The supernumerary B chromosome of maize: drive and genomic conflict

Global modulation of gene expression and transcriptome size in aneuploid combinations of maize

Genomic imbalance refers to the more severe phenotypic consequences of changing a single chromosome compared to changing the whole genomic set. Previous genomic imbalance studies in maize have identified gene expression modulation in aneuploids of single chromosome arms. Here, the modulation of gene expression in more complex aneuploids, e.g., monosomy of one chromosome arm and trisomy of another was examined to determine the extent that combination aneuploids were additive, multiplicative, or rebalanced in terms of their effect on gene expression. A series of genetic crosses was performed to produce one, two, and three copies of one arm with independent one, two, and three copies of the other arm in each of the three genotypes for the opposite arm. In total, 31 combinations were analyzed. By examining RNA modulation, we found that cis genes on varied chromosome arms are generally more dosage-compensated in aneuploid combinations than in single aneuploidy even though some showed a clear dosage effect. For genes on the unvaried chromosomes (trans), there is greater modulation in most of the aneuploidy combinations. Finally, 22 of the 31 combinations were found to have an altered transcriptome size, suggesting that extensive stoichiometric changes of genomic regions upset global messenger RNA (mRNA) transcription. Overall, these results have important implications for understanding the role of genomic stoichiometry for mechanisms of gene expression, the evolution of dosage-sensitive duplicated genes, the evolution of sex chromosomes, the rapid growth adaptation of aneuploid cancer cells, and the control of quantitative traits.

Aneuploidy and ploidy variation conditioned by the B chromosome of maize

The supernumerary B chromosome of maize has a drive mechanism to maintain itself in a population despite being dispensible. This involves nondisjunction of the B centromere at the second pollen mitosis that produces the two sperm followed by preferential fertilization of the egg by the B containing sperm during double fertilization. During an introgression of the supernumerary B chromosome into the inbred line B73, an unusually high frequency of trisomies for A chromosomes was observed. Due to parallels to the High Loss phenomenon in which three or more B chromosomes in a specific genetic background cause chromosomal breakage at heterochromatic knob sites during the second pollen mitosis as well as ploidy changes, this phenomenon was revisited. Examination of pollen of the High Loss line revealed a high frequency of single sperm in the presence of the B chromosomes, which was previously not realized. Crosses to tetraploid females confirmed that the single sperm were diploid and functional but also revealed the presence of diploids with their A chromosomes derived solely from the tetraploid parent indicating a "diploid induction". Collectively, the results reveal two backgrounds in which the B drive mechanism is not confined to this chromosome causing detrimental effects by adherence of heterochromatic knobs and apparently A centromeres at the mitosis preceding sperm development. In most genetic backgrounds this process is restricted to the B chromosome but in B73 and the High Loss line, there is spillover to the normal chromosomes in distinct ways.

Genome assembly of the maize B chromosome provides insight into its epigenetic characteristics and effects on the host genome

BACKGROUND

B chromosomes contribute to the genetic variation in numerous eukaryotes. Yet their genetic and epigenetic characteristics, as well as their effects on the host genome remain poorly understood.

RESULTS

Here, we present a comprehensive genome assembly of diploid maize B73 with two copies of B chromosomes using long-read sequencing. We annotate a total of 1124 high-confidence protein-coding genes and 119,579,190 bp repeat elements representing 88.55% of the B chromosome assembly. Using CENH3 ChIP-seq data, we accurately determined the position of the B chromosome centromere, which features a unique monomer-composed satellite array distinct from that found on the chromosome arms. Our research provides detailed genetic and epigenetic maps of the B chromosome, shedding light on its molecular landscape, including DNA sequence composition, DNA methylation patterns, histone modifications, and R-loop distributions across various chromatin regions. Consistent with the cytological morphology of the B chromosome, the less condensed euchromatin regions displayed high levels of H3K4me3, H3K9ac, gene expression, and dense R-loop distributions. DNA methylation on the B chromosome was primarily observed at CG sites. The centromeric region is notably enriched with H3K4me3 and H3K9ac histone modifications and has lower CHG methylation compared to the pericentromeric regions. Moreover, our findings reveal that B chromosome accumulation affects R-loop formation on A chromosomes, and exerts tissue-specific influences on A chromosome gene expression.

CONCLUSIONS

The accurate assembly and detailed epigenetic maps of the maize B chromosome will help understand the drive mechanism, reveal its conflict with the host genome, and accelerate the construction of artificial chromosomes.

Synthetic minichromosomes in plants: past, present, and promise

The status of engineered mini-chromosomes/artificial chromosomes/synthetic chromosomes in plants is summarized. Their promise is that they provide a means to accumulate foreign genes on an independent entity other than the normal chromosomes, which would facilitate stacking of novel traits in a way that would not be linked to endogenous genes and that would facilitate transfer between lines. Centromeres in plants are epigenetic, and therefore the isolation of DNA underlying centromeres and reintroduction into plant cells will not establish a functional kinetochore, which obviates this approach for in vitro assembly of plant artificial chromosomes. This issue was bypassed by using telomere-mediated chromosomal truncation to produce mini-chromosomes with little more than an endogenous centromere that could in turn be used as a foundation to build synthetic chromosomes. Site-specific recombinases and various iterations of CRISPR-Cas9 editing provide many tools for the development and re-engineering of synthetic chromosomes.

