Effect of B chromosomes on induced and spontaneous mutation frequencies in maize

Two hypotheses (the Bodyguard hypothesis and the ABCW hypothesis) have been proposed that predict that the amount and type of chromatin in the nucleus will affect induced mutation rates. The Bodyguard hypothesis proposes that a function of constitutive heterochromatin may be to protect euchromatin from chemical mutagens. The ABCW hypothesis, states that the mutation rate per locus from ionizing radiation is directly proportional to the haploid DNA content of a species. We altered the total amount of genomic DNA and also the amount of heterochromatin by adding supernumerary B chromosomes (which are largely composed of heterochromatin) to maize (Zea mays L.) cells. We compared induced mutation frequencies at the yellow-green2 (yg2) locus in near-isogenic plants that contained 0 (diploid) or 4 supernumerary B chromosomes (diploid + 4 Bs) to evaluate these hypotheses. We found that the chemical mutagen, EMS, caused significantly higher mutation frequencies in plants that contained 4 B chromosomes (and therefore additional constitutive heterochromatin) than in diploid controls. The Bodyguard hypothesis predicts precisely the opposite result. We also found that ionizing radiation caused significantly higher mutation frequencies in plants with 4 B chromosomes than in diploid control plants. This type of change is predicted by the ABCW hypothesis; however, the extent of the increase observed in this study is much higher than the ABCW hypothesis would predict. The higher mutation frequencies from EMS and radiation in plants that contained 4 B chromosomes was unanticipated, and is the first observation that cells may be more susceptible to mutagenesis when B chromosomes are present. We also compared spontaneous mutation frequencies at the waxy1 (wx1) locus in plants containing 0 or 4-5 B chromosomes, and found that the presence of B chromosomes had no detectable impact. However, the pollen abortion frequency was significantly increased by the presence of 5 B chromosomes.