Viçosa, Brazil

Universidade Federal de Viçosa

Cytogenetic mapping of sister chromatid exchange points in maize --Borges, CMO , Carvalho, CR In order to map the sister chromatid exchange (SCE) points in maize, image analysis resources by computational methods (Rasband. NIH-image 1.60 anonymous ftp from zippy.nimh.ninh.gov) were used in association with cytogenetic techniques. Maize seeds from the test line from Universidade Federal de Viçosa were germinated and the roots treated with a 100 µM 5-bromo-2`-deoxyuridine solution for 22 hours and then with a 0.02% colchicine solution for 2 to 3 hours, both done in the dark at 28 C. The roots were fixed in a methanol:acetic acid (3:1) solution and slides were prepared by the air-drying technique after enzymatic maceration (Carvalho, Saraiva. Heredity, 70:515-519,1993). The preparations were stained with Hoechst 33258 for 10 minutes, irradiated with a UV-254 nm light for 5 to 15 minutes and stained with a 2% Giemsa solution, for 10 minutes (Perry, Wolff. Nature, 251,156,1974). Images of chromosome figures were captured directly from the microscope by means of a video camera coupled to a computer and digitized with an image analysis software. To generate 256 gray value density profile plots, chromosome images were calibrated to the range of 0 (white) to 255 (black) and spatial calibration from chromosome length was performed in micrometers. This methodology permitted the identification of the SCE points with enough resolution to measure the relative and absolute positions of the exchanged segments (Fig. 1). These results were used to elaborate the maps of the positive and negative regions of the SCEs.
 
 

Figure 1. The plot pixel value of 256 gray scale of sister chromatid exchanges in maize chromosomes (numbers: 6, 9, 1 and 3). The background picture was set to a 0 gray value and chromosome length in micrometers. C = centromere.
 
 


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