B Chromosomes - A Matter of Chromosome Drive

Abstract

B chromosomes are supernumerary chromosomes which are often preferentially inherited, deviating from usual Mendelian segregation. The balance between the so-called chromosome drive and the negative effects that the presence of Bs applies on the fitness of their host determines the frequency of Bs in a particular population. Drive is the key for understanding most B chromosomes. Drive occurs in many ways at pre-meiotic, meiotic or post-meiotic divisions, but the molecular mechanism remains unclear. The cellular mechanism of drive is reviewed based on the findings obtained for the B chromosomes of rye, maize and other species. How novel analytical tools will expand our ability to uncover the biology of B chromosome drive is discussed.

Keywords: asymmetric cell division; non-disjunction; pericentromere; pollen mitosis; selfish element; supernumerary B chromosome.

FIGURE 1

Hand drawing of Hasegawa (1934) depicting for the first time the process of B chromosome drive at first pollen mitosis in rye (A) metaphase, (B) lagging B chromosome due to non-disjunction. (C) Disjoined sister chromatids of the B chromosome going to different poles, (D) chromosome drive occurs, the future generative nucleus receives both sister chromatids of the B, in contrast the vegetative nucleus contains only the 7 standard A chromosomes. Permission has been obtained for use of copyrighted material from the Japan Mendel Society.

FIGURE 2

Structure and drive of rye B chromosomes. The trans-acting non-disjunction control region (NCR) is enriched in B-specific satellite DNAs expressing non-coding RNA. During first pollen mitosis Bs undergo non-disjunction and both chromatids are included preferentially in the generative nucleus. In contrast, due to the absence of the NCR deficient Bs segregate during the first pollen mitosis like standard A chromosomes.