in the bioenergy shrub willow Salix purpurea. Six candidate master regulator genes for sex determination were heterologously expressed in Arabidopsis, followed by floral proteome analysis. In addition, 11 transcription factors with predicted roles in mediating sex dimorphism downstream of the SDR were tested using DAP-Seq in both male and female S. purpurea DNA. The results of this study provide further evidence to support models for the roles of ARR17 and GATA15 as master regulator genes of sex determination in S. purpurea, contributing to a regulatory system that is notably different from that of its sister genus Populus. Evidence was also obtained for the roles of two transcription factors, an AP2/ERF family gene and a homeodomain-like transcription factor, in downstream regulation of sex dimorphism.

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.

Genome-wide identification and expression analysis of MADS-box transcription factors reveal their involvement in sex determination of hardy rubber tree (Eucommia ulmoides oliv.)

Eucommia ulmoides is a famous rubber-producing and medicinal tree species that produces unisexual flowers on separate individuals from the earliest stage of stamen/pistil primordium formation. To explore the genetic regulation pathway of sex in E. ulmoides, comprehensive genome-wide analyses and tissue-/sex-specific transcriptome comparisons of MADS-box transcription factors were performed for the first time in this work. Quantitative real-time PCR technique was employed to further validate the expression of genes that are assigned to floral organ ABCDE model. A total of 66 non-redundant E. ulmoides MADS-box (EuMADS) genes were identified, they were classified into Type I (M-type, 17 genes) and Type II (MIKC, 49 genes). Complex protein-motif composition, exon-intron structure and phytohormone-response cis-elements were detected in MIKC-EuMADS genes. Furthermore, 24 differentially-expressed EuMADS genes (DEGs) between male and female flowers, and two DEGs between male and female leaves were revealed. Amongst the 14 floral organ ABCDE model-related genes, there were 6 (A/B/C/E-class) and 5 (A/D/E-class) genes displayed male- and female-biased expression respectively. In particular, one B-class gene EuMADS39 and one A-class gene EuMADS65 were almost exclusively expressed in male trees, no matter in flower or leaf tissues. Collectively, these results suggested a critical role of MADS-box transcription factors in sex determination of E. ulmoides, which is conducive to decoding the molecular regulation mechanism of sex in E. ulmoides.

Origin and chromatin remodeling of young X/Y sex chromosomes in catfish with sexual plasticity

Assembly of a complete Y chromosome is a significant challenge in animals with an XX/XY sex-determination system. Recently, we created YY-supermale yellow catfish by crossing XY males with sex-reversed XY females, providing a valuable model for Y-chromosome assembly and evolution. Here, we assembled highly homomorphic Y and X chromosomes by sequencing genomes of the YY supermale and XX female in yellow catfish, revealing their nucleotide divergences with only less than 1% and with the same gene compositions. The sex-determining region (SDR) was identified to locate within a physical distance of 0.3 Mb by F_ST scanning. Strikingly, the incipient sex chromosomes were revealed to originate via autosome-autosome fusion and were characterized by a highly rearranged region with an SDR downstream of the fusion site. We found that the Y chromosome was at a very early stage of differentiation, as no clear evidence of evolutionary strata and classical structure features of recombination suppression for a rather late stage of Y-chromosome evolution were observed. Significantly, a number of sex-antagonistic mutations and the accumulation of repetitive elements were discovered in the SDR, which might be the main driver of the initial establishment of recombination suppression between young X and Y chromosomes. Moreover, distinct three-dimensional chromatin organizations of the Y and X chromosomes were identified in the YY supermales and XX females, as the X chromosome exhibited denser chromatin structure than the Y chromosome, while they respectively have significantly spatial interactions with female- and male-related genes compared with other autosomes. The chromatin configuration of the sex chromosomes as well as the nucleus spatial organization of the XX neomale were remodeled after sex reversal and similar to those in YY supermales, and a male-specific loop containing the SDR was found in the open chromatin region. Our results elucidate the origin of young sex chromosomes and the chromatin remodeling configuration in the catfish sexual plasticity.