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

U-type exchanges in Lilium hybridsChromatid breakage is probably a genetic response to genomic shock caused byinterspecific hybridization in lily. Like radiation, interspecific hybridization can cause meioticinstability, which is common in many species hybrids. Our results showed that univalents,multivalents, non-homologous bivalents, bridges as well as ring chromosomes were presentduring meiosis of these lily hybrids. Similarly, univalent, chain and ring multivalents andanaphase bridges were found in the pollen mother cells of a F1 hybrid between Vignaumbellate and V. minima (Gopinathan and Babu 1986). Non-homologous chromosomepairing has also been found in the hybrids of Lolium temulentum × L. perenne. In the hybridsof Helianthus annuus × H. tuberosus, genomic alterations were revealed to be the response togenomic shock following the interspecific cross (Natali et al. 1998). These meioticabnormalities all involved chromatid breakage. Since normal meiosis can be found in both ofthe parents of the hybrids, the meiotic irregularity is probably due to interspecifichybridization. Indeed, during allopolyploid formation, interspecific hybridization, rather thanpolyploidization, is likely the reason of extensive genetic and epigenetic changes (Wang et al.2006). Furthermore, if chromosome breakage occurs at the centromere position, fusion of twobroken chromatids from one chromosome arm will probably lead to the production ofisochromosomes (see chapter 5), which has been also presumed as a mechanism leading to Bchromosomes.We propose that U-type exchanges in lily hybrids are DSBs and the repair mediated byNHEJ. It has been revealed that crossovers are indeed DSBs followed by the repair by HR(Keeney 2001; Puchta 2005; Szostak et al. 1983). In mitotic cells, DSB repair with the sisterchromatid appears to be preferred, whereas interhomolog recombination is favoured duringmeiosis (Pradillo and Santos 2011). Sequence repeats comprise a large fraction of lily genomeand, although they can be quite divergent from each other, their enormous number anddispersal throughout the genome also makes them potential repair templates. Increase of HRmediated events—such as unequal sister-chromatid exchange and ectopic HR between nonallelicrepeated DNA fragments can result in chromosomal rearrangements (Aguilera andGómez-González 2008). As a result, altered karyotypes in yeast have been explained as due toDSBs repaired either by reciprocal unequal sister chromatid recombination or ectopicrecombination between non-homologous chromosome (Loidl and Nairz 1997). However, suchexplanation doesn’t fit the current results for two reasons. Firstly, none of reciprocal unequalrecombination and ectopic recombination can produce bridges and fragments like what hasbeen found in lily (Fig. 4.1). Like in yeast, two mechanisms normally lead to variation ofchromosome size. Even there was an inversion, the chance that two fluorescence of the57