Number Of Crossovers Research Articles

The kinase ATR controls meiotic crossover distribution at the genome scale in Arabidopsis

Abstract Meiotic crossover, i.e., the reciprocal exchange of chromosome fragments during meiosis, is a key driver of genetic diversity. Crossover is initiated by the formation of programmed DNA double-strand breaks (DSBs). While the role of ATAXIA-TELANGIECTASIA AND RAD3-RELATED (ATR) kinase in DNA damage signaling is well-known, its impact on crossover formation remains understudied. Here, using measurements of recombination at chromosomal intervals and genome-wide crossover mapping, we showed that ATR inactivation in Arabidopsis (Arabidopsis thaliana) leads to dramatic crossover redistribution, with an increase in crossover frequency in chromosome arms and a decrease in pericentromeres. These global changes in crossover placement were not caused by alterations in DSB numbers, which we demonstrated by analyzing phosphorylated H2A.X foci in zygonema. Using the seed-typing technique, we found that hotspot usage remains mainly unchanged in atr mutants compared to wild-type individuals. Moreover, atr showed no change in the number of crossovers caused by two independent pathways, which implies no effect on crossover pathway choice. Analyses of genetic interaction indicate that while the effects of atr are independent of MMS AND UV SENSITIVE81 (MUS81), ZIPPER1 (ZYP1), FANCONI ANEMIA COMPLEMENTATION GROUP M (FANCM) and D2 (FANCD2), the underlying mechanism may be similar between ATR and FANCD2. This study extends our understanding of ATR’s role in meiosis, uncovering functions in regulating crossover distribution.

Interference length reveals regularity of crossover placement across species

Crossover interference is a phenomenon that affects the number and positioning of crossovers in meiosis and thus affects genetic diversity and chromosome segregation. Yet, the underlying mechanism is not fully understood, partly because quantification is difficult. To overcome this challenge, we introduce the interference length Lint that quantifies changes in crossover patterning due to interference. We show that it faithfully captures known aspects of crossover interference and provides superior statistical power over previous measures such as the interference distance and the gamma shape parameter. We apply our analysis to empirical data and unveil a similar behavior of Lint across species, which hints at a common mechanism. A recently proposed coarsening model generally captures these aspects, providing a unified view of crossover interference. Consequently, Lint facilitates model refinements and general comparisons between alternative models of crossover interference.

ASYNAPSIS3 has diverse dosage-dependent effects on meiotic crossover formation in Brassica napus.

Crossovers create genetic diversity and are required for equal chromosome segregation during meiosis. Crossover number and distribution are highly regulated by different mechanisms that are not yet fully understood, including crossover interference. The chromosome axis is crucial for crossover formation. Here, we explore the function of the axis protein ASYNAPSIS3. To this end, we use the allotetraploid species Brassica napus; due to its polyploid nature, this system allows a fine-grained dissection of the dosage of meiotic regulators. The simultaneous mutation of all 4 ASY3 alleles results in defective synapsis and drastic reduction of crossovers, which is largely rescued by the presence of only one functional ASY3 allele. Crucially, while the number of class I crossovers in mutants with 2 functional ASY3 alleles is comparable to that in wild type, this number is significantly increased in mutants with only one functional ASY3 allele, indicating that reducing ASY3 dosage increases crossover formation. Moreover, the class I crossovers on each bivalent in mutants with 1 functional ASY3 allele follow a random distribution, indicating compromised crossover interference. These results reveal the distinct dosage-dependent effects of ASY3 on crossover formation and provide insights into the role of the chromosome axis in patterning recombination.

The conserved ATPase PCH-2 controls the number and distribution of crossovers by antagonizing crossover formation in C. elegans.

