Biology subjects

Comai, L.

Publications and source records attributed to Comai, L..

16 recordsLinked to original sources

bioRxiv2025

Tobacco twin and single spots result from genome instability, chromosome breaks, and translocations

Somatic recombination can have profound consequences including clonal evolution. In plants, however, the contribution of homologous recombination (HR) to genomic instability remains unclear. The semidominant sulfur mutant of tobacco, named for its yellow foliage resulting from a chlorophyll defect, spontaneously develops green and white twin spots that have been attributed to mitotic crossovers. To test this hypothesis, we sequenced DNA from twin spots and surrounding tissues in su/+ F1 hybrids, expecting somatic crossovers to result in reciprocal chromosome arm exchange in the paired sectors. Contrary to expectations, 20 of 22 twin spots exhibited aneuploidy or reciprocal translocations between non-homologous chromosomes. One twin spot showed a pattern consistent with a reciprocal crossover. Single white or green spots were [~]10X more frequent than twin spots. Among 26 single spots, 18 resulted from terminal arm deletion, 2 from aneuploidy, and 4 were associated with translocations. Deletion and reciprocal exchange breakpoints clustered near the centromere. Together, these findings indicate that nearly all spots arise from deletions, translocations, and other rearrangements likely mediated by chromosome missegregation or nonhomologous repair rather than by HR. These rearrangements altered the ratio of mutant to wild-type alleles and consequently chlorophyll levels. We conclude that mitotic recombination between homologous chromosomes is rare, and that genome instability is the dominant driver of somatic chimerism in this system.

genetics↗

bioRxiv2023

A K-mer based Bulked Segregant Analysis approach to map seed traits in unphased heterozygous potato genomes

High-throughput sequencing-based methods for bulked segregant analysis (BSA) allow for the rapid identification of genetic markers associated with traits of interest. BSA studies have successfully identified qualitative (binary) and quantitative trait loci using QTL mapping. However, most traditional QTL mapping approaches require a reference genome. Here, we combine high throughput short read sequencing with bulk segregant analysis of k-mers (BSA-k-mer). This method can be applied to any organism and is particularly useful for species with genomes too different from the closest sequenced genome. It is also instrumental when dealing with highly heterozygous and polyploid genomes without phased haplotype assemblies and for which a single haplotype can control a trait. Finally, it is flexible in terms of population structure. Here, we present the application of the BSA-k-mer method for the rapid identification of candidate regions related to seed spot and seed size in diploid potato. While conventional QTL mapping of parental genotypes did not generate any signal, candidate loci were identified for each trait using the BSA-k-mer approach. The seed spot loci match with previously identified loci associated with pigmentation in potato. The loci associated with seed size are novel. Both sets of loci are potentially instrumental in future breeding towards true seeds in potato. Article SummaryIdentifying genes linked to agronomic traits in potatoes is challenging because potato genomes are complex and variable. We are investigating the genetic basis of seed size and color. Seeds were categorized as large or small, or spotted or not, based on simple visual observation. Next, DNA sequences from each individual were mined for association between random short sequences (k-mers) and those two traits. This more flexible method allowed us to identify regions of the potato genome associated with both traits.

plant biology↗

bioRxiv2023

Differential mutation rates in plant meristematic layers

The upper plant body is formed by the continued growth of shoot apical meristems. In angiosperms, meristems are organized in three cell layers that tend to remain clonally isolated. Somatic variants emerge when mutant cells overtake part or all of a meristem. During sexual reproduction, only one layer, the L2, contributes to the next generation by forming gametes. The L2 is known to frequently invade and replace the L3, while L1-L2 separation is persistent. The role of different meristem layers in mutation accumulation is unknown. We discovered a potato periclinal chimera in which the L2 and L3, but not the L1, are marked by a chromosomal translocation. This enabled the identification of plants regenerated from leaf protoplasts originating from either the L1 or L2+L3. Leveraging these layer-specific clones, we identified and compared mutations that accumulated in the layers in the clonal parent for several decades. Here we show that the L1 accumulates mutations at 1.9 times the rate of the L2, indicating that plants might protect the germline by mechanisms that reduce the rate of mutation accumulation in the L2. In contrast to these layer-specific mutations, we found no evidence of somatic mutation fixation in all three meristem layers. Our findings highlight how periclinal chimeras are formed by independent mutational processes in which a mutation-prone epidermal layer could increase clonal variation.

