The rDNA genes were also mapped on the pericentromeric region of homologous chromosomes 3 and 7 common to all individuals by FISH with biotin-labeled 18S rDNA. At least 250 cells from 12 individuals(6 males and 6 females) were evaluated for the presence of FISH hybridization signals (green fluorescent spots, summarized in Fig. 2). It can be seen from Fig. 2 that the signals on chromosome pair 7 were unique and restricted to the pericentromeric region(Fig. 2c), but it was slightly different on chromosome pair 3. On chromosome pair 3, the signals were restricted to the pericentromeric region (Fig. 2a and Fig. 2b) or away slightly from the centromere (Fig.2c). The signals in the former and the later situation were 78.3 % and 21.7 % respectively. No other chromosomal sites were labeled above the background. Because of the lack of genome compositional com-partmentalization in fish genomes compared with warm-blooded vertebrate, it is difficult to identify bands on the metaphase obtained with different banding techniques and localize genes accurately(BERNARDI 1993). In order to map rDNA genes accurately, ISH was carried out to the bivalents. More than 280 meiotic chromosomes from 14 males were analyzed to verify the presence of the hybridization signals (black spots, summarized in Fig. 3). Compared with the feature of G-banding (YU et al.1994), the signals were localized on bivalents 3q12-q24 and 7q14-q26. With regard to the slight differences
among positions on chromosome pair 3 obtained by FISH, this might result from a different condensation of the rDNA-containing chromatin caused by the utilization of colchicines or PHA. Compared with the metaphase, the bivalent has several advantages in gene mapping: 1) the number of the bivalents of an individual is half that in metaphase; 2) the bivalent is slimmer and longer,which is ideal material for banding and gene mapping. The good correspondence in the localization of the brighter signals in all individuals analyzed obtained by FISH to the metaphase and ISH to the bivalents clearly identifies the unique localization of rDNA on the pericentromeric region of homologous
3 and 7 (bivalents 3q12-q24 and 7q14-q26), and indicates that the better resolution can be obtained with ISH in the bivalents. Without doubt, the applications of ISH to the bivalents will advance gene mapping in M. albus and other fishes.