Figure 4.

(a) Depiction of stabilization of univalent transmission of the B chromosome in meiosis. González-Sánchez et al. showed that a B univalent can be out of the plate and with the orientation to the poles at metaphase I of meiosis [22]. At meiotic anaphase I, the B chromosome proceeds to the poles ahead of the regular chromosomes, as shown by Carlson [20] and González-Sánchez [22]. These behaviours of the B chromosome are thought to facilitate the transmission of the B univalent. (b,c) Diagram of the high loss (HL) phenomenon induced by the presence of B chromosomes. From Rhoades & Dempsey's conclusions [88,89], in the HL line without the B chromosome, chromosome 3 with a large knob on 3L divides normally during pollen meiosis and mitosis. However, when a microsporocyte contains multiple B chromosomes such that the resulting microspores would contain two copies of the B chromosome, chromatids in the knob region on the 3L remain attached at the anaphase of the second pollen mitosis, whereas the centromere of chromosome 3 proceeds towards opposite poles. This process fractures 3L. The pollen grain will have one sperm cell with a truncated chromosome 3 and the normal chromosome 3 in the other sperm cell. In some cases, the knob region can divide before the fracture of 3L and both sperm cells have one copy of an intact chromosome 3. Note that the assortment of the nondisjoined B chromosomes is random, which means the B chromosomes could proceed towards the same or opposite poles (shown here).