Chromosome Research最新文献

Most plant species have three or more SPO11/TOPOVIA homologs and two TOPOVIB homologs, which associate to trigger meiotic double-strand break (DSB) formation and subsequent meiotic recombination. In Zea mays L. (maize), ZmSPO11-1 and ZmMTOPVIB have been reported to be indispensable for the initiation of meiotic recombination, yet the function of ZmSPO11-2 remains unclear. In this study, we characterized meiotic functions of ZmSPO11-2 during male meiosis in maize. Two independent Zmspo11-1 knock-out mutants exhibited normal vegetative growth but both male and female sterility. The formation of meiotic DSBs of DNA molecules was fully abolished in the Zmspo11-2 plants, leading to the defective homologous chromosome paring, synapsis, recombination, and segregation. However, the bipolar spindle assembly was not noticeably affected in Zmspo11-2 meiocytes. Overall, our results demonstrate that as its partner ZmSPO11-1 and ZmMTOPVIB, ZmSPO11-2 plays essential roles in DSB formation and homologous recombination in maize meiosis.

Imprinted X chromosome inactivation (iXCI) balances the expression of X-linked genes in preimplantation embryos and extraembryonic tissues in rodents. Long noncoding Xist RNA drives iXCI, silencing genes and recruiting Xist-dependent chromatin repressors. Some domains on the inactive X chromosome include repressive modifications specific to constitutive heterochromatin, which show no direct link to Xist RNA. We explored the relationship between Xist RNA and chromatin silencing during iXCI in vole Microtus levis. We performed locus-specific activation of Xist transcription on the only active X chromosome using the dCas9-SAM system in XO vole trophoblast stem cells (TSCs), which allow modeling iXCI events to some extent. The artificially activated endogenous vole Xist transcript is truncated and restricted ~ 6.6 kb of the exon 1. Ectopic Xist RNA accumulates on the X chromosome and recruits Xist-dependent modifications during TSC differentiation, yet is incapable by itself repressing X-linked genes. Transcriptional silencing occurs upon ectopic Xist upregulation only when repressive marks spread from the massive telomeric constitutive heterochromatin to the X chromosome region containing genes. We hypothesize that the Xist RNA-induced propagation of repressive marks from the constitutive heterochromatin could be a mechanism involved in X chromosome inactivation.

Unisexual reproduction, which generates clonal offspring, is an alternative strategy to sexual breeding and occurs even in vertebrates. A wide range of non-sexual reproductive modes have been described, and one of the least understood questions is how such pathways emerged and how they mechanistically proceed. The Amazon molly, Poecilia formosa, needs sperm from males of related species to trigger the parthenogenetic development of diploid eggs. However, the mechanism, of how the unreduced female gametes are produced, remains unclear. Cytological analyses revealed that the chromosomes of primary oocytes initiate pachytene but do not proceed to bivalent formation and meiotic crossovers. Comparing ovary transcriptomes of P. formosa and its sexual parental species revealed expression levels of meiosis-specific genes deviating from P. mexicana but not from P. latipinna. Furthermore, several meiosis genes show biased expression towards one of the two alleles from the parental genomes. We infer from our data that in the Amazon molly diploid oocytes are generated by apomixis due to a failure in the synapsis of homologous chromosomes. The fact that this failure is not reflected in the differential expression of known meiosis genes suggests the underlying molecular mechanism may be dysregulation on the protein level or misexpression of a so far unknown meiosis gene, and/or hybrid dysgenesis because of compromised interaction of proteins from diverged genomes.

Studying the probability distribution of replication initiation along a chromosome is a huge challenge. Drosophila polytene chromosomes in combination with super-resolution microscopy provide a unique opportunity for analyzing the probabilistic nature of replication initiation at the ultrastructural level. Here, we developed a method for synchronizing S-phase induction among salivary gland cells. An analysis of the replication label distribution in the first minutes of S phase and in the following hours after the induction revealed the dynamics of replication initiation. Spatial super-resolution structured illumination microscopy allowed identifying multiple discrete replication signals and to investigate the behavior of replication signals in the first minutes of the S phase at the ultrastructural level. We identified replication initiation zones where initiation occurs stochastically. These zones differ significantly in the probability of replication initiation per time unit. There are zones in which initiation occurs on most strands of the polytene chromosome in a few minutes. In other zones, the initiation on all strands takes several hours. Compact bands are free of replication initiation events, and the replication runs from outer edges to the middle, where band shapes may alter.

