Accumulation of interspersed and sex-specific repeats in the non-recombining region of papaya sex chromosomes

Genomic profiling of dioecious Amaranthus species provides novel insights into species relatedness and sex genes

BACKGROUND

Amaranthus L. is a diverse genus consisting of domesticated, weedy, and non-invasive species distributed around the world. Nine species are dioecious, of which Amaranthus palmeri S. Watson and Amaranthus tuberculatus (Moq.) J.D. Sauer are troublesome weeds of agronomic crops in the USA and elsewhere. Shallow relationships among the dioecious Amaranthus species and the conservation of candidate genes within previously identified A. palmeri and A. tuberculatus male-specific regions of the Y (MSYs) in other dioecious species are poorly understood. In this study, seven genomes of dioecious amaranths were obtained by paired-end short-read sequencing and combined with short reads of seventeen species in the family Amaranthaceae from NCBI database. The species were phylogenomically analyzed to understand their relatedness. Genome characteristics for the dioecious species were evaluated and coverage analysis was used to investigate the conservation of sequences within the MSY regions.

RESULTS

We provide genome size, heterozygosity, and ploidy level inference for seven newly sequenced dioecious Amaranthus species and two additional dioecious species from the NCBI database. We report a pattern of transposable element proliferation in the species, in which seven species had more Ty3 elements than copia elements while A. palmeri and A. watsonii had more copia elements than Ty3 elements, similar to the TE pattern in some monoecious amaranths. Using a Mash-based phylogenomic analysis, we accurately recovered taxonomic relationships among the dioecious Amaranthus species that were previously identified based on comparative morphology. Coverage analysis revealed eleven candidate gene models within the A. palmeri MSY region with male-enriched coverages, as well as regions on scaffold 19 with female-enriched coverage, based on A. watsonii read alignments. A previously reported FLOWERING LOCUS T (FT) within A. tuberculatus MSY contig was also found to exhibit male-enriched coverages for three species closely related to A. tuberculatus but not for A. watsonii reads. Additional characterization of the A. palmeri MSY region revealed that 78% of the region is made of repetitive elements, typical of a sex determination region with reduced recombination.

CONCLUSIONS

The results of this study further increase our understanding of the relationships among the dioecious species of the Amaranthus genus as well as revealed genes with potential roles in sex function in the species.

Accumulation of retrotransposons contributes to W chromosome differentiation in the willow beauty Peribatodes rhomboidaria (Lepidoptera: Geometridae)

The W chromosome of Lepidoptera is typically gene-poor, repeat-rich and composed of heterochromatin. Pioneering studies investigating this chromosome reported an abundance of mobile elements. However, the actual composition of the W chromosome varies greatly between species, as repeatedly demonstrated by comparative genomic hybridization (CGH) or fluorescence in situ hybridization (FISH). Here we present an analysis of repeats on the W chromosome in the willow beauty, Peribatodes rhomboidaria (Geometridae), a species in which CGH predicted an abundance of W-enriched or W-specific sequences. Indeed, comparative analysis of male and female genomes using RepeatExplorer identified ten putative W chromosome-enriched repeats, most of which are LTR or LINE mobile elements. We analysed the two most abundant: PRW LINE-like and PRW Bel-Pao. The results of FISH mapping and bioinformatic analysis confirmed their enrichment on the W chromosome, supporting the hypothesis that mobile elements are the driving force of W chromosome differentiation in Lepidoptera. As the W chromosome is highly underrepresented in chromosome-level genome assemblies of Lepidoptera, this recently introduced approach, combining bioinformatic comparative genome analysis with molecular cytogenetics, provides an elegant tool for studying this elusive and rapidly evolving part of the genome.

GISH painting of the Y chromosomes suggests advanced phases of sex chromosome evolution in three dioecious Cannabaceae species (Humulus lupulus, H. japonicus, and Cannabis sativa)