Meiotic crossover recombination is essential for both accurate chromosome segregation and the generation of new haplotypes for natural selection to act upon. While the conserved role of the ATPase, PCH-2, during meiotic prophase has been enigmatic, a universal phenotype that is observed when pch-2 or its orthologs are mutated is a change in the number and distribution of meiotic crossovers. Here, we show that PCH-2 controls the number and distribution of crossovers by antagonizing crossover formation. This antagonism produces different effects at different stages of meiotic prophase: early in meiotic prophase, PCH-2 prevents double strand breaks from becoming crossovers, limiting crossovers at sites of initial DSB formation and homolog interactions. Later in meiotic prophase, PCH-2 winnows the number of crossover-eligible intermediates, contributing to the reinforcement of crossover-eligible intermediates, designation of crossovers and ultimately, crossover assurance. We also demonstrate that PCH-2 accomplishes this regulation through the meiotic HORMAD, HIM-3. Our data strongly support a model in which PCH-2's conserved role is to remodel meiotic HORMADs throughout meiotic prophase to destabilize crossover-eligible precursors, coordinate meiotic recombination with synapsis, and contribute to the progressive implementation of meiotic recombination, guaranteeing crossover control.

Genetic background affects the strength of crossover interference in house mice.

Meiotic recombination is required for faithful chromosome segregation in most sexually reproducing organisms and shapes the distribution of genetic variation in populations. Both the overall rate and the spatial distribution of crossovers vary within and between species. Adjacent crossovers on the same chromosome tend to be spaced more evenly than expected at random, a phenomenon known as crossover interference. Although interference has been observed in many taxa, the factors that influence the strength of interference are not well understood. We used house mice (Mus musculus), a well-established model system for understanding recombination, to study the effects of genetics and age on recombination rate and interference in the male germline. We analyzed crossover positions in 503 progeny from reciprocal F1 hybrids between inbred strains representing the three major subspecies of house mice. Consistent with previous studies, autosomal alleles from M. m. musculus tend to increase recombination rate, while inheriting a M. m. musculus X chromosome decreases recombination rate. Old males transmit an average of 0.6 more crossovers per meiosis (5.0%) than young males, though the effect varies across genetic backgrounds. We show that the strength of crossover interference depends on genotype, providing a rare demonstration that interference evolves over short timescales. Differences between reciprocal F1s suggest that X-linked factors modulate the strength of interference. Our findings motivate additional comparisons of interference among recently diverged species and further examination of the role of paternal age in determining the number and positioning of crossovers.

A familial, telomere-to-telomere reference for human de novo mutation and recombination from a four-generation pedigree.

Using five complementary short- and long-read sequencing technologies, we phased and assembled >95% of each diploid human genome in a four-generation, 28-member family (CEPH 1463) allowing us to systematically assess de novo mutations (DNMs) and recombination. From this family, we estimate an average of 192 DNMs per generation, including 75.5 de novo single-nucleotide variants (SNVs), 7.4 non-tandem repeat indels, 79.6 de novo indels or structural variants (SVs) originating from tandem repeats, 7.7 centromeric de novo SVs and SNVs, and 12.4 de novo Y chromosome events per generation. STRs and VNTRs are the most mutable with 32 loci exhibiting recurrent mutation through the generations. We accurately assemble 288 centromeres and six Y chromosomes across the generations, documenting de novo SVs, and demonstrate that the DNM rate varies by an order of magnitude depending on repeat content, length, and sequence identity. We show a strong paternal bias (75-81%) for all forms of germline DNM, yet we estimate that 17% of de novo SNVs are postzygotic in origin with no paternal bias. We place all this variation in the context of a high-resolution recombination map (~3.5 kbp breakpoint resolution). We observe a strong maternal recombination bias (1.36 maternal:paternal ratio) with a consistent reduction in the number of crossovers with increasing paternal (r=0.85) and maternal (r=0.65) age. However, we observe no correlation between meiotic crossover locations and de novo SVs, arguing against non-allelic homologous recombination as a predominant mechanism. The use of multiple orthogonal technologies, near-telomere-to-telomere phased genome assemblies, and a multi-generation family to assess transmission has created the most comprehensive, publicly available "truth set" of all classes of genomic variants. The resource can be used to test and benchmark new algorithms and technologies to understand the most fundamental processes underlying human genetic variation.

Test time affects Farnsworth D15 outcomes in practiced, but not unpracticed, subjects with color vision deficiency.

