RAD51AP2 is required for efficient meiotic recombination between X and Y chromosomes

RAD51AP2 is required specifically for efficient meiotic recombination to form crossover between X and Y chromosomes.

Unique structure and positive selection promote the rapid divergence of Drosophila Y chromosomes

Y chromosomes across diverse species convergently evolve a gene-poor, heterochromatic organization enriched for duplicated genes, LTR retrotransposons, and satellite DNA. Sexual antagonism and a loss of recombination play major roles in the degeneration of young Y chromosomes. However, the processes shaping the evolution of mature, already degenerated Y chromosomes are less well-understood. Because Y chromosomes evolve rapidly, comparisons between closely related species are particularly useful. We generated de novo long read assemblies complemented with cytological validation to reveal Y chromosome organization in three closely related species of the Drosophila simulans complex, which diverged only 250,000 years ago and share >98% sequence identity. We find these Y chromosomes are divergent in their organization and repetitive DNA composition and discover new Y-linked gene families whose evolution is driven by both positive selection and gene conversion. These Y chromosomes are also enriched for large deletions, suggesting that the repair of double-strand breaks on Y chromosomes may be biased toward microhomology-mediated end joining over canonical non-homologous end-joining. We propose that this repair mechanism contributes to the convergent evolution of Y chromosome organization across organisms.

An Infertile Azoospermic Male with 45, X T(Yp;15) Karyotype

Objective: 45, X is a very rare condition that usually results from Y/autosomal translocations or insertions. Here we present an infertile azoospermic man who had 45, X t(Yp;15) karyotype and deletion of AZF (azoospermia factor) gene region. Case report: A 35-year-old infertile azoospermic man with a typical male appearance came for infertility genetic counseling. He was infertile for more than ten years and had short height. High-resolution of metaphase chromosomes of 50 peripheral white blood cells were analyzed for karyotyping. Fluorescence in situ hybridization (FISH) analysis and Polymerase chain reaction (PCR) were done for SRY and AZF gene localization. Karyotyping and FISH analysis revealed 45, X t(Yp;15) karyotype and no mosaicism. More investigation on the Y chromosome revealed no deletion in the SRY region, but AZF a/b/c were deleted. It was revealed that Yp's subtelomeric region but not Yq was translocated to chromosome 15. Conclusion: This study shows that despite the lack of a complete Y chromosome in this person, the occurrence of secondary male traits is a result of the short arm translocation of the Y chromosome, which contains the (ex-determining region Y) SRY gene. Infertility is also due to the Y chromosomes long arm's deletion containing the AZF gene region.

Molecular mechanisms and topological consequences of drastic chromosomal rearrangements of muntjac deer

Muntjac deer have experienced drastic karyotype changes during their speciation, making it an ideal model for studying mechanisms and functional consequences of mammalian chromosome evolution. Here we generated chromosome-level genomes for Hydropotes inermis (2n = 70), Muntiacus reevesi (2n = 46), female and male M. crinifrons (2n = 8/9) and a contig-level genome for M. gongshanensis (2n = 8/9). These high-quality genomes combined with Hi-C data allowed us to reveal the evolution of 3D chromatin architectures during mammalian chromosome evolution. We find that the chromosome fusion events of muntjac species did not alter the A/B compartment structure and topologically associated domains near the fusion sites, but new chromatin interactions were gradually established across the fusion sites. The recently borne neo-Y chromosome of M. crinifrons, which underwent male-specific inversions, has dramatically restructured chromatin compartments, recapitulating the early evolution of canonical mammalian Y chromosomes. We also reveal that a complex structure containing unique centromeric satellite, truncated telomeric and palindrome repeats might have mediated muntjacs’ recurrent chromosome fusions. These results provide insights into the recurrent chromosome tandem fusion in muntjacs, early evolution of mammalian sex chromosomes, and reveal how chromosome rearrangements can reshape the 3D chromatin regulatory conformations during species evolution.

New Genes in the Drosophila Y Chromosome: Lessons from D. willistoni

Y chromosomes play important roles in sex determination and male fertility. In several groups (e.g., mammals) there is strong evidence that they evolved through gene loss from a common X-Y ancestor, but in Drosophila the acquisition of new genes plays a major role. This conclusion came mostly from studies in two species. Here we report the identification of the 22 Y-linked genes in D. willistoni. They all fit the previously observed pattern of autosomal or X-linked testis-specific genes that duplicated to the Y. The ratio of gene gains to gene losses is ~25 in D. willistoni, confirming the prominent role of gene gains in the evolution of Drosophila Y chromosomes. We also found four large segmental duplications (ranging from 62 kb to 303 kb) from autosomal regions to the Y, containing ~58 genes. All but four of these duplicated genes became pseudogenes in the Y or disappeared. In the GK20609 gene the Y-linked copy remained functional, whereas its original autosomal copy degenerated, demonstrating how autosomal genes are transferred to the Y chromosome. Since the segmental duplication that carried GK20609 contained six other testis-specific genes, it seems that chance plays a significant role in the acquisition of new genes by the Drosophila Y chromosome.