The B chromosome (B) is a dispensable component of the genome in many species. To evaluate the impact of Bs on the transcriptome of the standard A chromosomes (A), comparative RNA-seq analyses of rye and wheat anthers with and without additional rye Bs were conducted. In both species, 5-6% of the A-derived transcripts across the entire genomes were differentially expressed in the presence of  2Bs. The GO term enrichment analysis revealed that Bs influence A chromosome encoded processes like "gene silencing"; "DNA methylation or demethylation"; "chromatin silencing"; "negative regulation of gene expression, epigenetic"; "post-embryonic development"; and "chromosome organization." 244 B chromosome responsive A-located genes in + 2B rye and + B wheat shared the same biological function. Positively correlated with the number of Bs, 939 and 1391 B-specific transcripts were identified in + 2B and + 4B wheat samples, respectively. 85% of B-transcripts in + 2B were also found in + 4B transcriptomes. 297 B-specific transcripts were identified in + 2B rye, and 27% were common to the B-derived transcripts identified in + B wheat. Bs encode mobile elements and housekeeping genes, but most B-transcripts were without detectable similarity to known genes. Some of these genes are involved in cell division-related functions like Nuf2 and might indicate their importance in maintaining Bs. The transcriptome analysis provides new insights into the complex interrelationship between standard A chromosomes and supernumerary B chromosomes.

Meiotic drive occurs when one allele at a heterozygous site cheats its way into a disproportionate share of functional gametes, violating Mendel's law of equal segregation. This genetic conflict typically imposes a fitness cost to individuals, often by disrupting the process of gametogenesis. The evolutionary impact of meiotic drive is substantial, and the phenomenon has been associated with infertility and reproductive isolation in a wide range of organisms. However, cases of meiotic drive in humans remain elusive, a finding that likely reflects the inherent challenges of detecting drive in our species rather than unique features of human genome biology. Here, we make the case that house mice (Mus musculus) present a powerful model system to investigate the mechanisms and consequences of meiotic drive and facilitate translational inferences about the scope and potential mechanisms of drive in humans. We first detail how different house mouse resources have been harnessed to identify cases of meiotic drive and the underlying mechanisms utilized to override Mendel's rules of inheritance. We then summarize the current state of knowledge of meiotic drive in the mouse genome. We profile known mechanisms leading to transmission bias at several established drive elements. We discuss how a detailed understanding of meiotic drive in mice can steer the search for drive elements in our own species. Lastly, we conclude with a prospective look into how new technologies and molecular tools can help resolve lingering mysteries about the prevalence and mechanisms of selfish DNA transmission in mammals.

The ability to subvert independent assortment of chromosomes is found in many meiotic drivers, such as the t haplotype in house mice Mus musculus, in which the t-bearing chromosomal homolog is preferentially transmitted to offspring. This is explained by a poison-antidote system, in which developing + and t sperm in testes of + /t males are exposed to 'poison' coded by t loci, from which t sperm are protected, allowing t sperm an overwhelming fertilisation advantage in monogamous matings. This system is thought to result in poorly and normally motile sperm subpopulations within + /t sperm, leaving t sperm unharmed. Conversely, we found that the fastest quartile of sperm from + /t males swam more slowly, both forwards and along their travel path, and had reduced straightness and linearity, compared to the fastest quartile of + / + sperm. Moreover, sperm from + /t males had shorter tails and narrower heads than + / + sperm, and these morphological differences covaried with motility differences. Finally, + /t traits did not show evidence of bimodal distributions. We conclude that the t haplotype drive results in lasting damage to the motility of both + and t developing sperm, although previous studies indicate that + must be more harmed than t sperm. This damage to all sperm may explain the low success of + /t males in sperm competition with + / + males, seen in earlier studies. We propose that the harm the t causes to itself could be termed 'spiteful', which may also be common to other gamete-harming meiotic drive systems.