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

Chapter 4bridges is explained as U-type exchange between non-sister chromatids. In this process, twononsister chromatids from homoeologous chromosomes broke respectively, two brokenchromatids, which possessed centromeres, mismatched together and formed a bridge, and twoacentric chromatids formed a fragment at anaphase I (Fig. 4.2). The other type of bridgesand fragments involved sister-chromatids (Fig. 4.1d). This type of bridging not only happenedwith Asiatic chromosomes (Fig. 4.1d), but could also be detected in Longiflorumchromosomes (Fig. 4.1e). In this case, the bridges and fragments only showed onefluorescence. FISH experiments with telomere repeats and 45s rDNA as probes revealed thatthe fragments possessed two normal telomeres in all cases, and the two 45s rDNA loci on thebridge further confirmed that the bridge involved sister-chromatids, (Fig. 4.1g). According tothe configuration of the bridge-linked homoeologous chromosomes revealed by GISH andFISH, formation of this type of bridges & fragments involved a U-type exchange betweensister chromatids and a single crossing over between non-sister chromatids (Fig. 4.2).There were two additional indications for the occurrence of U-type exchanges. The firstone is that U-type exchanges occured not only between homoeologous chromosomes, but alsobetween two non-homologous chromosomes from Asiatic genome (Fig. 4.1f). In this case,both the bridge and the fragment showed the same fluorescence. The second proof was thepresence of a putative ring chromosome with an additional fragment, except another bridgeand fragment (Fig. 4.1h). This broken chromosome was recognized as chromosome number 2from Asiatic genome, which was sub-metacentric and showed a very strong 45s rDNA locuson the short arm near the second constriction. It was deduced that a U-type exchangehappened between two sister chromatids, and no crossover (or rarely with even number ofcrossovers) happened between non-sister chromatids of these two homoeologouschromosomes. As a result, one part of the broken chromosome formed a ring chromosomeand two other arm fragments fused together and formed an acentric fragment with twotelomere (Fig. 4.2). The ring chromosome will cause anaphase bridging in the second meioticdivision. In conclusion, the bridges produced at anaphase I during meiosis of the interspecificlily hybrids were the outcome of chromosome breakage and fusion, with or without crossingover between homoeologous chromatids.DiscussionIn the present study, anaphase bridges with fragments between sister and non-sisterchromatids were observed and the origin was found to be due to U-type exchanges. There area number of reasons for this conclusion: 1) chromosome breakage was found at metaphase I;54