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

General DiscussionTwo translocated chromosomes usually form ‘pseudolinkage’ (Albrecht and Chetelat 2009;Farré et al. 2010; Kamphuis et al. 2007). Meanwhile, since normal and translocated segmentslead to reduced crossover interference, distance between markers on normal and translocatedchromosome fragments will be wrongly estimated, marker order is ambiguous along themerged linkage groups and higher stringencies (increase the LODs) do not result in a divisioninto two balanced chromosomes (Albrecht and Chetelat 2009). Third, the existence ofduplication leads to erroneous location of markers in the linkage group associated with thechromosome with duplication (Fig. 6.1). Furthermore, chromosomes with two or more typesof structure variation make the genetic maps even more complicated. In conclusion,chromosome rearrangements not only cause reduced fertility, but also lead to errors whenestimating genetic distances between markers.Sexual polyploidization and its significance in polyploidy mappingWhen interspecific crosses are made between distantly related species, the resulting hybridsare generally sterile. This hybrid sterility is explained to be due to the failure of chromosomeassociation and the forthcoming error-disjoining during meiosis because of the parentaldivergence (Asano 1982). However, there is still a wide genetic variation, with someindividuals possessing a low fertility. These outstanding genotypes normally produceunreduced (2n) gametes, as well as fewer n gametes (Ramanna and Jacobsen 2003). Theprocess of restoring fertility through unreduced gametes is termed as sexual (meiotic)polyploidization, as a comparison with asexual (mitotic) polyploidization. The production ofunreduced gametes has been reported in many interspecific hybrids, such as Lilium (Van Tuylet al. 1989), Alstroemeria (Kamstra et al. 1999), Allium (Khrustaleva and Kik 1998) andothers. One of the main advantages of sexual polyploidization, compared with its counterpart,is the occurrence of intergenomic recombination during the production of unreduced gametes,which will lead to segregation and diversity in the next generation (Ramanna and Jacobsen2003). The segregation in the resulting population provides a possibility for genetic mapping.When detecting intergenomic alteration in sexual polyploidized allopolyploids, molecularcytogenetic techniques (GISH and FISH) are more powerful compared with molecularmarkers. The detecting efficiency of the two methods with respect to unreduced (2n) gametesis compared in Fig. 6.3. GISH can simultaneously detect intergenomic recombination,characterize the crossing over events and trace the origin of non-sister chromatid exchangeswhen combined with meiosis observation. However, molecular markers with multi-locusanalysis in crossing progenies cannot detect reciprocal crossing over, and quantification ofallele number by the intensity of bands is not always accurate (Gaeta and Pires 2010; Nicolaset al. 2007). For example, in four types of segregated products between homoeologouschromosomes with a single or two strand double crossover during FDR meiosis (Fig. 6.3),GISH can detect all the intergenomic recombinations, whereas molecular markers can onlydetect two types of them. In the recombinant chromosomes derived from different crossing77