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

Chapter 6been detected with GISH and FISH, and later on a U-type reunion led to the formation ofanaphase bridges and fragments (Chapter 4). Finally, microspores with different chromosomenumbers have also been detected after meiosis (Zhou et al. 2008a). In conclusion,intersectional lily hybrids show a range of abnormalities during different stages of meiosis.Some other kinds of meiotic abnormalities in interspecific lily hybrids have also beenreflected and emphasized by progeny analysis. The first evidence is the polyploidizedbackcross progenies. The resultant progenies from crosses involving interspecific lily hybridswere predominant triploids, indicating the functional gametes were unreduced gametes andthe mechanism has been identified as first division restitution (FDR) and indeterminatemeiotic restitution (IMR) (Lim et al. 2001a). The second feature in backcross progenies of lilyis aneuploidy. When analyzing the genomic composition of these triploid lily hybrids, a smallproportion of aneuploids has been found. The last character of the backcross progeny is thepresence of isochromosomes. In a few genotypes, resulting from some interspecific hybrids ofLA lilies, isochromosomes with different sizes were detected, and these newly-generatedsmall aberrant chromosomes were derived from the fusion of the two short arms of themissing chromosomes during meiosis, respectively (Chapter 5).Crossing over and introgression breedingThe role of crossing over during evolution and speciation has long been realized and studiedin flowering plants. Crossing over, which is one of the key features that distinguish meiosisfrom mitosis, not only facilitates the proper segregation of homologous chromosome in thefirst meiotic division, but also generates novel combinations of alleles via homologouschromosome exchanges. This process, in addition to maintaining the ploidy level duringsexual reproduction, contributes to genetic diversity, which is essential for introgressionbreeding.Crossing over between homoeologous chromatids has been proven to be less frequent ascompared with crossing over between homologous non-sister chromatids. In monosomicadditions of tomato, a homologous bivalent (II) together with a univalent was the mainmeiotic configuration, GISH has revealed that the number of rod bivalents (stands for singlecrossing over) was much higher compared with that of ring bivalents (stands for other types ofcrossing over which probably lead to chromosomes with two or more recombinant sites),indicating single crossing over was the predominant type of exchange between homologouschromosomes. While in the substitution line of tomato SL-8, reduction of homoeologousrecombination has been revealed by the considerable decrease of ring bivalent formation (Jiand Chetelat 2003). Similarly, results from several studies of homeologous recombinationbetween chromosomes of wheat and related species have showed the absence of multiplecrossovers (Dubcovsky et al. 1995; Lukaszewski 1995, 2000; Luo et al. 1996; Luo et al. 2000).Homoeologous crossing over has been proven to occur with different frequencies indifferent species hybrids. Although different types of crossing over events have been checked80