Imposing a time limit on the Farnsworth D15 test may prevent patients from compromising the test. This study aimed to investigate the effect of test time on the Farnsworth D15 color vision test in unpracticed and practiced subjects and determine an optimal test time. Twenty-one subjects (mean/standard deviation age, 33.1/9.3 years) with a range of congenital color vision deficiency participated in the study. Pseudoisochromatic plate screening, Farnsworth D15, and anomaloscope testing were performed for classification purposes. At each of 2 visits, 10 trials of the Farnsworth D15 were performed with a range in test times from 30 seconds to 10 minutes. Between visits, subjects practiced the test. Major crossovers were used as the outcome measure. A repeated-measures analysis of variance compared the scores across trials. Post hoc Dunnett's testing analyzed the pairwise data. Although no significant difference in the mean number of major crossovers was found across the 10 trials for the first visit ( F (9, 180) = 1.30, p=0.24), a significant difference was found for the second visit ( F (9, 180) = 4.77, p<0.001). The range of mean number of major crossovers for the second visit was 1.71 to 5.1, with the 30-second trial resulting in the largest number of major crossovers and the longest trial resulting in the smallest number of major crossovers. Analysis showed that a 2-minute time limit resulted in a Farnsworth D15 outcome that would be expected based on the anomaloscope for a majority of subjects. In this study, test time was found to affect performance in practiced subjects but not in unpracticed subjects. Based on this study, we recommend enforcing a time limit of 2 minutes to discourage those who try to pass the Farnsworth D15 through practice. Additional measures, such as recording patient behavior, can also be taken.

Analysis of Cassini Altimetric Crossovers on Titan

The Cassini spacecraft performed several flybys of Saturn’s largest moon, Titan, collecting valuable data. During several passes, altimetric data were acquired. Here, we focus on altimetric measurements collected by Cassini’s radar when flying over the same region at different epochs in order to correlate such measurements (crossovers) and investigate differences in altimetry. In our study, we assess altimetric errors associated with three distinct methods for extracting topography from Cassini’s radar data: the maximum likelihood estimator (MLE), the threshold method, and the first moment technique. Focusing on crossover events, during which Cassini revisited specific areas of Titan’s surface, we conduct a detailed examination of the consistency and accuracy of these three topography extraction methods. The proposed analysis involves closely examining altimetric data collected at different epochs over identical geographical regions, allowing us to investigate potential errors due to the variations in off-nadir angle, relative impact, uncertainties, and systematic errors inherent in the application of these methodologies. Our findings reveal that the correction applied for the off-nadir angle to the threshold and first moment methods significantly reduces the dispersion in the delta difference at the crossover, resulting in a dispersion of the order of 60 m, even lower than what is achieved with the MLE (~70 m). Additionally, an effort is made to assess the potential of Cassini for estimating the tidal signal on Titan. Considering the altimetric errors identified in our study and the relatively low number of crossovers performed by Cassini, our assessment indicates that it is not feasible to accurately measure the tidal signal on Titan using the currently available standard altimetry data from Cassini. Our assessment regarding the accuracy of the Cassini altimeter provides valuable insights for future planetary exploration endeavors. Our study advances the understanding of Titan’s complex landscape and contributes to refining topographical models derived from Cassini’s altimetry observations. These insights not only enhance our knowledge of Saturn’s largest moon but also open prospects for Titan surface and interior exploration using radar systems.

Work–family conflict and spouse’s job performance: when detaching from home is key

ABSTRACT Work–family conflict (WFC) is a common experience which frequently affects employees’ performance and wellbeing. But among dual-earner couples, is it possible that employees’ WFC relates to spouses’ job performance? Why does this occur and what are the associated consequences? Drawing on crossover literature and the stressor–detachment model, we explore a number of crossover and spillover effects. Specifically, we propose that employees’ WFC negatively affects spouses’ home detachment and job performance on the next day (crossover effects), which relates to spouses’ lower work detachment (spillover effects). We conducted a diary study among 145 working couples who responded to a daily survey over five consecutive working days (N = 1450 occasions). Our results from multilevel analyses largely supported our hypotheses. Employees’ WFC predicted lower home detachment of spouses on the following day, which, in turn, negatively affected spouses’ performance and work detachment. These findings show that WFC affects spouses beyond the family domain and that detachment from family-related issues is key to performing well and disconnecting from work.