B chromosome and its non-Mendelian inheritance in Atractylodes lancea

Supernumerary B chromosomes contribute to intraspecific karyotypic variation. B chromosomes have been detected in more than 2000 organisms; they possess unique and diverse features, including non-Mendelian inheritance. Here, we report one or more B chromosomes in the gynodioecious plant Atractylodes lancea. Among 54 A. lancea lines, 0-2 B chromosomes were detected in both hermaphroditic and female plants, with the B chromosomes appearing as DAPI-bright regions within the nuclei. Genomic in situ hybridization revealed that the B chromosomes had no conserved A chromosome DNA sequences, confirmed by fluorescence in situ hybridization probed with independently dissected B chromosomes. In male meiosis, the B chromosome did not pair with an A chromosome and was therefore eliminated; accordingly, only 20.1% and 18.6% of these univalent B chromosomes remained at the end of meiosis for the 1B lines of KY17-148 and KY17-118, respectively. However, we also found that B chromosomes were transmitted from male parents in 40.8%-44.2% and 47.2% of the next generation; although these transmission rates from male parents were not essentially different from Mendelian inheritance (0.5), the transmission of gametes carrying B chromosomes increased through fertilization or seed development. B chromosomes were transmitted from three of four 1B female parents to 64.3%-92.6% of the next generation, suggesting B chromosome accumulation. We propose that the B chromosome of A. lancea has a specific sequence and persists via non-Mendelian inheritance from female parents. Overall, A. lancea, with its unique characteristics, is a promising model for understanding the structure, evolution, and mechanism of non-Mendelian inheritance of B chromosomes.

Unravelling the unusual: chromosome elimination, nondisjunction and extra pollen mitosis characterize the B chromosome in wild sorghum

The B chromosomes exhibit diverse behaviour compared with conventional genetic models. The capacity of the B chromosome either to accumulate or to be eliminated in a tissue-specific manner is dependent on biological processes related to aberrant cell division(s), but here yet remains compatible with normal development. We studied B chromosome elimination in Sorghum purpureosericeum embryos through cryo-sections and demonstrated the B chromosome instability during plant growth using flow cytometry, molecular markers and fluorescent in situ hybridization techniques. Consequently, using B chromosome-specific probes we revealed the non-Mendelian inheritance of B chromosomes in developing pollen. We disclosed that the occurrence of the B chromosome is specific to certain tissues or organs. The distribution pattern is mainly caused by an extensive elimination that functions primarily during embryo development and persists throughout plant development. Furthermore, we described that B chromosome accumulation can occur either by nondisjunction at first pollen mitosis (PMI) or the initiation of extra nuclear division(s) during pollen development. Our study demonstrates the existence of a not-yet-fully described B chromosome drive process, which is likely under the control of the B chromosome.

What is a B chromosome? Early definitions revisited

Since the discovery of B chromosomes, multiple different definitions of these selfish genetic elements have been put forth. We reconsidered early definitions in light of recently published studies. While there are many characteristics that vary among different B chromosomes, such as their evolutionary origins, size, segregation behaviors, gene content, and function, there is one defining trait of all B chromosomes: they are nonessential for the organism. The points raised here may be useful for framing future B chromosome studies and help guide the categorization of new chromosomal elements that are uncovered in genomic studies.

PSRs: Selfish chromosomes that manipulate reproductive development

B chromosomes are intriguing "selfish" genetic elements, many of which exhibit higher-than-Mendelian transmission. This perspective highlights a group of B chromosomes known as Paternal Sex Ratio chromosomes (PSRs), which are found in several insects with haplo-diploid reproduction. PSRs harshly alter the organism's reproduction to facilitate their own inheritance. A manifestation of this effect is the conversion of female destined individuals into males. Key to this conversion is the mysterious ability of PSRs to cause elimination of the sperm-inherited half of the genome during zygote formation. Here we discuss how PSRs were discovered, what is known about how they alter paternal chromatin dynamics to cause sex conversion, and how PSR-induced genome elimination is different from other forms of programmed genome elimination in different insects. PSRs also stand out because their DNA sequence compositions differ in remarkable ways from their insect's essential chromosomes, a characteristic suggestive of interspecies origins. Broadly, we also highlight poorly understood aspects of PSR dynamics that need to be investigated.