In plants, dioecy is relatively rare, and it involves sex chromosome systems that often developed independently over time. These characteristics make dioecious plants an attractive model to study sex chromosome evolution. To clarify the patterns of plant sex chromosome evolution, studies should be performed on a wide range of dioecious species. It is interesting to study the sex chromosomes in related species that evolved during a long period of independent sex chromosome evolution. The Cannabaceae family includes three dioecious species with heteromorphic sex chromosomes. Cannabis sativa and Humulus lupulus use the XX/XY chromosome system, whereas Humulus japonicus contains multiple sex chromosomes (XX/XY₁Y₂). To better understand sex chromosome evolution and the level of genomic divergence of these three related species, we undertook self-GISH and comparative GISH analyses. The self-GISH allowed visualization of the Y chromosomes of C. sativa, H. lupulus, and H. japonicus. The self-GISH signal was distributed along the entire Y chromosome, excluding the pseudo-autosomal region (PAR). Our results indicate that the male-specific region of the Y chromosome (MSY) spans the overwhelming majority of the Y chromosomes of all three species studied. The self-GISH results reveal the accumulation of repetitive DNA sequences in the Y chromosomes of all three species studied. This sequences presented in autosomes and/or chromosome X at a lower copy number than in Y. In comparative GISH experiments where the probe DNA of one species was applied to another species, a weak signal was exclusively detected on 45S rDNA sites, indicating a high level of genomic differentiation of the species used in this study. We demonstrate small PAR size and opposing large MSY and its positions on Y chromosomes. We also found that these genomes are highly differentiated. Furthermore, the data obtained in this study indicate a long period of independent and advanced sex chromosome evolution. Our study provides a valuable basis for future genomic studies of sex and suggests that the Cannabaceae family offers a promising model to study sex chromosome evolution.

Do Ty3/Gypsy Transposable Elements Play Preferential Roles in Sex Chromosome Differentiation?

Transposable elements (TEs) comprise a substantial portion of eukaryotic genomes. They have the unique ability to integrate into new locations and serve as the main source of genomic novelties by mediating chromosomal rearrangements and regulating portions of functional genes. Recent studies have revealed that TEs are abundant in sex chromosomes. In this review, we propose evolutionary relationships between specific TEs, such as Ty3/Gypsy, and sex chromosomes in different lineages based on the hypothesis that these elements contributed to sex chromosome differentiation processes. We highlight how TEs can drive the dynamics of sex-determining regions via suppression recombination under a selective force to affect the organization and structural evolution of sex chromosomes. The abundance of TEs in the sex-determining regions originates from TE-poor genomic regions, suggesting a link between TE accumulation and the emergence of the sex-determining regions. TEs are generally considered to be a hallmark of chromosome degeneration. Finally, we outline recent approaches to identify TEs and study their sex-related roles and effects in the differentiation and evolution of sex chromosomes.

Identification of structural variation and polymorphisms of a sex co-segregating scaffold in spinach

Spinach is a common vegetable, and dioecy is maintained by a pair of XY sex chromosomes. Due to limited genomic resources and its highly repetitive genome, limited studies were conducted to investigate the genomic landscape of the region near sex-determining loci. In this study, we screened the structure variations (SVs) between Y-linked contigs and a 1.78-Mb X scaffold (Super_scaffold 66), which enabled the development of 12 sex co-segregating DNA markers. These markers were tested in one F₁ mapping population and 40 spinach accessions, which comprised 692 individual plants with the strong sex linkage pattern. In addition, we found that Super_scaffold 66 was highly repetitive along with the enriched LTR-RTs insertions and decreased microsatellite distribution compared with the rest genome, which matches extremely low gene density featured by only nine annotated genes. Synteny analysis between Y contigs and Superscaffold_66 revealed a 340-Kb accumulative Y contig (non-continuous) and a 500-Kb X counterpart along with SVs and wide-spread tandem duplications. Among the nine genes, one ABC transporter gene revealed noticeable SVs between Y contig and X counterpart, as an approximate 5-Kb recent Gypsy LTR-RT insertion in the Y-linked allele, but not the X allele. The gene paucity, SVs, and sex-linked polymorphisms attributed to the recombination suppression. We proposed that Super_scaffold 66 is part of the non-recombining region containing the sex determination genes. The spread of 12 sex co-segregating markers from this 1.78 Mb genomic region indicated the existence and expansion of sex determination region during progression of the Y chromosome.

