Sexually antagonistic evolution of mitochondrial and nuclear linkage

Across eukaryotes, genes encoding bioenergetic machinery are located in both mitochondrial and nuclear DNA, and incompatibilities between the two genomes can be devastating. Mitochondria are often inherited maternally, and theory predicts sex-specific fitness effects of mitochondrial mutational diversity. Yet how evolution acts on linkage patterns between mitochondrial and nuclear genomes is poorly understood. Using novel mito-nuclear population genetic models, we show that the interplay between nuclear and mitochondrial genes maintains mitochondrial haplotype diversity within populations, and it selects both for sex-independent segregation of mitochondrion-interacting genes and for paternal leakage. These effects of genetic linkage evolution can eliminate male-harming fitness effects of mtDNA mutational diversity. With maternal mitochondrial inheritance, females maintain a tight mitochondrial-nuclear match, but males accumulate mismatch mutations because of the weak statistical associations between the two genomic components. Sex-independent segregation of mitochondria-interacting loci improves the mito-nuclear match. In a sexually antagonistic evolutionary process, male nuclear alleles evolve to increase the rate of recombination, while females evolve to suppress it. Paternal leakage of mitochondria can evolve as an alternative mechanism to improve the mito-nuclear linkage. Our modelling framework provides an evolutionary explanation for the observed paucity of mitochondrion-interacting genes on mammalian sex chromosomes and for paternal leakage in protists, plants, fungi, and some animals.

Establishing a foundation for large DNA transfer to artificial minichromosomes and B insert platforms in maize

In plants, conventional genetic engineering methods limit the number of available traits that could potentially improve the quality of agriculture. Agrobacterium-mediated transformation and biolistic bombardment are tools used in transferring genes into plant cells, both of which result in random integrations into host genomes. The absence of targeting machinery, together with low DNA carrying capacity on most plasmid vectors, limit researchers to a few genes in a single modification experiment, a process that takes [about]1 year in most plant species. While stacking traits from independent genetic modifications allow for an increase in the number of transgenes in a single plant, recovery of all genes in subsequent generations becomes increasingly difficult due to independent segregation in meiosis. Alternatively, the use of binary bacterial artificial chromosomes (BiBACs), large insert cloning vectors, can maintain and transfer up to 300 kps, but are also subject to random integrations. Therefore, establishment of a BiBAC targeting system would be advantageous for researchers focusing on creating plant lines that contain several genes that work together to express complex traits, such as disease resistance clusters or whole biosynthetic pathways. Additionally, BiBAC targeting to a location outside the native chromosomal sets, such as an artificial minichromosome or B chromosome platform, would enable researchers to stack traits without disrupting endogenous sequences.

Two independent modes of chromosome organization are revealed by cohesin removal

The three-dimensional organization of chromosomes is tightly related to their biological function 1. Both imaging and chromosome conformation capture studies have revealed several layers of organization 2-4: segregation into active and inactive compartments at the megabase scale 5, and partitioning into domains (TADs) 6,7 and associated loops 8 at the sub-megabase scale. Yet, it remains unclear how these layers of genome organization form, interact with one another, and contribute to or result from genome activities. TADs seem to have critical roles in regulating gene expression by promoting or preventing interactions between promoters and distant cis-acting regulatory elements 9-14, and different architectural proteins, including cohesin, have been proposed to play central roles in their formation 15,16. However, experimental depletions of these proteins have resulted in marginal changes in chromosome organization 17-19. Here, we show that deletion of the cohesin-loading factor, Nipbl, leads to loss of chromosome-associated cohesin and results in dramatic genome reorganization. TADs and associated loops vanish globally, even in the absence of transcriptional changes. In contrast, segregation into compartments is preserved and even reinforced. Strikingly, the disappearance of TADs unmasks a finer compartment structure that accurately reflects the underlying epigenetic landscape. These observations demonstrate that the 3D organization of the genome results from the independent action of two distinct mechanisms: 1) cohesin-independent segregation of the genome into fine-scale compartment regions, defined by the underlying chromatin state; and 2) cohes-dependent formation of TADs possibly by the recently proposed loop extrusion mechanism 20,21, enabling long-range and target-specific activity of promiscuous enhancers. The interplay between these mechanisms creates an architecture that is more complex than a simple hierarchy of layers and can be central to guiding normal development.

Applying Mendelian rules in rapeseed (Brassica napus) breeding

Rapeseed is one of the most important sources of edible oil, raw material for industry, as well as feed. The yield and quality of rapeseed have significantly been improved in recent decades as a result of intensive breeding and optimized production technology. The application of Mendel's rules in introducing monogenic traits has also contributed to success in rapeseed breeding. Rule 1, which refers to the uniformity of F1 generation, is now the basis of widespread development of rapeseed hybrids. Rule 2, dealing with genetic segregation in the F2 generation, is the basis for understanding the process of breeding lines. Rule 3, regarding the independent segregation of genes and traits, while exempting linked traits, is the basis of combining different desirable properties by selection. In the last few decades, the systematic use of Mendel's rules has contributed to the improvement of many properties of rapeseed, including tolerance to biotic and abiotic stress, yield and seed quality. Particular progress has been made in breeding for resistance to diseases, including the identification of molecular markers for marker-assisted selection. The next objective of rapeseed breeding is to create varieties with improved tolerance to environmental stress (e.g. frost, heat, and drought). Based on Mendel's rules, classical breeding methods and the latest developments in the field of molecular genetics and breeding, future progress is expected in the field of rapeseed breeding with an emphasis on polygenic, quantitative traits such as biomass, seed, and oil yield.