plant biology↗

bioRxiv2023

Brain infection by wild-type SARS-CoV-2 and the B.1.617.2 and B.1.1.529 variants of concern is a severe outcome in K18-hACE2 transgenic mice

BackgroundSARS-CoV-2 is a respiratory virus with neurological complications including loss of smell and taste, headache, and confusion that can persist for months or longer. Severe neuronal cell damage has also been reported in some cases. The objective of this study was to compare the infectivity of Wild-type virus, Delta and Omicron variants in transgenic mice that express the human angiotensin-converting enzyme 2 (hACE2) receptor under the control of the keratin 18 promoter (K18) and characterize the progression of infection and inflammatory response in the lung, brain medulla oblongata and olfactory bulbs of these animals. We hypothesized that Wild-type, Delta and Omicron differentially infect K18-hACE2 mice, thereby inducing distinct cellular responses. MethodsK18-hACE2 female mice were intranasally infected with Wild-type, Delta, or Omicron variants and euthanized either at 3 days post-infection (dpi) or at the humane endpoint. None of the animals infected with the Omicron variant reached the humane endpoint and were euthanized at day 8 dpi. Virological and immunological analyses were performed in the lungs, olfactory bulbs, medulla oblongata, and brains. ResultsMice infected with Wild-type and Delta display higher levels of viral RNA in the lungs than mice infected with Omicron at 3dpi. Viral RNA levels in the brains of mice infected with the Wild-type virus were however significantly lower than those observed in mice infected with either Delta or Omicron at 3dpi. Viral RNA was also detected in the medulla oblongata of mice infected by all these virus strains at 3dpi. At this time point, mice infected with the Delta virus display a marked upregulation of inflammatory makers both in the lungs and brains. Upregulation of inflammatory markers was also observed in the brains of mice infected with Omicron but not in mice infected with the Wild-type virus, suggesting that during the initial phase of the infection only the Delta and Omicron variants induce strong inflammatory response in the brain. At the humane endpoint/8dpi, mice infected by any of these strains display elevated levels of viral RNA and upregulation of a subset of inflammatory markers in the lungs. There was also a significant increase in viral RNA in the brains of mice infected with Wild-type and Delta, as compared to 3dpi. This was accompanied by an increase in the expression of most cytokines and chemokines. In contrast, mice infected with the Omicron variant showed low levels of viral RNA and downregulation of cytokines and chemokines expression at 8dpi, suggesting that brain inflammation by this variant is attenuated. Reduced RNA levels and downregulation of inflammatory markers was also observed in the medulla oblongata and olfactory bulbs of mice infected with Omicron, while infection by Wild-type and Delta resulted in high levels of viral RNA and increased expression of inflammatory makers in these organs.

microbiology↗

bioRxiv2022

GABA-receptors are a new druggable target for limiting disease severity, lung viral load, and death in SARS-CoV-2 infected mice

GABA-receptors (GABA-Rs) are well-known neurotransmitter receptors in the central nervous system. GABA-Rs are also expressed by immune cells and lung epithelial cells and GABA-R agonists/potentiators reduce inflammatory immune cell activities and limit acute lung injuries. Notably, plasma GABA levels are reduced in hospitalized COVID-19 patients. Hence, GABA-R agonists may have therapeutic potential for treating COVID-19. Here, we show that oral GABA treatment initiated just after SARS-CoV-2 infection, or 2 days later near the peak of lung viral load, reduced disease severity, lung coefficient index, and death rates in K18-hACE2 mice. GABA-treated mice had a reduced viral load in their lungs and displayed shifts in their serum cytokine and chemokine levels that are associated with better outcomes in COVID-19 patients. Thus, GABA-R activation had multiple beneficial effects in this mouse model which are also desirable for the treatment of COVID-19. A number of GABA-R agonists are safe for human use and can be readily tested in clinical trials with COVID-19 patients. We also discuss their potential for limiting COVID-19-associated neuroinflammation.

