B chromosomes!
B chromosomes are extra chromosomes that are not duplicates of any of the essential A chromosomes.
Since their discovery over a century ago, B chromosomes have been identified in hundreds of species across a diverse range of taxa.
The formation and dynamics of B chromosomes can enhance our understanding of de novo chromosome formation and reveal key aspects of chromosome evolution.
B chromosomes have unique molecular and genetic features that have the potential to be exploited for advancements in biotechnology, agriculture, and human health.
https://www.cell.com/trends/genetics/fulltext/S0168-9525(26)00171-X