Terpene-augmented novasomal gels for the sustainment of rasagiline mesylate delivery; A new approach for treating Parkinson's disease induced by rotenone in rats

Rasagiline mesylate (RSM) is a potent anti-Parkinson's Disease (PD) drug; nevertheless, it has limited oral bioavailability due to hepatic first pass metabolism. RSM is a highly hydrophilic drug hurdled by several barriers to reach the central nervous system (CNS). This study aimed to prepare RSM-loaded novasomal gels for the transdermal delivery of the drug to CNS. Novasomes were prepared by ethanol injection method using terpenes as penetration enhancers. Design Expert® software was applied to select the optimum formulation by varying the type of terpene, amount of cholesterol, besides surface active agents (SAA): free fatty acid (FFA) ratio. The desirability calculations revealed that the optimum formulation composed of cholesterol (60 mg), SAA:FFA ratio (1:1) in the presence of eugenol. Particle size, zeta potential and entrapment efficiency of the optimum formulation were 265.90 ± 2.40 nm, −33.45 ± 0.49 mV and 83.09 ± 0.42%, respectively. TEM showed spherical well-defined nanovesicles. The optimum novasomal formulation was incorporated into gel system composed of 2% HPMC and 2% CMC. In vivo study revealed the feasibility of reversing PD symptoms regarding the behavioral, biochemical and histopathological analyses in rotenone-induced Parkinsons' rat model. A marked elevation in the number of crossovers, grooming score, frequency of rearing, gripe strength, and dopamine level besides a reduction in interleukin-17 and NLRP-3 inflammasome level when compared to positive control group were detected. No histopathological alterations were detected in examined brain sections of treated rats. In conclusion, the obtained results suggest that RSM-loaded novasomal gel formulations are promising carriers for the transdermal delivery of hydrophilic drugs like RSM.

Rec-1 loss of function increases recombination in the central gene clusters at the expense of autosomal pairing centers.

Meiotic control of crossover (CO) number and position is critical for homologous chromosome segregation and organismal fertility, recombination of parental genotypes, and the generation of novel genetic combinations. We here characterize the recombination rate landscape of a rec-1 loss of function modifier of CO position in Caenorhabditis elegans, one of the first ever modifiers discovered. By averaging CO position across hermaphrodite and male meioses and by genotyping 203 single-nucleotide variants covering about 95% of the genome, we find that the characteristic chromosomal arm-center recombination rate domain structure is lost in the loss of function rec-1 mutant. The rec-1 loss of function mutant smooths the recombination rate landscape but is insufficient to eliminate the nonuniform position of CO. Lower recombination rates in the rec-1 mutant are particularly found in the autosomal arm domains containing the pairing centers. We further find that the rec-1 mutant is of little consequence for organismal fertility and egg viability and thus for rates of autosomal nondisjunction. It nonetheless increases X chromosome nondisjunction rates and thus male appearance. Our findings question the maintenance of recombination rate heritability and genetic diversity among C. elegans natural populations, and they further suggest that manipulating genetic modifiers of CO position will help find quantitative trait loci located in low-recombining genomic regions normally refractory to discovery.