microbiology↗

bioRxiv2022

Chromoanagenesis in the asy1 meiotic mutant of Arabidopsis

Chromoanagenesis is a catastrophic event that involves localized chromosomal shattering and reorganization. In this study, we report a case of chromoanagenesis resulting from defective meiosis in the MEIOTIC ASYNAPTIC MUTANT 1 (asy1) background in Arabidopsis thaliana. We provide a detailed characterization of the genomic structure of this individual with a severely shattered segment of chromosome 1. We reveal more than 300 candidate novel DNA junctions in the affected region, confirming that asy1-related defective meiosis is a potential trigger for chromoanagenesis. This is the first example of chromoanagenesis associated with female meiosis and indicates the potential for genome evolution during oogenesis.

genomics↗

bioRxiv2022

The Arabidopsis Hop1 homolog ASY1 mediates cross-over assurance and interference

The chromosome axis plays a crucial role in meiotic recombination. Here, we study the function of ASY1, the Arabidopsis homolog of the yeast chromosome axis associated component Hop1. Specifically, we characterized cross-over (CO) distribution in female and male meiosis by deep sequencing of the progeny of an allelic series of asy1 mutants. Combining data from nearly 1000 individual plants, we find that reduced ASY1 activity leads to genomic instability and sometimes drastic genomic rearrangements. We further observed that COs are less frequent and appear in more distal chromosomal regions in plants with no or reduced ASY1 activity, consistent with previous analyses. However, our sequencing approach revealed that the reduction in CO number is not as dramatic as suggested by cytological analyses. Analysis of double mutants of asy1 with mutants with three other CO factors, MUS81, MSH4 and MSH5 as well as the determination of foci number of the CO regulator MLH1 demonstrates that the majority of the COs in asy1, similar to the situation in the wildtype, largely belong to the class I, which are subject to interference. However, these COs are redistributed in asy1 mutants and typically appear much closer than in the wildtype. Hence, ASY1 plays a key role in CO interference that spaces COs along a chromosome. Conversely, since a large proportion of chromosomes do not receive any CO, we conclude that CO assurance, the process that ensures the obligatory assignment of one CO per chromosome, is also affected in asy1 mutants. Significant statementThe regulation of the number and placement of cross-overs (COs) during meiosis is critical to ensure meiotic fidelity and promote new genetic combinations. Here, we investigated the function of one of the proteins of the chromosome axis, which plays a key role in CO formation: ASY1. Our results show that COs in asy1 mutants are positioned closer to each other than in the wildtype and that, despite a roughly similar number of COs, not every chromosome receives a CO. With this, our results shed light on the mechanisms regulating two important but still poorly understood aspects of meiosis: CO assurance, which safeguards at least one CO per chromosome pair, and CO interference, which prevents two COs from occurring close to each other.

cell biology↗

bioRxiv2021

Accurate Direct PCR with Arabidopsis and rice

Plant sampling methods for PCR that avoid costly and time-consuming plant DNA purification can be accurate but have not been widely adopted, partly because the efficacy of the methods is unclear. Here, we introduce new sampling methods for Direct PCR in Arabidopsis and rice and compare them to previously published methods. CutTip, stabbing a pipette tip into a plant and depositing the tip of the pipette tip into the reaction buffer, yielded high accuracy for genotyping Arabidopsis and rice. This did not require visible tissue fragments in the reactions. We found that using short thin segments of fishing line, called Line-PCR, to be even more practical and highly accurate. Compared to traditional and other Direct PCR preparation methods, Line-PCR is faster, inexpensive, easier, uses less plastic and chemicals, and causes the least tissue damage. CutTip and Line-PCR require no modified PCR reagents. Line-PCR particularly lacks any complicated steps. Probably, the most difficult step in Line-PCR is a wipe of the tweezer between samples. These methods can help address the challenge of genotyping at different scales with high accuracy. CutTip and Line-PCR also simplify the application of PCR in the field.

