Juiz de Fora-MG, Brazil
Universidade Federal de Juiz de Fora
Viçosa-MG, Brazil
Universidade Federal de Viçosa Relationship between chromosome breaks and knob heterochromatin in maize meristematic cells resulting from irradiated pollen --Viccini, LF, de Carvalho, CR Relationships between chromosome breaks and heterochromatin regions have been discussed in the literature. While some authors have reported the occurrence of random breaks, others suggest that chromosome breaks occur, mainly, in heterochromatic regions. With the objective of verifying the relationship between chromosomic breaks induced by gamma irradiation and heterochromatic regions, maize line L-869 (of the Federal University of Viçosa) pollen grains were exposed to 36 and 72 Gy of gamma radiation and soon after used for pollination. Seeds that originated from irradiated pollen were germinated in Petri dishes with a film of distilled water in the dark at 29 C. Root tips ranging from 0.5 to 1.0 cm in length were fixed in fresh cold methanol-acetic acid (3:1). Slides were prepared by the air drying technique with enzymatic maceration. The slides were submitted to C-banding technique to identify the heterochromatic regions. After drying on a hot plate, the slides were stained with Giemsa solution. The percentage of abnormal anaphase cells carrying chromosomal bridges was evaluated. It was observed that the occurrence of bridges was not directly related to the presence of heterochromatic regions (Figure 1). In the case of lower radiation dosages, about 66% of the anaphases showed evidence of heterochromatin on the bridges, while for the higher dosage, only 42% of them presented heterochromatin (Figure 2). Fifty-two percent of the analysed anaphases presented bridges with strongly stained regions, indicating heterochromatic regions. Considering that maize C-banding technique identifies heterochromatic regions present in the knobs, it is possible that, for the anaphases without strongly stained regions on the bridges, the involved chromosomes do not have knobs. This fact hinders the establishment of a straight relationship between the occurrence of bridges and the presence of heterochromatin. These observations suggest that other mechanisms could be involved in the origin of chromosomal structural rearrangements and that the presence of the heterochromatin should not be considered as necessary for the occurrence of bridges.

Figure 1. Mitotic anaphases of a maize line L-869, which was derived from irradiated pollen, lacking evidence of heterochromatin on the bridge. Note the regions deeply stained (heterochromatin) on the chromosomes already separated (bar = 10 µm)

Figure 2. Mitotic anaphases of a maize line L-869, which was derived from irradiated pollen, with evidence of heterochromatin on the bridge (bar = 10 µm)
 
 

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