Female meiotic drive in plants: mechanisms and dynamics

Female meiosis is fundamentally asymmetric, creating an arena for genetic elements to compete for inclusion in the egg to maximize their transmission. Centromeres, as mediators of chromosomal segregation, are prime candidates to evolve via 'female meiotic drive'. According to the centromere-drive model, the asymmetry of female meiosis ignites a coevolutionary arms race between selfish centromeres and kinetochore proteins, the by-product of which is accelerated sequence divergence. Here, I describe and compare plant models that have been instrumental in uncovering the mechanistic basis of female meiotic drive (maize) and the dynamics of active selfish centromeres in nature (monkeyflowers). Then, I speculate on the mechanistic basis of drive in monkeyflowers, discuss how centromere strength influences chromosomal segregation in plants, and describe new insights into the evolution of plant centromeres.

Meiotic drive of noncentromeric loci in mammalian meiosis II eggs

The germline produces haploid gametes through a specialized cell division called meiosis. In general, homologous chromosomes from each parent segregate randomly to the daughter cells during meiosis, providing parental alleles with an equal chance of transmission. Meiotic drivers are selfish elements who cheat this process to increase their transmission rate. In female meiosis, selfish centromeres and noncentromeric drivers cheat by preferentially segregating to the egg cell. Selfish centromeres cheat in meiosis I (MI), while noncentromeric drivers can cheat in both meiosis I and meiosis II (MII). Here, we highlight recent advances on our understanding of the molecular mechanisms underlying these genetic cheating strategies, especially focusing on mammalian systems, and discuss new models of how noncentromeric selfish drivers can cheat in MII eggs.

Cell biology: Selfish B chromosomes unleashed by a dysfunctional chromosome segregation system

A study in the fruit fly Drosophila melanogaster shows that a defective chromosome segregation system allows non-essential B chromosomes to transmit at higher-than-Mendelian frequencies.

Genomic imbalance modulates transposable element expression in maize

Genomic imbalance refers to the more severe phenotypic consequences of changing part of a chromosome compared with the whole genome set. Previous genome imbalance studies in maize have identified prevalent inverse modulation of genes on the unvaried chromosomes (trans) with both the addition or subtraction of chromosome arms. Transposable elements (TEs) comprise a substantial fraction of the genome, and their reaction to genomic imbalance is therefore of interest. Here, we analyzed TE expression using RNA-seq data of aneuploidy and ploidy series and found that most aneuploidies showed an inverse modulation of TEs, but reductions in monosomy and increases in disomy and trisomy were also common. By contrast, the ploidy series showed little TE modulation. The modulation of TEs and genes in the same experimental group were compared, and TEs showed greater modulation than genes, especially in disomy. Class I and II TEs were differentially modulated in most aneuploidies, and some superfamilies in each TE class also showed differential modulation. Finally, the significantly upregulated TEs in three disomies (TB-7Lb, TB9Lc, and TB-10L19) did not increase the proportion of adjacent gene expression when compared with non-differentially expressed TEs, indicating that modulations of TEs do not compound the effect on genes. These results suggest that the prevalent inverse TE modulation in aneuploidy results from stoichiometric upset of the regulatory machinery used by TEs, similar to the response of core genes to genomic imbalance.

Comprehending the dynamism of B chromosomes in their journey towards becoming unselfish

Investigated for more than a century now, B chromosomes (Bs) research has come a long way from Bs being considered parasitic or neutral to becoming unselfish and bringing benefits to their hosts. B chromosomes exist as accessory chromosomes along with the standard A chromosomes (As) across eukaryotic taxa. Represented singly or in multiple copies, B chromosomes are largely heterochromatic but also contain euchromatic and organellar segments. Although B chromosomes are derived entities, they follow their species-specific evolutionary pattern. B chromosomes fail to pair with the standard chromosomes during meiosis and vary in their number, size, composition and structure across taxa and ensure their successful transmission through non-mendelian mechanisms like mitotic, pre-meiotic, meiotic or post-meiotic drives, unique non-disjunction, self-pairing or even imparting benefits to the host when they lack drive. B chromosomes have been associated with cellular processes like sex determination, pathogenicity, resistance to pathogens, phenotypic effects, and differential gene expression. With the advancements in B-omics research, novel insights have been gleaned on their functions, some of which have been associated with the regulation of gene expression of A chromosomes through increased expression of miRNAs or differential expression of transposable elements located on them. The next-generation sequencing and emerging technologies will further likely unravel the cellular, molecular and functional behaviour of these enigmatic entities. Amidst the extensive fluidity shown by B chromosomes in their structural and functional attributes, we perceive that the existence and survival of B chromosomes in the populations most likely seem to be a trade-off between the drive efficiency and adaptive significance versus their adverse effects on reproduction.

The non-Mendelian behavior of plant B chromosomes

B chromosomes, also known as supernumerary chromosomes, are dispensable elements in the genome of many plants, animals, and fungi. Many B chromosomes have evolved one or more drive mechanisms to transmit themselves at a higher frequency than predicted by Mendelian genetics, and these mechanisms counteract the tendency of non-essential genetic elements to be lost over time. The frequency of Bs in a population results from a balance between their effect on host fitness and their transmission rate. Here, we will summarize the findings of the drive process of plant B chromosomes, focusing on maize and rye.