Formation of Two Generative and One Tube Nucleus in Pollen

The microsporocyte with the normal chromosome 9 haploid will undergo two mitotic divisions. The first mitotic division results in a vegetative (tube) nucleus and a generative nucleus. The vegetative nucleus does not undergo a second mitotic division. However, the generative nucleus undergoes a mitotic division, which results in two sperm nuclei.

The microsporocyte with the deficient haploid chromosome 9 will undergo two mitotic divisions. The tube nucleus will be dysfunctional due to the deficiency.

McClintock (1941) suggested that the two microsporocytes with a duplication for the haploid chromosome 9 would develop a dicentric chromosome with a resulting bridge and break during mitosis. If this bridge-break occurred, the two generative nuclei would have different chromatids, one with a deficiency and one with a duplication. The tube nucleus might be without a deficiency and could fertilize the egg.

Double-fertilization

The duplicate haploid chromosome 9 has a ‘sticky’ end. The sticky end that fertilizes the embryo ‘heals’ and is no longer sticky. The embryo is 2n and does not undergo the breakage-bridge-fusion cycle. The endosperm tissue is 3n and the duplicate chromosome 9 haploid sperm nucleus that fertilizes the endosperm nucleus retains the sticky end. During interphase of mitosis each homologous chromosome replicates to form sister chromatids. Sister chromatids with sticky ends fuse. This initiates the breakage-bridge-fusion cycle during successive mitotic divisions of endosperm cells. Variegate sectors in the endosperm result from breakage of the dicentric chromosome.

Telephase of mitosis in endosperm tissue:

Variegated sectors arise from cell lineage derived from this cell which is deficient in locus 1 (Assuming the female parent carried a recessive allele).