Dynamic Response of Transmission Tower-Line Systems Due to Ground Vibration Caused by High-Speed Trains

With the continuous expansion of the scale of power grid and transportation infrastructure construction, the number of crossovers between transmission lines and high-speed railways continues to increase. At present, there is a lack of systematic research on the dynamic characteristics of transmission tower-line structures crossing high-speed railways under vehicle-induced ground vibration. This article focuses on the phenomenon of accidents such as line drops when crossing areas in recent years and establishes a high-speed train track foundation soil finite element model in ABAQUS that considers track irregularity. The three-dimensional vibration characteristics and attenuation law of train ground vibration are analyzed. Acceleration data for key points are also extracted. A separate finite element model of the transmission tower-line system is established in ANSYS, where acceleration is applied as an excitation to the transmission tower-line system, and the coupling effect between the tower and the line is considered to analyze its dynamic response. Subsequently, modal analysis is conducted on the tower-line system, providing the vibration modes and natural frequencies of the transmission tower-line structure. The effects of factors such as train speed, soil quality, and distance from the tower to the track on the dynamic response of the transmission tower-line system under vehicle-induced ground vibration are studied. The results show that the speed range (300 km/h–400 km/h) and track distance range (4.5 m–30 m) with the greatest impacts are obtained. The research results can provide a reference for the reasonable design of transmission tower-line systems in high-speed railway sections.

Crossover shortage in potato is caused by StMSH4 mutant alleles and leads to either highly uniform unreduced pollen or sterility.

The balanced segregation of homologous chromosomes during meiosis is essential for fertility and is mediated by crossovers (COs). A strong reduction of CO number leads to the unpairing of homologous chromosomes after the withdrawal of the synaptonemal complex. This results in the random segregation of univalents during meiosis I and ultimately to the production of unbalanced and sterile gametes. However, if CO shortage is combined with another meiotic alteration that restitutes the first meiotic division, then uniform and balanced unreduced male gametes, essentially composed of nonrecombinant homologs, are produced. This mitosis-like division is of interest to breeders because it transmits most of the parental heterozygosity to the gametes. In potato, CO shortage, a recessive trait previously referred to as desynapsis, was tentatively mapped to chromosome 8. In this article, we have fine-mapped the position of the CO shortage locus and identified StMSH4, an essential component of the class I CO pathway, as the most likely candidate gene. A 7 base-pair insertion in the second exon of StMSH4 was found to be associated with CO shortage in our mapping population. We also identified a second allele with a 3,820 base-pair insertion and confirmed that both alleles cannot complement each other. Such nonfunctional alleles appear to be common in potato cultivars. More than half of the varieties we tested are carriers of mutational load at the StMSH4 locus. With this new information, breeders can choose to remove alleles associated with CO shortage from their germplasm to improve fertility or to use them to produce highly uniform unreduced male gametes in alternative breeding schemes.

The human fungal pathogen Aspergillus fumigatus can produce the highest known number of meiotic crossovers.

Sexual reproduction involving meiosis is essential in most eukaryotes. This produces offspring with novel genotypes, both by segregation of parental chromosomes as well as crossovers between homologous chromosomes. A sexual cycle for the opportunistic human pathogenic fungus Aspergillus fumigatus is known, but the genetic consequences of meiosis have remained unknown. Among other Aspergilli, it is known that A. flavus has a moderately high recombination rate with an average of 4.2 crossovers per chromosome pair, whereas A. nidulans has in contrast a higher rate with 9.3 crossovers per chromosome pair. Here, we show in a cross between A. fumigatus strains that they produce an average of 29.9 crossovers per chromosome pair and large variation in total map length across additional strain crosses. This rate of crossovers per chromosome is more than twice that seen for any known organism, which we discuss in relation to other genetic model systems. We validate this high rate of crossovers through mapping of resistance to the laboratory antifungal acriflavine by using standing variation in an undescribed ABC efflux transporter. We then demonstrate that this rate of crossovers is sufficient to produce one of the common multidrug resistant haplotypes found in the cyp51A gene (TR34/L98H) in crosses among parents harboring either of 2 nearby genetic variants, possibly explaining the early spread of such haplotypes. Our results suggest that genomic studies in this species should reassess common assumptions about linkage between genetic regions. The finding of an unparalleled crossover rate in A. fumigatus provides opportunities to understand why these rates are not generally higher in other eukaryotes.

