Chromosome damage studies in the onion plant Allium cepa L.

The onion plant (Allium cepa L.) is a suitable indicator plant for the determination of potential genotoxic agents in the samples taken from the environment. The genotoxic level of the agent under study is reflected by structural changes of the chromosomes and their changed numbers. The chromosomes under study are taken from the meristem cells of the young growing onion roots. A healthy normal onion cell has 16 (2n = 16) chromosomes. They are relatively large and so very appropriate for the detection of morphological changes. Prior to the chromosome study the root tip cells were immersed in a 0.1% aquatic solution of colchicine which stopped the mitotic cycle continuing beyond metaphase. The changes in morphology varied from a single distortion of a single chromosome up to several morphological changes observed on many chromosomes. We identified 15 categories of morphological aberrations which are classified into three groups: chromatid damage (CtD), centromere damage (CmD) and chromosome damage (CsD). CtD includes: single break chromatid, double break chromatid, isochromatid break, multiple break chromatid, gap chromatid, centric ring chromatid, acentric ring chromatid and triradial chromosomes. CmD includes: break centromere, gap centromere, single break centromere, double break centromere and multiple break centromere. CsD includes: ring chromosomes and dicentric chromosomes. Sometimes also the chromosome number changed which occurred as aneuploidy with monosomy 2n = 15 (2n - 16 - 1) and euploidy - increased number of the basic chromosome number (2n - 6x - 8x). We identified also the translocation: t(3p-; 5p+).