Cloning and physical localization of male-biased repetitive DNA sequences in Spinacia oleracea (Amaranthaceae)

Spinach (Spinacia oleracea Linnaeus, 1753) is an ideal material for studying molecular mechanisms of early-stage sex chromosome evolution in dioecious plants. Degenerate oligonucleotide-primed polymerase chain reaction (DOP-PCR) technique facilitates the retrotransposon-relevant studies by enriching specific repetitive DNA sequences from a micro-dissected single chromosome. We conducted genomic subtractive hybridization to screen sex-biased DNA sequences by using the DOP-PCR amplification products of micro-dissected spinach Y chromosome. The screening yielded 55 male-biased DNA sequences with 30 576 bp in length, of which, 32 DNA sequences (12 049 bp) contained repeat DNA sequences, including LTR/Copia, LTR/Gypsy, simple repeats, and DNA/CMC-EnSpm. Among these repetitive DNA sequences, four DNA sequences that contained a fragment of Ty3-gypsy retrotransposons (SP73, SP75, SP76, and SP77) were selected as fluorescence probes to hybridization on male and female spinach karyotypes. Fluorescence in situ hybridization (FISH) signals of SP73 and SP75 were captured mostly on the centromeres and their surrounding area for each homolog. Hybridization signals primarily appeared near the putative centromeres for each homologous chromosome pair by using SP76 and SP77 probes for FISH, and sporadic signals existed on the long arms. Results can be served as a basis to study the function of repetitive DNA sequences in sex chromosome evolution in spinach.

Multiple independent recombinations led to hermaphroditism in grapevine

We studied the grape sex-determining region (SDR) in 12 Vitis genomes and demonstrated its conservation across 556 genotypes including 193 accessions from 47 world-wide wild grapevine species and 363 accessions of cultivated grapevine. Although the grape SDR is recombination free in all wild species, we found two distinct hermaphrodite (H) haplotypes (H1 and H2) among the cultivated grapevines, both chimeras of male (M) and female (f) haplotypes. The two independent recombinations carry different genetic signatures which long predate the domestication of grapevine, suggesting independent evolutions of this trait in wild European grapevine gene pools prior to human domestication.

Hermaphroditic (perfect) flowers were a key trait in grapevine domestication, enabling a drastic increase in yields due to the efficiency of self-pollination in the domesticated grapevine (Vitis vinifera L. ssp. vinifera). In contrast, all extant wild Vitis species are dioecious, each plant having only male or female flowers. In this study, we identified the male (M) and female (f) haplotypes of the sex-determining region (SDR) in the wild grapevine species V. cinerea and confirmed the boundaries of the SDR. We also demonstrated that the SDR and its boundaries are precisely conserved across the Vitis genus using shotgun resequencing data of 556 wild and domesticated accessions from North America, East Asia, and Europe. A high linkage disequilibrium was found at the SDR in all wild grape species, while different recombination signatures were observed along the hermaphrodite (H) haplotype of 363 cultivated accessions, revealing two distinct H haplotypes, named H1 and H2. To further examine the H2 haplotype, we sequenced the genome of two grapevine cultivars, 'Riesling' and 'Chardonnay'. By reconstructing the first two H2 haplotypes, we estimated the divergence time between H1 and H2 haplotypes at ∼6 million years ago, which predates the domestication of grapevine (∼8,000 y ago). Our findings emphasize the important role of recombination suppression in maintaining dioecy in wild grape species and lend additional support to the hypothesis that at least two independent recombination events led to the reversion to hermaphroditism in grapevine.

The Genomic Architecture and Evolutionary Fates of Supergenes

Supergenes are genomic regions containing sets of tightly linked loci that control multi-trait phenotypic polymorphisms under balancing selection. Recent advances in genomics have uncovered significant variation in both the genomic architecture as well as the mode of origin of supergenes across diverse organismal systems. Although the role of genomic architecture for the origin of supergenes has been much discussed, differences in the genomic architecture also subsequently affect the evolutionary trajectory of supergenes and the rate of degeneration of supergene haplotypes. In this review, we synthesize recent genomic work and historical models of supergene evolution, highlighting how the genomic architecture of supergenes affects their evolutionary fate. We discuss how recent findings on classic supergenes involved in governing ant colony social form, mimicry in butterflies, and heterostyly in flowering plants relate to theoretical expectations. Furthermore, we use forward simulations to demonstrate that differences in genomic architecture affect the degeneration of supergenes. Finally, we discuss implications of the evolution of supergene haplotypes for the long-term fate of balanced polymorphisms governed by supergenes.

