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

Intergenomic recombination and chromosome translocation in Lilium hybridsthe detection of not only reductional and equational segregation of chromosomes in somecases but also identify different types of cross-overs between the non-sister chromatids ofhomoeologous chromosome pairs. Assuming normal chromosome pairing and crossing-overin LA hybrids, one can expect different types cross-overs to occur, viz., two strand single, twostrand double, three strand double, four strand double and multiple cross-overs (Fig. 3.3). Allthe expected types have been found in the present study (Fig. 3.2a, b, c and d). Then thequestion arises whether these exchanges of segments between the non-sister chromatids ofeach pair of homoeologous chromosomes should be considered as recombinations ortranslocations? Undoubtedly the latter possibility must be ruled out because cross-over eventsare locus specific events between homoeologous pairs of chromosomes. Moreover thefrequencies of exchanges per cell are so high that it is inconceivable that such high rates ofchromosome mutations (translocations) can occur spontaneously in any organism.Based on difficulty of crossing, F1 hybrid sterility, reduced chromosome pairing andclearly differential fluorescence of chromosomes, the genomes of L. longiflorum and Asiaticspecies are well differentiated. Despite this, however, there is almost completely normalcrossing-over between some of the pairs of homoeologous chromosomes. Considering thecrossover types observed in this study as well as those reported earlier (Zhou et al. 2008a), thegenomes of different groups of Lilium species appear to be homo-sequential. So much so, thatit has enabled the construction of cytological maps of three different genomes, viz.,Longiflorum, Asiatic and Oriental groups (Khan et al. 2009a). In view of the high degree ofhomoeology between the genomes, the exchanges of chromosome segments through crossingoverevents cannot be considered as translocations.The occurrence of either duplication or translocation in the cultivar ‘Connecticut King’ isof considerable importance because attempts have been made to construct molecular maps inrecent years (Abe et al. 2002; Khan 2009; Shahin et al. 2011; Van Heusden et al. 2002). Amain problem to establish molecular maps is that the number of linkage groups exceed thebasic chromosome number. One cause of this problem is the presence of very large genomesin Lilium species with a large basic chromosome number (x=12), which means larger numberof markers and mapping individuals are needed. On the other hand, chromosomerearrangements provide another candidate reason for the mapping problem of lily. Althoughthe relationship between chromosome structure variation and genetic mapping has not beenwell studied, limited reports have showed that reciprocal translocation can also cause thevariation of linkage groups (Farré et al. 2010; Kamphuis et al. 2007; Larson et al. 2011). Untilnow, none of the chromosomes of Lilium species have been associated with any of the genesor molecular markers so far. In this context, it might be essential to carefully analyse thegenome of ‘Connecticut King’ and identify the aberration that has been encountered.41