Muller’s ratchet of the Y chromosome with gene conversion

Muller’s ratchet is a process in which deleterious mutations are fixed irreversibly in the absence of recombination. The degeneration of the Y chromosome, and the gradual loss of its genes, can be explained by Muller’s ratchet. However, most theories consider single-copy genes, and may not be applicable to Y chromosomes, which have a number of duplicated genes in many species, which are probably undergoing concerted evolution by gene conversion. We developed a model of Muller’s ratchet to explore the evolution of the Y chromosome. The model assumes a non-recombining chromosome with both single-copy and duplicated genes. We used analytical and simulation approaches to obtain the rate of gene loss in this model, with special attention to the role of gene conversion. Homogenization by gene conversion makes both duplicated copies either mutated or intact. The former promotes the ratchet, and the latter retards, and we ask which of these counteracting forces dominates under which conditions. We found that the effect of gene conversion is complex, and depends upon the fitness effect of gene duplication. When duplication has no effect on fitness, gene conversion accelerates the ratchet of both single-copy and duplicated genes. If duplication has an additive fitness effect, the ratchet of single-copy genes is accelerated by gene duplication, regardless of the gene conversion rate, whereas gene conversion slows the degeneration of duplicated genes. Our results suggest that the evolution of the Y chromosome involves several parameters, including the fitness effect of gene duplication by increasing dosage and gene conversion rate.

Unusual Mammalian Sex Determination Systems: A Cabinet of Curiosities

Therian mammals have among the oldest and most conserved sex-determining systems known to date. Any deviation from the standard XX/XY mammalian sex chromosome constitution usually leads to sterility or poor fertility, due to the high differentiation and specialization of the X and Y chromosomes. Nevertheless, a handful of rodents harbor so-called unusual sex-determining systems. While in some species, fertile XY females are found, some others have completely lost their Y chromosome. These atypical species have fascinated researchers for over 60 years, and constitute unique natural models for the study of fundamental processes involved in sex determination in mammals and vertebrates. In this article, we review current knowledge of these species, discuss their similarities and differences, and attempt to expose how the study of their exceptional sex-determining systems can further our understanding of general processes involved in sex chromosome and sex determination evolution.

Sex chromosomes, sex ratios and sex gaps in longevity in plants

In animals, males and females can display markedly different longevity (also called sex gap in longevity, SGLs). Recent work has revealed that sex chromosomes contribute to establishing these SGLs. X-hemizygosity and toxicity of the Y chromosomes are two mechanisms that have been suggested to reduce male longevity (Z-hemizygosity and W toxicity in females in ZW systems). In plants, SGLs are known to exist but the role of sex chromosomes remains to be established. Here, by using adult sex ratio as a proxy for measuring SGLs, we explored the relationship between sex chromosome and SGLs across 43 plant species. Based on the knowledge recently accumulated in animals, we specifically asked whether: (i) species with XY systems tend to have female-biased sex ratios (reduced male longevity) and species with ZW ones tend to have male-biased sex ratios (reduced female longevity), and (ii) this patterns was stronger in heteromorphic systems compared to homomorphic ones. Our results tend to support these predictions although we lack statistical power because of a small number of ZW systems and the absence of any heteromorphic ZW system in the dataset. We discuss the implications of these findings, which we hope will stimulate further research on sex-differences in lifespan and ageing across plants.

A procedure to detect 6 basic STSs and 11 extended STSs in the AZF region using multiplex PCR

Microdeletions of Y chromosomes frequently occur in 3 subregions of the AZF, namely, AZFa, AZFb, and AZFc, with 6 basic STS marker sequences, which are sY84, sY86 (AZFa), sY127, sY134 (AZFb), and sY254, sY255 (AZFc). According to EAA/EMNQ guidelines, 11 additional AZFabc marker sequences should be used to determine the extent of the microdeletion in the AZF region of infertile men, which is known as 11 extended STSs. By applying mPCR, the authors develop an optimal detection procedure for the 6 basic STS and 11 extended STS using 3 multiplex PCR reactions. The first multiplex PCR reaction includes 6 basic STS plus the 2 control sequences sex-determining region Y (SRY) and zinc finger protein X/Y-linked (ZFX/Y). The second multiplex PCR reaction includes the 6 extended STS sY88, sY1182, sY105, sY121, sY1191, and sY1291 and the 2 control sequences SRY and ZFX/Y. The third multiplex PCR reaction includes the 5 extended STS sY153, sY160, sY82, sY143, and sY83 and the 2 control sequences SRY and ZFX/Y. Six basic primer sequences and eleven extended primer sequences are redesigned to simultaneously pair and amplify STS in the same multiplex reaction: set of 8 primers for 6 basic STS: 6 basic STS + 2 (SRY, ZFX/Y), 8 extension primers set E1: 6 extended STS + 2 (SRY, ZFX/Y), and 7 extension primers set E2: 5 extended STS + 2 (SRY, ZFX/Y). We successfully designed primer pairs with high specificity and stability and successfully amplified 6 basic STS and 11 extended STS, which ensures that the STSs have the correct sequence as recommended by EAA/EMQN and are consistent with the NCBI gene bank. This study has successfully developed a procedure to simultaneously detect 17 STSs, including 6 basic STSs and 11 extended STSs in the AZF region using 3 multiplex PCR reactions.