Figure 2.

Components of the B drive mechanism. (a) Diagram of nondisjunction of the B chromosome in maize. The B chromosome (blue with a magenta centromere) is shown in the generative nucleus (G) after the first pollen mitosis. After replication, the two chromatids proceed to the same pole (red arrow) at the second pollen mitosis in the vast majority of divisions. Thus, most mature pollen grains contain two sperm cells (S) with only one containing the B chromosomes. (b) Depiction of preferential fertilization. For most lines of maize, the sperm with the two B chromosomes will preferentially fertilize the egg (E), as compared with the central cell (C) in the process of double fertilization. The fertilized egg develops into the next generation embryo and the fertilized central cell develops into the endosperm.