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

Chromosome rearrangements in Lilium hybridsIntroductionThe occurrence of profound changes in newly synthesized polyploids (neopolyploids) hasbeen recognized for a long time in many plant species (see review, Ramsey and Schemske2002). Such changes occur in both auto- and allopolyploids and exhibit meiotic complexityincluding multivalent pairing, multisomic inheritance and the production of unbalancedgametes. More recent investigations have indicated that extensive “chromosomalrearrangements” commonly occur in neopolyploids of some plant species, the chromosomalrearrangements in these cases include translocations, duplications and deletions. Someexamples of neopolyploids that have been analysed in detail are: Brassica species hybrids(Nicolas et al. 2007; Osborn et al. 2003; Song et al. 1995; Udall et al. 2005) and hybridsbetween wheat and its related species (David et al. 2004; Feldman et al. 1997; Zhang et al.2008). The implications of such chromosomal rearrangements for the evolution of polyploidshave been reviewed (Leitch and Bennett 1997, 2004; Wendel 2000). Moreover, if extensivechromosomal rearrangements do occur, they might have implications for the speciation ofneopolyploids.Apart from other observations on neopolyploids, molecular cytogenetic analyses usinggenomic in situ hybridization (GISH) technique on some of the allopolyploid crops and theirrelatives have revealed the occurrence of several intergenomic translocations in theircomplements. For example, in tobacco (Nicotiana tabaccum L.) nine intergenomictranslocations have been detected (Kenton et al. 1993); in Avena maroccana Gand. five and incultivated oat (A. sativa L.) as many as 18 intergenomic translocations have been identified(Chen and Armstrong 1994; Jellen et al. 1994). It is concluded that such translocations mayoccur following polyploid formation (Leitch and Bennett 1997). In the case of tobacco andwheat there is convincing evidence that these translocations involve nonhomologouschromosomes of different genomes (Parokonny and Kenton 1995; Zhang et al. 2008).Unlike translocations that involve nonhomologous chromosomes, the occurrence of socalled“homeologous translocations” have been reported in the case of neopolyploids ofBrassica napus L. (2n = 4x = 38) (Nicolas et al. 2007; Osborn et al. 2003; Udall et al. 2005).The neopolyploids used in these analyses were produced by crossing dihaploids of B. napus(2n = 2x = 19) as female parents with tetraploid male parents. The progenies in these casesoriginated through the functioning of 2n eggs from the dihaploids and 2x pollen from theeuploid parent. As expected, the progenies were tetraploid. By genotyping theseneopolyploids with molecular markers, extensive chromosomal rearrangements that included“homeologous nonreciprocal translocations (HNRT), duplications and deletions wereobserved (Nicolas et al. 2007). The origin of chromosomal rearrangements was explained asdue to recombination between the two distinct but related genomes of B. napus (AACC), i.e.,A = B. rapa (x = 10) and C = B. oleracea (x = 9) during the formation of 2n eggs in thedihaploids. Thus, based on the examples of wheat and tobacco on the one hand and B. napus17