Figure 3

Aberrant meiotic disjunction of the X-chromosome in mei-W68 homozygous ( dmspo11 -deficient) Drosophila oocytes induced by plant SPO11s. (A) Normal chromosome segregation in meiosis. Black and white bold lines represent homologous chromosomes, such as X-chromosomes. Each pair of black or white bold lines indicates a parental chromosome duplicated by premeiotic DNA replication, a milestone of the start of meiosis, and each pair remains together until meiosis II. Black and white ovals represent their centromeres. All pairs of duplicated homologous chromosomes (left; indicated as two black and two white lines) are physically connected by at least a chiasma (indicated by a cross) formed by meiotic recombination, and then are segregated into sister cells (middle). After meiosis II, each recombinant or nonrecombinant chromosome is received by each of the four meiotic products (Egg). (B) In nondisjunction at meiosis I, one of the sister cells receives both X-chromosomes and the other receives no X-chromosome. In meiosis II, the former sister cell generates two diplo-X eggs, and the latter yields two nullo-X eggs. Thus, each non-disjunction event of the X-chromosome in meiosis I generates two diplo-X eggs and two nullo-X eggs. (C) If a DSB(s) is introduced into one of the X-chromosomes, but is not repaired at prophase of meiosis I, then the broken X-chromosome is lost. When random segregation follows in meiosis I, one of the sister cells receives the single X-chromosome and the other receives the duplicated X-chromosome. In meiosis II, the sister cell that received the single X-chromosome generates one nullo-X egg and one normal egg, and the other sister cell that received the duplicated X-chromosome generates two normal eggs. Thus, the X-chromosome aberration in meiosis generates more nullo-X eggs than diplo-X eggs.