An Effective Approach for Stepping-Stone Intrusion Detection Resistant to Intruders’ Chaff-Perturbation via Packet Crossover

Today’s intruders usually send attacking commands to a target system through several stepping-stone hosts, for the purpose of decreasing the probability of being caught. Using stepping-stone intrusion (SSI), the intruder’s identity is very difficult to discover as it is concealed by a long interactive connection chain of hosts. An effective approach for SSI detection (SSID) is to determine how many connections are contained in a connection chain. This type of method is called network-based SSID. Most existing network-based SSID only worked for network traffic without intruders’ session manipulation. These known SSID algorithms are either weak to resist intruders’ chaff-perturbation manipulation or have very limited capability in resisting attacker’s session manipulation. This paper develops a novel network-based SSID algorithm resistant to intruders’ chaff-perturbation by using packet crossover. The SSID approach proposed in this paper is simple and easy to implement as the number of packet crossovers can be easily computed. Our proposed algorithm is verified by rigorous technical proofs as well as well-designed network experiments. Our experimental results show that the proposed SSID algorithm works effectively and perfectly in resisting intruders’ chaff-perturbation up to a chaff rate of 50%.

What happens at work does not always stay at work: Daily job crafting and detachment among colleagues

AbstractThrough job crafting, employees proactively change or modify their tasks, thus reducing adverse job demands or protecting resources. There is still a lack of understanding of the impact that job crafting may have on colleagues at work (crossover effect), and how this may affect their ability to disconnect from work (spillover effect). In the present daily diary study, we examine these two processes among 82 dyads of colleagues (N = 164 employees) over five consecutive working days (N = 820 observations). We found a number of crossover and differential spillover effects. For example, when the focal employee starts new challenging projects, their colleague reacts by reducing the number of stressful tasks. This, in turn, affects psychological detachment from work. Specifically, whereas increasing challenging demands hinders daily detachment, decreasing hindering demands facilitates it. Taken together, these findings demonstrate that the impact of job crafting goes beyond the focal employee and beyond the work domain.

DMC1 stabilizes crossovers at high and low temperatures during wheat meiosis.

Effective chromosome synapsis and crossover formation during meiosis are essential for fertility, especially in grain crops such as wheat. These processes function most efficiently in wheat at temperatures between 17-23 °C, although the genetic mechanisms for such temperature dependence are unknown. In a previously identified mutant of the hexaploid wheat reference variety 'Chinese Spring' lacking the long arm of chromosome 5D, exposure to low temperatures during meiosis resulted in asynapsis and crossover failure. In a second mutant (ttmei1), containing a 4 Mb deletion in chromosome 5DL, exposure to 13 °C led to similarly high levels of asynapsis and univalence. Moreover, exposure to 30 °C led to a significant, but less extreme effect on crossovers. Previously, we proposed that, of 41 genes deleted in this 4 Mb region, the major meiotic gene TaDMC1-D1 was the most likely candidate for preservation of synapsis and crossovers at low (and possibly high) temperatures. In the current study, using RNA-guided Cas9, we developed a new Chinese Spring CRISPR mutant, containing a 39 bp deletion in the 5D copy of DMC1, representing the first reported CRISPR-Cas9 targeted mutagenesis in Chinese Spring, and the first CRISPR mutant for DMC1 in wheat. In controlled environment experiments, wild-type Chinese Spring, CRISPR dmc1-D1 and backcrossed ttmei1 mutants were exposed to either high or low temperatures during the temperature-sensitive period from premeiotic interphase to early meiosis I. After 6-7 days at 13 °C, crossovers decreased by over 95% in the dmc1-D1 mutants, when compared with wild-type plants grown under the same conditions. After 24 hours at 30 °C, dmc1-D1 mutants exhibited a reduced number of crossovers and increased univalence, although these differences were less marked than at 13 °C. Similar results were obtained for ttmei1 mutants, although their scores were more variable, possibly reflecting higher levels of background mutation. These experiments confirm our previous hypothesis that DMC1-D1 is responsible for preservation of normal crossover formation at low and, to a certain extent, high temperatures. Given that reductions in crossovers have significant effects on grain yield, these results have important implications for wheat breeding, particularly in the face of climate change.

Nascent evolution of recombination rate differences as a consequence of chromosomal rearrangements.

