Properties and Prospects of B Chromosomes

Nonessential, supernumerary B chromosomes have been an intriguing anomaly known to researchers for over a century. B chromosomes were first identified in insects and have since been discovered in hundreds of species across many different taxa. Their prevalence has made it difficult to strictly define the nature of a B chromosome due to the variety of sizes, genetic compositions, and segregation behaviors; however, with modern molecular technologies, the impact, origin, and evolutionary trajectory of these B chromosomes are coming into focus. In this review, we consolidate what is currently known about B chromosomes, including their life cycle and evolutionary trajectory, the impact of B chromosomes on their host, and how B chromosomes are transmitted to the next generation. We highlight several prominent B chromosome model systems that have led to significant advances in B chromosome biology, serving as a prologue for the exciting future of the B chromosome field and its many unknowns.