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

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Chapter 5Centric breakage and fusion lead to the production of isochromosomes in progeny of LA ×AA crossAs mentioned before, a common feature of the three genotypes with isochromosomes in theprogeny of LA × AA cross is that they were all aneuploids with a loss of a normalchromosome. Interestingly, GISH results revealed that the new generated isochromosomeswere from the same genome as the missing chromosome in all of the three genotypes. As anext step, a comparison was made between the isochromosomes and the correspondingmissing chromosomes according to the arm length and FISH analysis. Results showed that thearm length of the new generated chromosome was the same as the short arm length of themissing chromosome in these three genotypes respectively (Table 5.2). Furthermore, 45SrDNA signals were detected in the proximal position on the isochromosome in genotype074051-9, which was exactly the same as the short arm of the missing chromosome L4 (Fig.5.3). In view of these facts, it was proposed that these new generated isochromosomesoriginated from a centric breakage and fusion of two short arms of the missing chromosomesduring meiosis.DiscussionThe small aberrant chromosomes in three genotypes of LA hybrids have been proposed tooriginate from centric fission and fusion of two short arms of the missing chromosomes.Firstly, there are reliable indications that the small aberrant chromosomes in the progeny ofLA × AA backcross have originated independently in the BC1 generation. None of the parentspossessed any small aberrant chromosome comparable to those observed in the three progenyBC1 plants; secondly, all three genotypes that possessed aberrant chromosomes had ananeuploid chromosome number of 35 instead of the expected 36 As; thirdly, the smallaberrant chromosome and the missing chromosome in each of the three genotypes arerespectively from the same genotype; fourthly, as what has been shown in the results part, thesimilarity of arm length relationship and 45S rDNA distribution also strongly support thehypothesis; and finally, chromosome breakage and fusion have been found during meiosis ofinterspecific hybrids of Longiflorum × Asiatic (LA) lilies (see chapter 4). All these evidenceindicates that due to misdivision of the centromere, two telocentric chromosomes are formed.The telocentric long arm is probably eliminated whereas the short arm has given rise to anisochromosome which has survived. This survival might be due to the fact that, in one step, astable chromosome with a functional centromere and telomeres at both ends are formed. Itmeans that the species of the genus Lilium are well positioned to generate aberrant smallchromosomes such as the ones reported in this study. This is because, the karyotypes ofLilium species possess two pairs of median or sub-median chromosomes while the other 10pairs are highly asymmetrical with very small or minute short arms relative to the long arms(Lim et al. 2001b; Stewart 1947). Furthermore, there are some other proofs to support that68
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
Page 3 and 4:
A molecular cytogenetic analysis of
Page 5:
Table of ContentsChapter 1General I
Page 8 and 9:
Chapter 1LilyLilies belong to genus
Page 10 and 11:
Chapter 1counterparts (Finnegan 200
Page 12 and 13:
Chapter 1renewed interest in detect
Page 14 and 15:
Chapter 1recombination, mechanisms
Page 16 and 17:
Chapter 1is only an equational segr
Page 18 and 19:
Chapter 1quite divergent with vario
Page 21 and 22:
Chapter 2An assessment of chromosom
Page 23 and 24: Chromosome rearrangements in Lilium
Page 37 and 38: Chapter 3Elucidation of intergenomi
Page 39 and 40: Intergenomic recombination and chro
Page 49: Intergenomic recombination and chro
Page 52 and 53: Chapter 4AbstractMeiotic abnormalit
Page 54 and 55: Chapter 4There are some criteria to
Page 56 and 57: Chapter 4FISH experiments were perf
Page 58 and 59: Chapter 4was apparently shorter, wi
Page 60 and 61: Chapter 4bridges is explained as U-
Page 62 and 63: Chapter 4of the sexual polyploidize
Page 64 and 65: Chapter 4fragment have the same len
Page 66 and 67: Chapter 5AbstractSupernumerary (B)
Page 68 and 69: Chapter 5their origin, the structur
Page 70 and 71: Chapter 5Fig. 5.1. Discovery of B c
Page 72 and 73: Chapter 5In order to investigate th
Page 76 and 77: Chapter 5It has not, however, been
Page 78 and 79: Chapter 6The results presented in t
Page 80 and 81: Chapter 6model for molecular cytoge
Page 82 and 83: Chapter 6Fig. 6.2. The meiosis proc
Page 84 and 85: Chapter 6over events during FDR mei
Page 86 and 87: Chapter 6been detected with GISH an
Page 88 and 89: Chapter 6Another feature caused by
Page 91 and 92: ReferencesAbe, H.A., Nakano, M.N.,
Page 93 and 94: ReferencesChen, Q., and Armstrong,
Page 95 and 96: ReferencesHartlerode, A.J., and Scu
Page 97 and 98: ReferencesLarson, S.R., Kishii, M.,
Page 99 and 100: ReferencesMcClintock, B. 1931. Cyto
Page 101 and 102: ReferencesRai, R., Zheng, H., He, H
Page 103 and 104: ReferencesStewart, R.N. 1947. The m
Page 105: ReferencesZhang, L., Pickering, R.,
Page 108 and 109: Summarychromosome rearrangements. T
Page 111 and 112: SamenvattingLelie (Lilium) is in de
Page 113 and 114: Samenvattingaantal 35 met daarnaast
Page 115 and 116: 摘要百合系百合科百
Page 117 and 118: Acknowledgements淡看世事去
Page 119: Curriculum VitaeSonglin Xie was bor
Page 123: Education Statement of the Graduate
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