Abstract
B chromosomes are supernumerary chromosomes found in addition to the normal standard chromosomes (A chromosomes). B chromosomes are well known to accumulate several distinct types of repeated DNA elements. Although the evolution of B chromosomes has been the subject of numerous studies, the mechanisms of accumulation and evolution of repetitive sequences are not fully understood. Recently, new genomic approaches have shed light on the origin and accumulation of different classes of repetitive sequences in the process of B chromosome formation and evolution. Here we discuss the impact of repetitive sequences accumulation on the evolution of plant B chromosomes.
Highlights
Supernumerary B chromosomes (Bs) are not required for the normal development of organisms and are assumed to represent a specific type of selfish genetic elements
Bs may vary in structure and chromatin properties in a species-specific way, their de novo formation is probably a rare event, because the occurrence of similar B chromosome variants within related species suggests that they arose from a single origin either from the same or from a related species [7,8]
Because the two blocks of heterochromatin that contain StarkB have persisted over many generations, their presence may play a role in B chromosome transmission [56]
Summary
Supernumerary B chromosomes (Bs) are not required for the normal development of organisms and are assumed to represent a specific type of selfish genetic elements. First analyses of the DNA composition of B chromosomes were based on renaturation kinetics and gradient density centrifugation These approaches showed that in rye the heterogeneity of repeats and ratio in Bs did not differ from As [12]. The second strategy requires sequencing of two whole genome datasets of the same species, one containing Bs (+B) and one without Bs (0B) It is an indirect approach because B-derived sequences are compared against the 0B-derived sequences as an additional step. Identification of B chromosome-enriched sequences, like satellite repeats, mobile elements or organelle-derived sequences by similarity-based clustering of generation sequence reads was achieved using the RepeatExplorer software [16,17] for rye [1] and Plantago lagopus [18]. Mention of specific programs in this review denotes only previous use in cited studies and does not imply endorsement
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