molecular biology↗

bioRxiv2021

LD-CNV: rapid and simple discovery of chromosomal translocations using linkage disequilibrium between copy number variable loci

Large scale structural variations, such as chromosomal translocations, can have profound effects on fitness and phenotype, but are difficult to identify and characterize. Here, we describe a simple and effective method aimed at identifying translocations using only the dosage of sequence reads mapped on the reference genome. We binned reads on genomic segments sized according to sequencing coverage and identified instances when copy number segregated in populations. For each dosage-polymorphic 1Mb bin, we tested linkage disequilibrium with other variable bins. In nine potato (Solanum tuberosum) dihaploid families translocations affecting pericentromeric regions were common and in two cases were due to genomic misassembly. In two populations, we found evidence for translocation affecting euchromatic arms. In cv. PI 310467, a non-reciprocal translocation between chromosome 7 and 8 resulted in a 5-3 copy number change affecting several Mb at the respective chromosome tips. In cv. "Alca Tarma", the terminal arm of chromosome 4 translocated to the tip of chromosome 1. Using oligonucleotide-based fluorescent in situ hybridization painting probes (oligo-FISH), we tested and confirmed the predicted arrangement in PI 310467. In 192 natural accessions of Arabidopsis thaliana, dosage haplotypes tended to vary continuously and resulted in higher noise, but we identified pericentromeric LD suggesting the effect of repeats. This method should be useful in species where translocations are suspected because it tests linkage without the need for genotyping.

genomics↗

bioRxiv2021

Biased removal and loading of centromeric histone H3 during reproduction underlies uniparental genome elimination

Uniparental genome elimination is a dramatic case of centromeric failure, resulting in the postzygotic loss of a parental chromosome set. Genome partitioning during cell division relies on centromere attachment to spindle fibers through kinetochores. Centromeres are epigenetically specified by CENP-A (CENH3), a conserved centromeric specific histone H3 variant. In Arabidopsis, CENH3 modification results in haploid inducers, whose genome is eliminated frequently when crossed to the wild type. To investigate the underlying mechanism, we dissected the timing and molecular features of genome elimination. In zygotes and early embryos from genome elimination crosses, CENH3 occupied only the centromeres contributed by the wild-type parent. Haploid inducer chromosomes had defective kinetochores and missegregated, often forming micronuclei. This uniparental loss of centromere identity is initiated by the removal of altered CENH3 at fertilization, while wild-type CENH3 persists and maintains strong centromeric identity. Weak centromeres were capable of rebuilding functional kinetochores, but often failed when in competition with normal ones. We induced a similar weak state by mitotic dilution of wild-type CENH3. Furthermore, weakness was suppressed by crosses of haploid inducers to other variants of haploid inducers, and enhanced by mutations in VIM1, a ubiquitin ligase known to modify CENH3 and centromeric DNA methylation.. The differential stability of altered CENH3 during reproduction has important genetic and evolutionary implications.

cell biology↗

bioRxiv2020

Rare instances of haploid inducer DNA in potato dihaploids and ploidy-dependent genome instability

In cultivated tetraploid potato, reduction to diploidy (dihaploidy) allows hybridization to diploid germplasm, introgression breeding, and may facilitate the production of inbreds. Pollination with haploid inducers yields maternal dihaploids, as well as triploid and tetraploid hybrids. It is not known if dihaploids result from parthenogenesis, entailing development of embryos from unfertilized eggs, or genome elimination, entailing missegregation and loss of paternal chromosomes. A sign of genome elimination is the occasional persistence of haploid inducer DNA in some of the dihaploids. We characterized the genomes of 1,001 putative dihaploids and 134 hybrids produced by pollinating tetraploid clones with three haploid inducers, IVP35, IVP101, and PL4. We detected inheritance of full or partial chromosomes from the haploid inducer parent in 0.87% of the overall dihaploid progeny, irrespective of the combination of parental genotypes. Chromosomal breaks commonly affected the paternal genome in the dihaploid and tetraploid progeny, but not in the triploid progeny. Residual haploid inducer DNA is consistent with genome elimination as the mechanism of haploid induction. Further, the fact that paternal chromosome breaks are specific to dihaploids and tetraploid progeny suggests that they may be specific to 2x sperms, and supports the hypothesis that 2x sperms facilitate genome elimination.

