B Chromosome Dynamics in Natural Populations

B chromosomes are dispensable genomic elements found alongside the standard complement in a wide range of plants, animals and fungi. These supernumerary chromosomes exhibit extensive variation in number, structure and sequence composition both within and among natural populations. Historically regarded as selfish or parasitic entities, they are now recognised as dynamic components that can carry functional genes, influence host gene expression and sometimes confer adaptive advantages. Their persistence is driven by non-Mendelian inheritance mechanisms—collectively termed chromosome drive—that bias their transmission to offspring, counteracting the tendency of non-essential elements to be eliminated. Host genomes, in turn, have evolved suppression systems to mitigate potential fitness costs. The interplay between drive efficiency, host suppression and environmental context shapes B chromosome frequencies and morphologies in the wild. Recent advances in chromosome-scale assemblies, comparative transcriptomics and cytogenetic mapping have begun to elucidate the molecular underpinnings of drive, the repertoire of genes resident on B chromosomes and their impact on genome regulation. These discoveries underscore the dual nature of B chromosomes as both genomic parasites and potential facilitators of ecological and evolutionary innovation.

Research from Nature Portfolio

A chromosome-level assembly of a rye B chromosome has revealed a suite of candidate genes implicated in its targeted nondisjunction during first pollen mitosis. Among these, a microtubule-associated protein gene shows elevated expression specifically at the generative nucleus, suggesting a direct role in mediating drive. Comparative analyses indicate that this gene family has undergone serial duplications and neo-functionalisation on B chromosomes across related species. In parallel, work on the grasshopper Eyprepocnemis plorans has uncovered ten protein-coding genes located on its B chromosome, five of which are actively transcribed in B-carrying individuals. Notably, up-regulated genes encode factors involved in cell division, hinting that B chromosome success may hinge on modulating host mitotic and meiotic machinery.

B Chromosome Dynamics in Natural Populations publication trend

The graph below shows the total number of articles in b chromosome dynamics in natural populations across all publications each year (not limited to Nature Index journals).

Technical terms

B chromosome: An accessory chromosome extra to the standard set (A chromosomes) that is non-essential but may influence host biology and inheritance patterns.

Chromosome drive: A suite of processes by which certain chromosomes bias their transmission to progeny beyond standard Mendelian expectations.

Non-Mendelian inheritance: Patterns of genetic transmission that deviate from the typical 50:50 segregation ratio dictated by Mendel’s laws.

Heterochromatin: Densely packed chromosomal regions rich in repetitive DNA, often transcriptionally inactive but important in structural functions.

Non-disjunction: Failure of chromosome pairs to separate properly during cell division, which can be exploited by B chromosomes to achieve biased segregation.

References

1. The genetic mechanism of B chromosome drive in rye illuminated by chromosome-scale assembly. Nature Communications (2024).
2. B chromosomes reveal a female meiotic drive suppression system in Drosophila melanogaster. Current Biology (2023).
3. Karyotype Variability in Wild Narcissus poeticus L. Populations from Different Environmental Conditions in the Dinaric Alps †. Plants (2024).
4. The Modern View of B Chromosomes Under the Impact of High Scale Omics Analyses. Cells (2019).
5. Protein-coding genes in B chromosomes of the grasshopper Eyprepocnemis plorans. Scientific Reports (2017).