Shams, I. and Raskina, O. 2020. Supernumerary B chromosomes and plant genome changes: A snapshot of wild populations of Aegilops speltoides Tausch (Poaceae, Triticeae). International Journal of Molecular Sciences, 21 (11): 3768

In various eukaryotes, supernumerary B chromosomes (Bs) are an optional genomic component. The notable features of Bs are variability in numbers among individuals, species-specific organization, and enrichments with different types of repetitive DNA; and the inheritance of the B chromosome is not subject to Mendelian laws. The origin of Bs is still obscure, but studies employing recent advances in next-generation sequencing indicate that they arise as a result of A chromosome rearrangements, and some genes of standard A chromosomes remain transcriptionally active in Bs.

In the present study, the impact of Bs on the current changes in the genome of goatgrass, Aegilops speltoides, which is a wild progenitor of the B and G genomes of allopolyploid wheats, was addressed. We focused on the cytogenetic features of Bs and the intra-organismal copy number dynamics of retrotransposons (TEs) and species-specific tandem repeat (TR) in plants with and without Bs in highly heterogeneous populations of Ae. speltoides. Our data indicate that fluctuations in TE and TR copy numbers are associated with DNA damage and repair processes during cell proliferation and differentiation, and ectopic recombination is one of the mechanisms by which Bs play a role in genome changes.

Variability in the copy numbers of the Angela, Wilma, and Stasy retrotransposons and Spelt1 tandem repeat in somatic and generative tissues of the spike in individual genotypes

Fluctuations in the total TE and TR copy number between different tissues of one spike is phenomenon that is indicative of the efficacy of DNA repair during cell proliferation and differentiation.

Somatic recombination is associated with DNA replication and repair processes and is tightly interlinked with individual chromosome positioning and dynamics in interphase nuclei. The involvement of ectopic repetitive sequences in the repair of DNA damage is a complementary process both in plant mitosis and meiosis.

Rearrangements of B chromosomes (C) and ectopic

B-A chromosome associations (D) in somatic cells.

Ectopic association between A and B chromosomes in meiosis at the anaphase I stage (arrow).

Ectopic recombination, which causes chromosomal rearrangements, may also prevent harmful aberrations, and, above all, double-stranded DNA breaks (DSBs), and therefore, it is vitally important for the cell.

In this regard, supernumerary B chromosomes serve as an additional template in DNA repair, especially in heterochromatin and clusters of various repetitive sequences, specifically, various transposable elements that are distributed throughout the euchromatin. Vice versa, standard A chromosomes contribute to ectopic DNA repair in Bs, and contribute to the accumulation of sequences of standard chromosomes in Bs during both somatic and meiotic cell proliferation.