Reshuffling of genetic variation occurs both by independent assortment of chromosomes and by homologous recombination. Such reshuffling can generate novel allele combinations and break linkage between advantageous and deleterious variants which increases both the potential and the efficacy of natural selection. Here we used high-density linkage maps to characterize global and regional recombination rate variation in two populations of the wood white butterfly (Leptidea sinapis) that differ considerably in their karyotype as a consequence of at least 27 chromosome fissions and fusions. The recombination data were compared to estimates of genetic diversity and measures of selection to assess the relationship between chromosomal rearrangements, crossing over, maintenance of genetic diversity and adaptation. Our data show that the recombination rate is influenced by both chromosome size and number, but that the difference in the number of crossovers between karyotypes is reduced as a consequence of a higher frequency of double crossovers in larger chromosomes. As expected from effects of selection on linked sites, we observed an overall positive association between recombination rate and genetic diversity in both populations. Our results also revealed a significant effect of chromosomal rearrangements on the rate of intergenic diversity change between populations, but limited effects on polymorphisms in coding sequence. We conclude that chromosomal rearrangements can have considerable effects on the recombination landscape and consequently influence both maintenance of genetic diversity and efficiency of selection in natural populations.

Partial cytological diploidization of neoautotetraploid meiosis by induced cross-over rate reduction

Polyploids, which arise from whole-genome duplication events, have contributed to genome evolution throughout eukaryotes. Among plants, novel features of neopolyploids include traits that can be evolutionarily or agriculturally beneficial, such as increased abiotic stress tolerance. Thus, in addition to being interesting from an evolutionary perspective, genome duplication is also increasingly recognized as a promising crop improvement tool. However, newly formed (neo)polyploids commonly suffer from fertility problems, which have been attributed to abnormal associations among the multiple homologous chromosome copies during meiosis (multivalents). Here, we test the long-standing hypothesis that reducing meiotic cross-over number may be sufficient to limit multivalent formation, favoring diploid-like bivalent associations (cytological diploidization). To do so, we developed Arabidopsis thaliana lines with low cross-over rates by combining mutations for HEI10 and TAF4b. Double mutants showed a reduction of ~33% in cross-over numbers in diploids without compromising meiotic stability. Neopolyploids derived from the double mutant show a cross-over rate reduction of about 40% relative to wild-type neotetraploids, and groups of four homologs indeed formed fewer multivalents and more bivalents. However, we also show that the reduction in multivalents comes with the cost of a slightly increased frequency of univalents and that it does not rescue neopolyploid fertility. Thus, while our results do show that reducing cross-over rates can reduce multivalent frequency in neopolyploids, they also emphasize that there are additional factors affecting both meiotic stability and neopolyploid fertility that will need to be considered in solving the neopolyploid fertility challenge.

Long-range linkage effects in adapting sexual populations

In sexual populations, closely-situated genes have linked evolutionary fates, while genes spaced far in genome are commonly thought to evolve independently due to recombination. In the case where evolution depends essentially on supply of new mutations, this assumption has been confirmed by mathematical modeling. Here I examine it in the case of pre-existing genetic variation, where mutation is not important. A haploid population with N genomes, L loci, a fixed selection coefficient, and a small initial frequency of beneficial alleles {f}_{0} is simulated by a Monte-Carlo algorithm. When the number of loci, L, is larger than a critical value of {text{4log}}^{2} left( {Nf_{0} } right), simulation demonstrates a host of linkage effects that decrease neither with the distance between loci nor the number of recombination crossovers. Due to clonal interference, the beneficial alleles become extinct at a fraction of loci 1-2mathrm{log}left(N{f}_{0}right)/{L}^{0.5}. Due to a genetic background effect, the substitution rate varies broadly between loci, with the fastest value exceeding the one-locus limit by the factor of {[{L}^{0.5}/mathrm{log}left(Nsright)]}^{0.75}. Thus, the far-situated parts of a long genome in a sexual population do not evolve as independent blocks. A potential link between these findings and the emergence of new Variants of Concern of SARS-CoV-2 is discussed.