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

Small aberrant chromosomes and B chromosomes in Lilium hybridschromosome centric fission and fusion lead to the production of isochromosomes. In maizeand wheat, meiotic univalents not only randomly move to one pole when segregating atanaphase I, but also have a tendency to misdivide at the centromere (Lukaszewski 2010).Such centromere misdivision gives rise to centric translocation, production of telocentric andisochromosomes (Kaszas et al. 2002; Lukaszewski 2010).The occurrence of telocentrics and isochromosomes has been reported previously in Liliumspecies (Brandram 1967). They have been called accessory chromosomes. Whether theybehave similar to B chromosomes from other species is not known. Because the origin of Bshas been considered as a ‘mystery’, it might be worthwhile to investigate the origin of thesesmall aberrant chromosomes as the ones observed in this study in more detail. One instance inwhich the mode of origin of a B chromosome has been investigated is in Plantago lagopuswhich involves the formation of a minichromosome, amplification of 5S rDNA, stabilizationof telomeric repeats and formation of an isochromosome (Dhar et al. 2000; Jones et al. 2008b).Compared to this mode of origin, the formation of isochromosomes from the short armsfollowing misdivision of the centromere, as described in this investigation, is a more simplemechanism for the potential origin of Bs.The presence of rDNA repeats in two cases deserves a comment. In more than 30 plantspecies the presence of rDNA sequences on Bs has been recorded (Dhar et al. 2002; Donald etal. 1995; Flavell and Rimpau 1975; Friebe et al. 1995; Jones 1995; Maluszynska andSchweizer 1989). A good example resembling the aberrant chromosome in genotype 074051-9 is the B chromosome found in Allium cernuum. Using Ag-NOR banding, the Bchromosome was found to be median and possessed rDNA sites with nucleoral activity onboth arms (Friebe 1989). Furthermore, there is information suggesting that NOR regions areprone to chromosome breakage and this may provide a mechanism behind the appearance ofB chromosome following interspecific hybridization (Beukeboom 1994; Camacho et al. 2000;Jones and Houben 2003). It is not known whether rDNA sites of As are more vulnerable forbreakage compared to other chromosome regions. It may be pointed out that such breakagemay not result in a chromosome fragment that can survive on its own, a centromere isabsolutely necessary. For this reason, it might be logical to assume that a chromosome armthat possesses a secondary constriction or nucleolus organizer, is probably more susceptiblefor breakage, or centromere misdivision. In tomato, the origin of an isochromosome of theshort arm of chromosome 2 (2S) is instructive in this connection. Moens (1965) reported theoccurrence of an isochromosome of 2S in Lycopersicon esculentum which had resulted fromthe misdivision of the centromere in a trisomic of chromosome 2. In addition to beingheterochromatic, 2S also carries the nucleolus organizer. Although this isochromosomepossessed a functional centromere, telomeres in addition to nucleolus organiser, it was notstable morphologically (Quiros 1976) but was transmitted to the progenies, accumulating asmany as eight copies in some of the progenies. In a later study, the isochromosomes of 2Swere shown to be highly unstable due to breakage-fusion-bridge cycle (Ramanna et al. 1985).69