Three research groups from the University of Missouri, the Institute of
Experimental Botany of the Czech Academy of Sciences, and the Institute
of Genetics and Developmental Biology of the Chinese Academy of Sciences
have recently reported a reference sequence for the supernumerary B
chromosome inmaize.

Supernumerary B chromosomes are present in thousands of plant and animal
genomes despite being nonessential. They are maintained in populations
by mechanisms of "drive" that make them inherited at higher than typical
Mendelian rates. However, their key properties such as their origin,
evolution, and the molecular mechanism for their accumulation in maize
have remained unclear even though such chromosomes have been a potent
tool for studying maize genetics.

The researchers discovered the rich availability of deletion derivatives
ensured strong scaffolding and vetting of assembly. In addition, 758
protein-coding genes were identified from the 125.9-Mb of chromosome
sequence, of which at least 88 are expressed. They also discovered that
the current gene content is a result of continuous transfer from the A
chromosomal complement over an extended evolutionary period. Annotation
results show that transposable elements in the B chromosome are shared
with the standard A chromosome set. However, the failure of multiple
lines of evidence to reveal a syntenic region in the B chromosome with
any A chromosome indicates that this chromosome has been present in the
evolutionary lineage for millions of years since any such synteny has
disintegrated.

Researchers Report Reference Genome for Maize B Chromosome----Chinese
Academy of Sciences (cas.cn)
[english.cas.cn]