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

Chapter 6Another feature caused by interspecific hybridization is the occurrence of univalentsduring meiosis in interspecific hybrids. Univalents, which arose from association failure, havebeen found in many interspecific hybrids, and are considered to be the main reason of thereduced fertility (Asano 1982; Lee et al. 2011; Lukaszewski 2010; Sears 1950). Meioticunivalents not only randomly move to one pole when segregating at anaphase I, but also havea tendency to misdivide at the centromere (Lukaszewski 2010). Centromere misdivision givesrise to centric translocation, production of telocentric and isochromosomes, which have beenfound in maize and wheat (Kaszas et al. 2002; Lukaszewski 2010). Chapter 5 reported theproduction of isochromosomes, which were derived from centric fission and fusion duringmeiosis of the maternal parent. Meanwhile, chromosome breakage has been found not only inunivalents, but also in bivalents (Chapter 4). As a result, it is still not known whether thenewly-generated isochromosomes in the backcross progenies are the result of centric breakageand fusion from either an univalent or a bivalent.B chromosomes, which extensively exist in many flowering plants, are probably derivedfrom aberrant chromosomes. It is already well accepted that B chromosomes originate frommeiotic errors in which interspecific hybridization provides an ideal platform, and this type ofchromosomes are deduced to be escaped from standard chromosomes (Jones and Houben2003). However, what should be noticed is that the origin of B chromosomes is not a one-stepprocess, which has been shown by Dhar et al. (Dhar et al. 2002) in Plantago. Combined withthe fact that most of the species are involved in at least one round polyploidization, it can beconcluded that B chromosomes arose in the process of speciation of polyploids ininterspecific hybrids, which has been shown by the production of small aberrantchromosomes during sexual polyploidization of lily hybrids.Conclusions and future perspectivesAs presented in this thesis, it has been shown that not only intergenomic recombination whichis derived from crossovers, but also chromosome rearrangements causes genetic variation inbackcross progenies of lily. Moreover, chromosome breakage and fusion lead to theproduction of chromosome bridges at anaphase I stage during meiosis and the generation ofsmall aberrant chromosomes in the backcross progenies. However, to apply these results inpractical breeding, the following research should also be done in the future:Although crossover events have been studied in this thesis, it is necessary to study it on thelevel of individual chromosomes. Since recombination sites on different chromosomes arehighly uneven (Khan et al. 2009a), it is a precondition to make an accurate identification ofindividual chromosomes. Traditional methods to identify chromosomes are based onchromosome length, arm length, arm length ratio and so on, which makes it difficult torecognize chromosomes with short arms since most lily chromosomes are morphologicalsimilar (Noda 1978; Stewart 1947). Later on, a few efforts were made to distinguishchromosomes with different banding techniques (Smyth et al. 1989; Von Kalm and Smyth82