Fundamentally different repetitive element composition of sex chromosomes in Rumex acetosa

BACKGROUND AND AIMS

Dioecious species with well-established sex chromosomes are rare in the plant kingdom. Most sex chromosomes increase in size but no comprehensive analysis of the kind of sequences that drive this expansion has been presented. Here we analyse sex chromosome structure in common sorrel (Rumex acetosa), a dioecious plant with XY1Y2 sex determination, and we provide the first chromosome-specific repeatome analysis for a plant species possessing sex chromosomes.

METHODS

We flow-sorted and separately sequenced sex chromosomes and autosomes in R. acetosa using the two-dimensional fluorescence in situ hybridization in suspension (FISHIS) method and Illumina sequencing. We identified and quantified individual repeats using RepeatExplorer, Tandem Repeat Finder and the Tandem Repeats Analysis Program. We employed fluorescence in situ hybridization (FISH) to analyse the chromosomal localization of satellites and transposons.

KEY RESULTS

We identified a number of novel satellites, which have, in a fashion similar to previously known satellites, significantly expanded on the Y chromosome but not as much on the X or on autosomes. Additionally, the size increase of Y chromosomes is caused by non-long terminal repeat (LTR) and LTR retrotransposons, while only the latter contribute to the enlargement of the X chromosome. However, the X chromosome is populated by different LTR retrotransposon lineages than those on Y chromosomes.

CONCLUSIONS

The X and Y chromosomes have significantly diverged in terms of repeat composition. The lack of recombination probably contributed to the expansion of diverse satellites and microsatellites and faster fixation of newly inserted transposable elements (TEs) on the Y chromosomes. In addition, the X and Y chromosomes, despite similar total counts of TEs, differ significantly in the representation of individual TE lineages, which indicates that transposons proliferate preferentially in either the paternal or the maternal lineage.

Sex Chromosome Evolution: So Many Exceptions to the Rules

Genomic analysis of many nonmodel species has uncovered an incredible diversity of sex chromosome systems, making it possible to empirically test the rich body of evolutionary theory that describes each stage of sex chromosome evolution. Classic theory predicts that sex chromosomes originate from a pair of homologous autosomes and recombination between them is suppressed via inversions to resolve sexual conflict. The resulting degradation of the Y chromosome gene content creates the need for dosage compensation in the heterogametic sex. Sex chromosome theory also implies a linear process, starting from sex chromosome origin and progressing to heteromorphism. Despite many convergent genomic patterns exhibited by independently evolved sex chromosome systems, and many case studies supporting these theoretical predictions, emerging data provide numerous interesting exceptions to these long-standing theories, and suggest that the remarkable diversity of sex chromosomes is matched by a similar diversity in their evolution. For example, it is clear that sex chromosome pairs are not always derived from homologous autosomes. In addition, both the cause and the mechanism of recombination suppression between sex chromosome pairs remain unclear, and it may be that the spread of recombination suppression is a more gradual process than previously thought. It is also clear that dosage compensation can be achieved in many ways, and displays a range of efficacy in different systems. Finally, the remarkable turnover of sex chromosomes in many systems, as well as variation in the rate of sex chromosome divergence, suggest that assumptions about the inevitable linearity of sex chromosome evolution are not always empirically supported, and the drivers of the birth-death cycle of sex chromosome evolution remain to be elucidated. Here, we concentrate on how the diversity in sex chromosomes across taxa highlights an equal diversity in each stage of sex chromosome evolution.

Southern Species From the Biodiversity Hotspot of Central Chile: A Source of Color, Aroma, and Metabolites for Global Agriculture and Food Industry in a Scenario of Climate Change

Two interesting plants within the Chilean flora (wild and crop species) can be found with a history related to modern fruticulture: Fragaria chiloensis subsp. chiloensis (Rosaceae) and Vasconcellea pubescens (Caricaceae). Both species have a wide natural distribution, which goes from the Andes mountains to the sea (East-West), and from the Atacama desert to the South of Chile (North-South). The growing locations are included within the Chilean Winter Rainfall-Valdivian Forest hotspot. Global warming is of great concern as it increases the risk of losing wild plant species, but at the same time, gives a chance for usually longer term genetic improvement using naturally adapted material and the source for generating healthy foods. Modern agriculture intensifies the attractiveness of native undomesticated species as a way to provide compounds like antioxidants or tolerant plants for climate change scenario. F. chiloensis subsp. chiloensis as the mother of commercial strawberry (Fragaria × ananassa) is an interesting genetic source for the improvement of fruit flavor and stress tolerance. On the other hand, V. pubescens produces fruit with high level of antioxidants and proteolytic enzymes of interest to the food industry. The current review compiles the botanical, physiological and phytochemical description of F. chiloensis subsp. chiloensis and V. pubescens, highlighting their potential as functional foods and as source of compounds with several applications in the pharmaceutical, biotechnological, and food science. The impact of global warming scenario on the distribution of the species is also discussed.

