Rice Genetics IV - IRRI books - International Rice Research Institute

parative organization of the Pooideae and Panicoideae chromosomes. The involvementof R10 in two different insertion events would suggest that an independent R10,as in rice itself, is the more primitive configuration.Although it is tempting to suggest that the extent of evolutionary chromosomalrearrangements will be correlated with evolutionary time (Paterson et al 1996), it isbecoming clear that the different chromosome structures can become fixed at varyingrates in different species. For example, the rearrangements between relatively closelyrelated Triticeae genomes that have been isolated by speciation for less than 10 millionyears, such as wheat, rye (Devos et al 1993), or Aegilops umbellulata (Zhang etal 1998), may be almost as extensive as those between wheat and rice. Almost norearrangements are apparent between wheat and barley, however, which are moredistantly related than wheat and rye (Fig. 2).Segmental chromosome duplication is also becoming recognized as being morecommon than we originally thought. Extensive duplication will be a hindrance tomany applications, such as in cross-genome map-based cloning in which “walks”could get deflected by inadvertent jumping from one segment to its duplicate. Inaddition, duplicated genes will probably be more prone to divergence and deletion, orsilencing prior to eventual deletion. These processes will all give rise to apparentlocal disruptions in colinearity. Information on the extent of duplication emerges slowlywhere it depends on genetic mapping of duplicated pairs of genes; however, where anorganism has been completely sequenced, the analysis is much more straightforward.1HH. vulgare–wheat2H 4H 3H6H5HS. cereale–wheat Ae. umbellulata–wheat4U7H 1R 2R 3R 4R7R6R 1U5U5R2U 6U3U 7UFig. 2. Chromosome rearrangements within the Triticeae tribe—the rye and Aegilops umbellulatagenomes relative to wheat. Although the basic chromosome number is constant at 2x=14throughout the tribe, the juxtaposition of linkage blocks carried on individual chromosomes canbe considerably rearranged in some species relative to the cultivated wheat and barley genomes.The extent of rearrangement appears not to be a reflection of evolutionary time orbreeding system—S. cereale is an outbreeder, whereas Ae. umbellulata is an inbreeder. (Reprintedfrom Gale et al, 2001, with permission from the publisher.)Rice: a central genome for the genetics of all cereals 83