Abstract
Accessory, supernumerary, or—most simply—B chromosomes, are found in many eukaryotic karyotypes. These small chromosomes do not follow the usual pattern of segregation, but rather are transmitted in a higher than expected frequency. As increasingly being demonstrated by next-generation sequencing (NGS), their structure comprises fragments of standard (A) chromosomes, although in some plant species, their sequence also includes contributions from organellar genomes. Transcriptomic analyses of various animal and plant species have revealed that, contrary to what used to be the common belief, some of the B chromosome DNA is protein-encoding. This review summarizes the progress in understanding B chromosome biology enabled by the application of next-generation sequencing technology and state-of-the-art bioinformatics. In particular, a contrast is drawn between a direct sequencing approach and a strategy based on a comparative genomics as alternative routes that can be taken towards the identification of B chromosome sequences.
Highlights
Accessory, supernumerary, or—most —B chromosomes, are found in many eukaryotic karyotypes
The outcome of applying next-generation sequencing (NGS) and extensive cytogenetic analyses has been that the B chromosomes, despite their being non-essential, have been shown to share much in common with A chromosomes, and that they evolved in the various taxa in comparable ways
There were 26 duplicated regions, harboring 34 intact and 21 partial gene sequences. The presence of both the proto-oncogenes cKIT and RET [25] in the M. gouazoubira B chromosomes suggests that the A chromosome genomic regions that become involved in B formation are not random [28]
Summary
The origin and evolution of the B chromosomes, which appear to make a non-essential contribution to the overall genome, have puzzled cytogeneticists for over a century. The acquisition of sequence from flow-sorted B chromosomes has enabled a demonstration that both harbor mainly repetitive DNA, with some representation of autosomal sequences undergoing pseudogenization and of amplified non-repetitive sequences Both the composition of the repetitive DNA and the spectrum of A-chromosome-derived sequences present differed greatly between the two species. Those in C. pygargus harbored at least two duplicated A chromosome regions containing three genes, and the level of heterozygosity and the number of haplotypes was high. There were 26 duplicated regions, harboring 34 intact and 21 partial gene sequences The presence of both the proto-oncogenes cKIT and RET [25] in the M. gouazoubira B chromosomes suggests that the A chromosome genomic regions that become involved in B formation are not random [28]. The involvement of sex chromosomes has been suggested in the frog species Leiopelma hochstetteri [31]
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