Retrotransposons in Plant Genomes: Structure, Identification, and Classification through Bioinformatics and Machine Learning

Transposable elements (TEs) are genomic units able to move within the genome of virtually all organisms. Due to their natural repetitive numbers and their high structural diversity, the identification and classification of TEs remain a challenge in sequenced genomes. Although TEs were initially regarded as "junk DNA", it has been demonstrated that they play key roles in chromosome structures, gene expression, and regulation, as well as adaptation and evolution. A highly reliable annotation of these elements is, therefore, crucial to better understand genome functions and their evolution. To date, much bioinformatics software has been developed to address TE detection and classification processes, but many problematic aspects remain, such as the reliability, precision, and speed of the analyses. Machine learning and deep learning are algorithms that can make automatic predictions and decisions in a wide variety of scientific applications. They have been tested in bioinformatics and, more specifically for TEs, classification with encouraging results. In this review, we will discuss important aspects of TEs, such as their structure, importance in the evolution and architecture of the host, and their current classifications and nomenclatures. We will also address current methods and their limitations in identifying and classifying TEs.

Degenerative Expansion of a Young Supergene

Long-term suppression of recombination ultimately leads to gene loss, as demonstrated by the depauperate Y and W chromosomes of long-established pairs of XY and ZW chromosomes. The young social supergene of the Solenopsis invicta red fire ant provides a powerful system to examine the effects of suppressed recombination over a shorter timescale. The two variants of this supergene are carried by a pair of heteromorphic chromosomes, referred to as the social B and social b (SB and Sb) chromosomes. The Sb variant of this supergene changes colony social organization and has an inheritance pattern similar to a Y or W chromosome because it is unable to recombine. We used high-resolution optical mapping, k-mer distribution analysis, and quantification of repetitive elements on haploid ants carrying alternate variants of this young supergene region. We find that instead of shrinking, the Sb variant of the supergene has increased in length by more than 30%. Surprisingly, only a portion of this length increase is due to consistent increases in the frequency of particular classes of repetitive elements. Instead, haplotypes of this supergene variant differ dramatically in the amounts of other repetitive elements, indicating that the accumulation of repetitive elements is a heterogeneous and dynamic process. This is the first comprehensive demonstration of degenerative expansion in an animal and shows that it occurs through nonlinear processes during the early evolution of a region of suppressed recombination.

Molecular insights into the non-recombining nature of the spinach male-determining region

Spinach (Spinacia oleracea L.) is a dioecious plant with male heterogametic sex determination and homomorphic sex chromosomes (XY). The dioecism is utilized for producing commercial hybrid seeds, and hence understanding the molecular-genetic basis of the species' sex determining locus is an important issue for spinach breeding. In this study, seven dominant DNA markers were shown to completely co-segregate with the male-determining gene in segregating spinach populations comprising > 1500 plants. In addition, these seven dominant DNA markers were completely associated with the male-determining gene in over 100 spinach germplasm accessions and cultivars. These observations suggest that, in spinach, a Y-chromosomal region around the male-determining locus does not (or almost not) recombine with a counterpart region on the X chromosome. Using five of the seven DNA markers, five bacterial artificial chromosome (BAC) clone contigs with a total length of approximately 690 kbp were constructed. Full sequencing of six representative BAC clones (total insert length 504 kbp) from the five contigs and a transcriptome analysis by RNA-seq revealed that the Y-chromosomal region around the male-determining locus contains large amounts of repetitive elements, suggesting that the region might be poor in gene content. Most of the repeats found in this region are novel Ty1-copia-like and its derivative elements that accumulate predominantly in heterochromatic regions. Our findings may provide valuable insight into spinach genome structure and clues for future research into the evolution of the sex determining locus.

