B Chromosomes in Wheat: Evolution, Functions and Breeding Potential

Supernumerary B chromosomes (Bs) have been documented in over 2000 species across the plant, animal, and fungal kingdoms, representing one of the most intriguing components of the genome that have consistently garnered significant attention in the past. These Bs form a distinct category of chromosomes that are not deemed essential. While coexisting with the standard set of chromosomes in an organism, the copy numbers of Bs can vary among individuals within a population or even among cells within a single organism. Multiple processes, including chromosomal rearrangement, uneven segregation, chromatid non-disjunction, and deletion in specific regions, lead to the formation of the Bs from the A chromosome. These dispensable Bs possess their own genetic content and do not undergo recombination with A chromosomes. Although various roles of Bs in breeding have been explored in the literature, their specific role in wheat improvement remains unclear. The understanding of their origins, composition, and evolutionary history in wheat is still incomplete. Therefore, this review discusses the current knowledge of the genetic makeup of Bs, their functions, their relationship with A chromosomes, and the morphogenetic consequences in the context of wheat.