Fluorescence in situ hybridization (FISH)
Slides used for chromosome number analysis were re-fixed in a mixture of 99.9 % Et-OH : glacial acetic acid (3:1) for 2–3 min to remove the DAPI, rinsed with 99.9 % ethanol,air dried and used for FISH experiments. Slides were incubated in 100 lg/mL RNase in 2xSSC for 1 h at 37 C and rinsed three times in 2 9 SSC at room temperature for 5 min. They were then incubated in 10 mM HCl for 5 min at room temperature, treated with an 0.01 mg/ml pepsin solution for 15 min at 37 C and washed in 2 9 SSC.Subsequently, the slides were fixed in 1 % formaldehyde in 1 9 PBS for 10 min and again rinsed three times in 2 9 SSC for 5 min; then dehydrated in 70, 90 and 100 %
ethanol and air dried. The hybridization mixture containing 1 ng/lL of each DNA probe, 50 % formamide, 10 % dextran sulphate, 0.5 % sodium dodecyl sulphate and 2xSSC was pre-denatured at 75 C for 10 min and stabilised on ice for 10 min. A 38 lL aliquot of the hybridization mixture was applied to the chromosome preparations, covered with a plastic coverslip and denatured
together at 75 C for 5 min in an Omnislide Thermal Cycler. Hybridization was carried out at 37 C in a humid chamber for 24 h. Stringent washing of slides at 42 C involved 2 9 SSC for 5 min, two changes of 20 % formamide in 0.1 9 SSC for 5 min and three changes of 2 9 SSC for 3 min. The Coplin jar was cooled to room temperature and slides were rinsed in 2 9 SSC and 4 9 SSC/Tween 20. In the case of the digoxygenin-labelled probe, 180 lL of a 5 % milk-blocking reagent was
applied to each slide and incubated in a humid chamber at room temperature for 30 min, then 45 lL of fluorescein (FITC) conjugated anti-digoxigenin Fab fragments (Roche), at a concentration 1:11 in the 5 % milk-blocking reagent was added. The slides were then incubated at 37 C for 1 h. After incubation, slides were washed in three changes of 4 9 SSC/Tween 20 at 37 C, dehydrated and air dried. A Vectashield mounting medium containing 2.5 lg/mL DAPI was applied to the dry slides before adding coverslips. Slides were analyzed with the use of an epifluorescence microscope (OLYMPUS) with the appropriate filters. Pictures were taken with a CCD camera and
processed with Adobe Photoshop software using only the functions that are applied equally to all pixels of the image.