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

U-type exchanges in Lilium hybrids2) bridges involved, not only non-sister chromatids, but also sister-chromatid; 3) breakage andfusion between sister chromatids without crossing over (or rarely even number of crossovers)lead to a ring chromosome together with a fragment, indicating that for U-type exchangecrossing over is not always needed; 4) chromosome breakage and fusion occured not onlybetween two nonsister chromatids of two homoeologous chromosomes, but also between thetwo non-sister chromatids of two non-homologous chromosomes; and 5) in view of thevariation in fragment size, paracentric inversion was excluded. As a result, bridges andfragments at anaphase I during meiosis of these lily hybrids were derived from spontaneouschromosome breakage and fusion, and similar to many species hybrids in which inversionheterozygote was usually expected, judge these bridges from paracentric inversion isarbitrary.DNA in situ hybridization is a powerful technique in studying bridges, which enables thediscrimination of sister U-type exchanges from non-sister U-type exchanges. GISH revealedthat anaphase I bridges and fragments are not sufficient proof for non-sister U-type exchanges.In classical cytogenetics, non-sister U-type exchange was characterized by the configurationof anaphase I bridges and fragments, while sister U-type exchanges were recognised by theanaphase I loops with fragments, univalent loops and univalent bridges at meiosis (Haga 1953;Jones and Brumpton 1971; Jones 1969; Karp and Jones 1983; Walters 1956). Our resultsrevealed that not only non-sister U-type exchanges, but also sister chromatid U-typeexchanges, with a single crossing over, can also give rise to the production of an anaphase Ibridge together with an acentric fragment (Fig. 4.1d and 1e; Fig. 4.2). The only difference ofthese two is that bridges and fragments derived from non-sister U-type exchanges are themerger of chromatid segments from two genomes that are differentially labelled by thefluorescence labelling . On the other hand, anaphase I loops with fragments, univalent loopsand univalent bridges at meiosis are of course proof of sister U-type exchanges, but loops aredifficult to be identified at meiosis since chromosomes are so condensed. However, FISHwith a telomere repeat as probe can simultaneously detect the number of telomeres onindividual chromosomes, which provides convincing proof for ring chromosomes.Anaphase bridging in interspecific hybrids results in reduced fertility, aneuploidy andprobably the production of isochromosomes in the progeny. During male meiosis which willgive rise to haploid pollen, a chromatid bridge will break at one or multiple locations,resulting in chromosome structural changes and/or loss of chromosome material. This willcause half of the gametes from the pollen mother cell to be unbalanced and lethal, and explainthe remarkable reduction of fertility. Since unreduced gametes can endure aneuploidy, some55