plant biology↗

bioRxiv2020

Unequal contribution of two paralogous centromeric histones to function the cowpea centromere

The legume cowpea (Vigna unguiculata, 2n=2x=22) has significant tolerance to drought and heat stress. Here we analysed and manipulated cowpea centromere-specific histone H3 (CENH3) genes, aiming to establish a centromere-based doubled-haploid method for use in genetic improvement of this dryland crop in future. Cowpea encodes two functional CENH3 variants (CENH3.1 and CENH3.2) and two CENH3 pseudogenes. Phylogenetic analysis suggests that gene duplication of CENH3 occurred independently during the speciation of V. unguiculata and the related V. mungo without a genome duplication event. Both functional cowpea CENH3 variants are transcribed, and the corresponding proteins are intermingled in subdomains of different types of centromere sequences in a tissue-specific manner together with the outer kinetochore protein CENPC. CENH3.2 is removed from the generative cell of mature pollen, while CENH3.1 persists. Differences between both CENH3 paralogs are restricted to the N-terminus. The complete CRISPR/Cas9-based inactivation of CENH3.1 resulted in delayed vegetative growth and sterility, indicating that this variant is needed for plant development and reproduction. By contrast, CENH3.2 knockout individuals did not show obvious defects during vegetative and reproductive development, suggesting that the gene is an early stage of subfunctionalization or pseudogenization. One-sentence summaryThe two paralogous centromeric histones (CENH3) of cowpea contribute unequal to the function of the centromere.

genetics↗

bioRxiv2019

Genomic Outcomes of Haploid Induction Crosses in Potato (Solanum tuberosum L.)

The challenges of breeding autotetraploid potato (Solanum tuberosum) have motivated the development of alternative breeding strategies. A common approach is to obtain uniparental dihaploids from a tetraploid of interest through pollination with S. tuberosum Andigenum Group (formerly S. phureja) cultivars. The mechanism underlying haploid formation of these crosses is unclear, and questions regarding the frequency of paternal DNA transmission remain. Previous reports described aneuploid and euploid progeny, which, in some cases, displayed genetic markers from the haploid inducer. Here, we surveyed a population of 167 presumed dihaploids for large-scale structural variation that would underlie chromosomal addition from the haploid inducer, and for small-scale introgression of genetic markers. In 19 progeny, we detected ten of the twelve possible trisomies and, in all cases, demonstrated the non-inducer parent origin of the additional chromosome. Deep sequencing indicated that occasional, short-tract signals appearing of haploid inducer origin were better explained as technical artifacts. Leveraging recurring CNV patterns, we documented sub-chromosomal dosage variation indicating segregation of polymorphic maternal haplotypes. Collectively, 52% of assayed chromosomal loci were classified as dosage variable. Our findings help elucidate the genomic consequences of potato haploid induction and suggest that most potato dihaploids will be free of residual pollinator DNA.

genomics↗

bioRxiv2019

The persimmon genome reveals clues to the evolution of a lineage-specific sex determination system in plants

