Fig 8. Proposed model illustrating the steps leading to chromoanagenesis following pollen irradiation.

Gamma irradiation of binucleate pollen induces double stranded DNA breaks in the generative cell, and results in chromosome lagging or in bridge formation [12] during the second pollen mitosis. The lagging chromosome is excluded from the main nucleus and forms a micronucleus. The sperm cell carrying the micronucleus undergoes karyogamy with the egg cell, and produces a zygote with a (2n-1) nucleus and a micronucleus containing a single paternal chromosome. DNA replication in micronuclei is delayed and leads to chromoanagenesis via two possible mechanisms, chromothripsis and chromoanasynthesis, which were both observed in our poplar lines. Chromothripsis involves fragmentation and random reassembly, while chromoanasynthesis results from replication fork stalling and template switching. The highly rearranged chromosome is eventually released from the micronucleus and reunites with the main nuclear genome during mitotic division. The shattered chromosome is thereafter retained in the main nucleus. SC: sperm cell; EC: egg cell; ECN: egg cell nucleus; CCN: central cell nucleus.