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

Chapter 4There are some criteria to distinguish the bridges and fragments with respect to theirdifferent origins. Bridges from paracentric inversion and U-type exchanges result in differentmeiotic configuration at the first meiotic division, which can be recognised through a criticalmeiotic observation. The first difference is that bridges and fragments from inversionheterozygote involve non-sister chromatids, while U-type exchanges can happen betweenboth sister and non-sister chromatids. Another feature caused by inversion is the invariablesize of the fragments. No matter where the crossover happened in the inversion loop, theresultant acentric fragments should be of constant size. On the contrary, asymmetricalbivalents, fragment size variation and side arm bridges are all evidence for the occurrence ofU-type exchanges. Moreover, bridges and fragments in some species and species hybrids,which had been considered to originate from inversion, have been proven to be derived fromU-type exchanges. In the species of Tradescantia and Paeonia brownii, “the occurrence ofinversion was presumptive and circumstantial” and the presence of bridges and fragmentshave finally been explained as due to U-type exchanges (Lewis and John 1963). In conclusion,inversion heterozygote, as well as U-type exchanges, lead to anaphase bridging with differentconfiguration at meiosis.Meiotic bridges not only occur spontaneous, but can also be induced by genomic shock,including radiation treatment and interspecific hybridization. Radiation treatment, which isprobably the most efficient method, leads to chromosome breakage and various types ofanaphase bridging in a number of species like Lilium longiflorum (Mitra 1958), Zea mays(Viccini and De Carvalho 2002), Triticum (Wu and Yu 2001) and many others. Interspecifichybridization is another cause of the dicentric bridge production in a wide range of specieshybrids like Vigna umbellate × V. minima (Gopinathan and Babu 1986), Pinus hybrids(Saylor and Smith 1966) and so on. Interestingly, the bridges and fragments in F1 hybridsfound between some species in the genus Chorthippus, which were once thought to haveoriginated from paracentric inversion, have been proven to arise from spontaneouschromosome breakage and reunion (Lewis and John 1966). In all of the above mentionedspecies and species hybrids, meiotic bridges were studied using traditional cytogeneticmethods.Genomic in situ hybridization (GISH) and fluorescence in situ hybridization (FISH) havethe potential to give more convincing results about the origin of anaphase bridging. These twomethods, which enable the localization of labelled probes after DNA hybridization, can notonly distinguish non-sister chromatids in hybrids and allopolyploids (GISH), but also check48