Impact of Repetitive Elements on the Y Chromosome Formation in Plants

In contrast to animals, separate sexes and sex chromosomes in plants are very rare. Although the evolution of sex chromosomes has been the subject of numerous studies, the impact of repetitive sequences on sex chromosome architecture is not fully understood. New genomic approaches shed light on the role of satellites and transposable elements in the process of Y chromosome evolution. We discuss the impact of repetitive sequences on the structure and dynamics of sex chromosomes with specific focus on Rumex acetosa and Silene latifolia. Recent papers showed that both the expansion and shrinkage of the Y chromosome is influenced by sex-specific regulation of repetitive DNA spread. We present a view that the dynamics of Y chromosome formation is an interplay of genetic and epigenetic processes.

Chromosome Evolution in Connection with Repetitive Sequences and Epigenetics in Plants

Chromosome evolution is a fundamental aspect of evolutionary biology. The evolution of chromosome size, structure and shape, number, and the change in DNA composition suggest the high plasticity of nuclear genomes at the chromosomal level. Repetitive DNA sequences, which represent a conspicuous fraction of every eukaryotic genome, particularly in plants, are found to be tightly linked with plant chromosome evolution. Different classes of repetitive sequences have distinct distribution patterns on the chromosomes. Mounting evidence shows that repetitive sequences may play multiple generative roles in shaping the chromosome karyotypes in plants. Furthermore, recent development in our understanding of the repetitive sequences and plant chromosome evolution has elucidated the involvement of a spectrum of epigenetic modification. In this review, we focused on the recent evidence relating to the distribution pattern of repetitive sequences in plant chromosomes and highlighted their potential relevance to chromosome evolution in plants. We also discussed the possible connections between evolution and epigenetic alterations in chromosome structure and repatterning, such as heterochromatin formation, centromere function, and epigenetic-associated transposable element inactivation.

Impact of repetitive DNA on sex chromosome evolution in plants

Structurally and functionally diverged sex chromosomes have evolved in many animals as well as in some plants. Sex chromosomes represent a specific genomic region(s) with locally suppressed recombination. As a consequence, repetitive sequences involving transposable elements, tandem repeats (satellites and microsatellites), and organellar DNA accumulate on the Y (W) chromosomes. In this paper, we review the main types of repetitive elements, their gathering on the Y chromosome, and discuss new findings showing that not only accumulation of various repeats in non-recombining regions but also opposite processes form Y chromosome. The aim of this review is also to discuss the mechanisms of repetitive DNA spread involving (retro) transposition, DNA polymerase slippage or unequal crossing-over, as well as modes of repeat removal by ectopic recombination. The intensity of these processes differs in non-recombining region(s) of sex chromosomes when compared to the recombining parts of genome. We also speculate about the relationship between heterochromatinization and the formation of heteromorphic sex chromosomes.

Repetitive sequences and epigenetic modification: inseparable partners play important roles in the evolution of plant sex chromosomes

The present review discusses the roles of repetitive sequences played in plant sex chromosome evolution, and highlights epigenetic modification as potential mechanism of repetitive sequences involved in sex chromosome evolution. Sex determination in plants is mostly based on sex chromosomes. Classic theory proposes that sex chromosomes evolve from a specific pair of autosomes with emergence of a sex-determining gene(s). Subsequently, the newly formed sex chromosomes stop recombination in a small region around the sex-determining locus, and over time, the non-recombining region expands to almost all parts of the sex chromosomes. Accumulation of repetitive sequences, mostly transposable elements and tandem repeats, is a conspicuous feature of the non-recombining region of the Y chromosome, even in primitive one. Repetitive sequences may play multiple roles in sex chromosome evolution, such as triggering heterochromatization and causing recombination suppression, leading to structural and morphological differentiation of sex chromosomes, and promoting Y chromosome degeneration and X chromosome dosage compensation. In this article, we review the current status of this field, and based on preliminary evidence, we posit that repetitive sequences are involved in sex chromosome evolution probably via epigenetic modification, such as DNA and histone methylation, with small interfering RNAs as the mediator.

