A molecular cytogenetic analysis of chromosome behavior in Lilium ...

Chapter 5In order to investigate the structure of these Bs in more detail, FISH was applied using 5SrDNA and 45S rDNA as probes (Fig. 5.2a). In addition, the telomeric repeat sequence wasused as a probe to detect the status of the chromosome ends. A notable feature was that FISHclearly detected several hybridization sites of both 5S rDNA (green) and 45S rDNA (red) sitesin the standard chromosomes (Fig. 5.2a). The B chromosomes clearly contained two equalarms and possessed blocks of 5S rDNA on both arms (inset in Fig. 5.2a) flanked by telomericsequences (result not shown). In view of the identical morphology of both arms of these Bchromosomes it was concluded that they are isochromosomes.Small aberrant chromosomes in progeny of LA × AA crossOut of 25 triploid progenies derived from LA × AA cross, 22 genotypes were euploids withthe expected 36 chromosomes. None of these normal triploids possessed any extrachromosomes or fragments, similar to both of the parents from which the progeny wasderived (results not shown). In the other three genotypes of this progeny, viz., 074051-9,084798-2 and 084798-6, however, all the somatic cells possess 35 chromosomes togetherwith a small chromosome in all the somatic cells. Because the small chromosome occurred inall three genotypes together with 35, instead of 36 normal chromosomes, the small structureswhich are probably related to the missing A chromosomes in each genotype are called‘aberrant’. The size of the aberrant chromosomes varied from 4.9 to 8.4 μm in differentgenotypes (Table 5.2). The structural organization of these three small chromosomes wasinvestigated through GISH and FISH using 5S rDNA, 45S rDNA and telomeric sequences asprobes. The results of GISH and FISH analyses of the aberrant chromosome in genotype074051-9, are shown in Fig. 5.2b, c and d. GISH results indicated that the small aberrantchromosome in genotype 074051-9 (Fig. 5.2b) and 084798-2 originated from Longiflorumwhereas the aberrant chromosome in 084798-6 was derived from Asiatic genome. By usingtwo probes, 45S rDNA and 5S rDNA, different hybridization sites were detected throughFISH in the complement (Fig. 5.2c). The striking feature, however, was that the smallaberrant chromosome in genotype 074051-9 possessed a hybridization site of 45S rDNArepeat (red fluorescence) on each of its two arms in proximal positions (arrow and inset in Fig.5.2c). When probed with telomeric sequences, FISH clearly demonstrated the presence oftelomeres in the small aberrant chromosome (Fig. 5.2d, arrow and inset). Thus, each arm ofthis aberrant chromosome has a block of 45S rDNA proximally followed by a non-hybridizedregion and a telomere. In two other genotypes, 084798-2 and 08798-6, the small aberrantchromosomes were clearly median chromosomes without any rDNA repeats but possessednormal telomeres as revealed by FISH (results not shown). In view of the similar morphologyof the arms of small chromosomes in all three genotypes, they were concluded to beisochromosomes as well.66