Most angiosperms bear hermaphroditic flowers, but a few species have evolved outcrossing strategies, such as dioecy, the presence of separate male and female individuals. We previously investigated the mechanisms underlying dioecy in diploid persimmon (D. lotus) and found that male flowers are specified by repression of the autosomal gene MeGI by its paralog, the Y-encoded pseudo-gene OGI. This mechanism is thought to be lineage-specific, but its evolutionary path remains unknown. Here, we developed a full draft of the diploid persimmon genome (D. lotus), which revealed a lineage-specific genome-wide paleoduplication event. Together with a subsequent persimmon-specific duplication(s), these events resulted in the presence of three paralogs, MeGI, OGI and newly identified Sister of MeGI (SiMeGI), from the single original gene. Evolutionary analysis suggested that MeGI underwent adaptive evolution after the paleoduplication event. Transformation of tobacco plants with MeGI and SiMeGI revealed that MeGI specifically acquired a new function as a repressor of male organ development, while SiMeGI presumably maintained the original function. Later, local duplication spawned MeGIs regulator OGI, completing the path leading to dioecy. These findings exemplify how duplication events can provide flexible genetic material available to help respond to varying environments and provide interesting parallels for our understanding of the mechanisms underlying the transition into dieocy in plants.\n\nAuthor summaryPlant sexuality has fascinated scientists for decades. Most plants can self-reproduce but not all. For example, a small subset of species have evolved a system called dioecy, with separate male and female individuals. Dioecy has evolved multiple times independently and, while we do not understand the molecular mechanisms underlying dioecy in many of these species yet, a picture is starting to emerge with recent progress in several dioecious species. Here, we focused on the evolutionary events leading to dioecy in persimmon. Our previous work had identified a pair of genes regulating sex in this species, called OGI and MeGI. We drafted the whole genome sequence of diploid persimmon to investigate their evolutionary history. We discovered a lineage-specific genome duplication event, and observed that MeGI underwent adaptive evolution after this duplication. Transgenic analyses validated that MeGI newly acquired a male-suppressor function, while the other copy of this gene, SiMeGI, did not. The regulator of MeGI, OGI, resulted from a second smaller-scale duplication event, finalizing the system. This study sheds light on the role of duplication as a mechanism that promote flexible genes functions, and how it can affect important biological functions, such as the establishment of a new sexual system.

evolutionary biology↗

bioRxiv2018

Translocation and duplication from CRISPR-Cas9 editing in Arabidopsis thaliana

Cut DNA ends in plants may recombine to form novel molecules. We asked whether CRISPR-Cas9 expression in plants could induce nonhomologous recombination between diverse and heterologous broken DNA ends. We induced two breaks separated by 2.3 or by 8.5 kilobases leading to duplication of the intervening DNA and meiotic transmission of the 2.3kb duplication. Two or more dsDNA breaks in nonhomologous chromosomes led to ligation of breakpoints consistent with chromosome arm translocations. Screening 881 primary transformants we obtained 195 PCR products spanning independent, expected translocation junctions involving ends produced by cutting different loci. Sequencing indicated a true positive rate of 84/91 and demonstrated the occurrence of different junction alleles. A majority of the resulting structures would be deleterious and none were transmitted meiotically. Ligation of interchromosomal, heterologous dsDNA ends suggest that the CRISPR-Cas9 can be used to engineer plant genes and chromosomes in vivo.\n\nSignificance StatementWe explored how genome editing tools such as CRISPR-Cas9 could provide new ways to tailor novel genomic combinations and arrangements. We show that distant cut ends often precisely come together, that cuts in different chromosomes can result in translocations, and that two cuts within a chromosome often result in the duplication of the intervening segment. Formation of multiple structures with precise junctions will enable engineered rearrangements that can be predicted with accuracy.

genetics↗

bioRxiv2018

Widespread genome instability in Solanum tuberosum plants regenerated from protoplasts

Non-transgenic genome editing in regenerable protoplasts, cell-wall free plant cells, could revolutionize crop improvement because it reduces regulatory and technical complexity. But, plant tissue culture is known to engender frequent unwanted variation. To evaluate the contribution of genome instability to this phenomenon, we analyzed large scale copy number changes in potatoes regenerated from protoplasts by comparison of Illumina read depth. While a control set of plants that had been propagated by cuttings displayed no changes, the protoplast regenerants were affected by pervasive aneuploidy. In addition, certain chromosomes displayed segmental deletions and duplications ranging from one to many. Resampling the same plant found different dosage profiles in different leaves, indicating frequent persistence of instability.

genomics↗