The genomics of plant sex chromosomes

Around six percent of flowering species are dioecious, with separate female and male individuals. Sex determination is mostly based on genetics, but morphologically distinct sex chromosomes have only evolved in a few species. Of these, heteromorphic sex chromosomes have been most clearly described in the two model species - Silene latifolia and Rumex acetosa. In both species, the sex chromosomes are the largest chromosomes in the genome. They are hence easily distinguished, can be physically separated and analyzed. This review discusses some recent experimental data on selected model dioecious species, with a focus on S. latifolia. Phylogenetic analyses show that dioecy in plants originated independently and repeatedly even within individual genera. A cogent question is whether there is genetic degeneration of the non-recombining part of the plant Y chromosome, as in mammals, and, if so, whether reduced levels of gene expression in the heterogametic sex are equalized by dosage compensation. Current data provide no clear conclusion. We speculate that although some transcriptome analyses indicate the first signs of degeneration, especially in S. latifolia, the evolutionary processes forming plant sex chromosomes in plants may, to some extent, differ from those in animals.

Origin and domestication of papaya Yh chromosome

Sex in papaya is controlled by a pair of nascent sex chromosomes. Females are XX, and two slightly different Y chromosomes distinguish males (XY) and hermaphrodites (XY(h)). The hermaphrodite-specific region of the Y(h) chromosome (HSY) and its X chromosome counterpart were sequenced and analyzed previously. We now report the sequence of the entire male-specific region of the Y (MSY). We used a BAC-by-BAC approach to sequence the MSY and resequence the Y regions of 24 wild males and the Y(h) regions of 12 cultivated hermaphrodites. The MSY and HSY regions have highly similar gene content and structure, and only 0.4% sequence divergence. The MSY sequences from wild males include three distinct haplotypes, associated with the populations' geographic locations, but gene flow is detected for other genomic regions. The Y(h) sequence is highly similar to one Y haplotype (MSY3) found only in wild dioecious populations from the north Pacific region of Costa Rica. The low MSY3-Y(h) divergence supports the hypothesis that hermaphrodite papaya is a product of human domestication. We estimate that Y(h) arose only ∼ 4000 yr ago, well after crop plant domestication in Mesoamerica >6200 yr ago but coinciding with the rise of the Maya civilization. The Y(h) chromosome has lower nucleotide diversity than the Y, or the genome regions that are not fully sex-linked, consistent with a domestication bottleneck. The identification of the ancestral MSY3 haplotype will expedite investigation of the mutation leading to the domestication of the hermaphrodite Y(h) chromosome. In turn, this mutation should identify the gene that was affected by the carpel-suppressing mutation that was involved in the evolution of males.

Identification of sex-linked SNP markers using RAD sequencing suggests ZW/ZZ sex determination in Pistacia vera L

Background

Pistachio (Pistacia vera L.) is a dioecious species that has a long juvenility period. Therefore, development of marker-assisted selection (MAS) techniques would greatly facilitate pistachio cultivar-breeding programs. The sex determination mechanism is presently unknown in pistachio. The generation of sex-linked markers is likely to reduce time, labor, and costs associated with breeding programs, and will help to clarify the sex determination system in pistachio.

Results

Restriction site-associated DNA (RAD) markers were used to identify sex-linked markers and to elucidate the sex determination system in pistachio. Eight male and eight female F₁ progenies from a Pistacia vera L. Siirt × Bağyolu cross, along with the parents, were subjected to RAD sequencing in two lanes of a Hi-Seq 2000 sequencing platform. This generated 449 million reads, comprising approximately 37.7 Gb of sequences. There were 33,757 polymorphic single nucleotide polymorphism (SNP) loci between the parents. Thirty-eight of these, from 28 RAD reads, were detected as putative sex-associated loci in pistachio. Validation was performed by SNaPshot analysis in 42 mature F₁ progenies and in 124 cultivars and genotypes in a germplasm collection. Eight loci could distinguish sex with 100% accuracy in pistachio. To ascertain cost-effective application of markers in a breeding program, high-resolution melting (HRM) analysis was performed; four markers were found to perfectly separate sexes in pistachio. Because of the female heterogamety in all candidate SNP loci, we report for the first time that pistachio has a ZZ/ZW sex determination system. As the reported female-to-male segregation ratio is 1:1 in all known segregating populations and there is no previous report of super-female genotypes or female heteromorphic chromosomes in pistachio, it appears that the WW genotype is not viable.

Conclusion

Sex-linked SNP markers were identified and validated in a large germplasm and proved their suitability for MAS in pistachio. HRM analysis successfully validated the sex-linked markers for MAS. For the first time in dioecious pistachio, a female heterogamety ZW/ZZ sex determination system is suggested.