DNA content, repeatome composition and origin of the Zea mays micronuclei

Micronuclei originate from DNA damage generated by clastogenic and/or by aneugenic effects. Depending on the pattern of damage, they may have distinct genomic origin and composition. Sequences of the centromere, telomere and rDNA have been identified in plant micronuclei. However, other DNA sequences may also be present in the micronuclei, as well as their DNA contents may be different. Here, we investigate the DNA content, genomic composition and origin of micronuclei induced in Zea mays by methyl methanesulfonate (MMS). DNA contents showed a wide range of distribution, suggesting their diverse genomic origins and illustrating how much of the nuclear genome can be lost due to mutagen effects. Micronuclei diversity was also evidenced by in situ probing with different DNA sequences (5S and 18S rDNAs, 180-bp knob and Grande LTR-retrotransposon) and by 6-diamidino-2 phenylindole (DAPI) fluorochrome. Perhaps these sequences are hotspots for MMS damage, especially the Grande LTR-retrotransposon, 5S and 18S rDNAs, which are rich in guanine. In addition, probe pools were constructed from individual genomic DNA of two microdissected micronuclei. These probe pools hybridized on all Z. mays chromosomes. However, the centromere, knob and secondary constriction were hybridized by only one probe pool, evidencing the distinct genomic composition of the micronuclei. We illustrate the micronuclei genomic diversity as they originated from several different chromosomes following the MMS treatment, and demonstrate the extent of the genotoxic damage to the genome. We provide some insights into micronuclei structure and diversity, and show that they can be further explored in mutagenesis research.