Kinetochore-independent chromosome segregation driven by lateral microtubule bundles

During cell division, chromosomes attach to spindle microtubules at sites called kinetochores, and force generated at the kinetochore-microtubule interface is the main driver of chromosome movement. Surprisingly, kinetochores are not required for chromosome segregation on acentrosomal spindles in Caenorhabditis elegans oocytes, but the mechanism driving chromosomes apart in their absence is not understood. In this study, we show that lateral microtubule–chromosome associations established during prometaphase remain intact during anaphase to facilitate separation, defining a novel form of kinetochore-independent segregation. Chromosome dynamics during congression and segregation are controlled by opposing forces; plus-end directed forces are mediated by a protein complex that forms a ring around the chromosome center and dynein on chromosome arms provides a minus-end force. At anaphase onset, ring removal shifts the balance between these forces, triggering poleward movement along lateral microtubule bundles. This represents an elegant strategy for controlling chromosomal movements during cell division distinct from the canonical kinetochore-driven mechanism.

Cohesin-independent segregation of sister chromatids in budding yeast

Cohesin generates cohesion between sister chromatids, which enables chromosomes to form bipolar attachments to the mitotic spindle and segregate. Cohesin also functions in chromosome condensation, transcriptional regulation, and DNA damage repair. Here we analyze the role of acetylation in modulating cohesin functions and how it affects budding yeast viability. Previous studies show that cohesion establishment requires Eco1p-mediated acetylation of the cohesin subunit Smc3p at residue K113. Smc3p acetylation was proposed to promote establishment by merely relieving Wpl1p inhibition because deletion of WPL1 bypasses the lethality of an ECO1 deletion (eco1Δ wpl1Δ). We find that little, if any, cohesion is established in eco1Δ wpl1Δ cells, indicating that Eco1p performs a function beyond antagonizing Wpl1p. Cohesion also fails to be established when SMC3 acetyl-mimics (K113Q or K112R,K113Q) are the sole functional SMC3s in cells. These results suggest that Smc3p acetylation levels affect establishment. It is remarkable that, despite their severe cohesion defect, eco1Δ wpl1Δ and smc3-K112R,K113Q strains are viable because a cohesin-independent mechanism enables bipolar attachment and segregation. This alternative mechanism is insufficient for smc3-K113Q strain viability. Smc3-K113Q is defective for condensation, whereas eco1Δ wpl1Δ and smc3-K112R,K113Q strains are competent for condensation. We suggest that Smc3p acetylation and Wpl1p antagonistically regulate cohesin's essential role in condensation.

Independent Segregation of the Two Arms of the Escherichia coli ori Region Requires neither RNA Synthesis nor MreB Dynamics

ABSTRACT The mechanism of Escherichia coli chromosome segregation remains elusive. We present results on the simultaneous tracking of segregation of multiple loci in the ori region of the chromosome in cells growing under conditions in which a single round of replication is initiated and completed in the same generation. Loci segregated as expected for progressive replication-segregation from oriC, with markers placed symmetrically on either side of oriC segregating to opposite cell halves at the same time, showing that sister locus cohesion in the origin region is local rather than extensive. We were unable to observe any influence on segregation of the proposed centromeric site, migS, or indeed any other potential cis-acting element on either replication arm (replichore) in the AB1157 genetic background. Site-specific inhibition of replication close to oriC on one replichore did not prevent segregation of loci on the other replichore. Inhibition of RNA synthesis and inhibition of the dynamic polymerization of the actin homolog MreB did not affect ori and bulk chromosome segregation.

Molecular characterization of a rare G1P[19] rotavirus strain from India: evidence of reassortment between human and porcine rotavirus strains

This study pertains to the characterization of a human rotavirus strain (NIV929893) with a rare specificity of G1P[19]. Three structural genes (VP4, VP6 and VP7) and one non-structural gene (NSP4) of strain NIV929893 were subjected to RT-PCR for amplification of entire coding regions. All of the amplicons were sequenced to carry out phylogenetic analysis. The complete amino acid sequences of the VP7 and VP4 gene products showed clustering of the VP7 gene with G1 strains of human origin and the VP4 gene with P[19] strains of porcine origin. The two viral proteins VP6 and NSP4, described previously as genetically linked proteins, were shown to be subgroup II and genotype B of human and porcine origins, respectively. The findings of this study provide evidence of reassortment between VP7/VP6 genes of humans and VP4/NSP4 genes of porcine species and an independent segregation of VP6 and NSP4 genes in a group A human rotavirus strain with G1P[19] specificity.