Rice Genetics IV - IRRI books - International Rice Research Institute

The synthesis shown in Figure 1 is derived from comparisons of genetic maps,particularly RFLP marker data. It is now possible to explore these relationships at themegabase and even DNA sequence levels. Initial results indicate that gene order isgenerally conserved but that small local duplications and other rearrangements arealso common (Bennetzen 2000). Comparisons of the sh2/a1 regions of rice and sorghum(Chen et al 1998) showed strong conservation of gene content and gene order.However, analysis of maize, sorghum, and rice Adh regions indicated considerablegene rearrangements (Tikhonov et al 1999, Tarchini et al 2000). In this latter study,sorghum and maize share nine genes in a colinear order but a further three genesappear to have been deleted from the maize genome. Comparisons of the region ofwheat chromosome 5B containing the Ph1 gene with the homoeologous region ofrice chromosome 9 (Foote et al 1997) demonstrated good colinearity over a 30-Mbregion of rice but also showed evidence for the duplication of one region containingthree markers to a location some 10 cM distant on the same chromosome. Feuillet andKeller (1999) showed commonalities between the regions of wheat chromosome 3,barley chromosome 3H, rice chromosome 1, and maize chromosome 8 containinghomoeologues of receptor-like kinases Lrk10 and Tak10. Nevertheless, some duplicationswere observed and, in wheat and barley, the whole region was found duplicatedon the short arms of the group 1 chromosomes. Interestingly, a lot of the evidencefor rearrangement at the megabase level appears to be associated with diseaseresistance genes and their analogues. It is possible that these genes in particular areprone to more rapid evolution than most genes for adaptation, since they have torespond to pathogens that evolve rapidly themselves.Poaceae evolutionThe differences in segmental chromosome organization of the various economic grasscrop species, expressed relative to the present-day rice genome, can be used to trackevolutionary relationships. This approach exposed maize as an almost complete tetraploid(Fig. 1) very early on. Other major rearrangements, dating back 60 millionyears, can be identified that define two major subfamilies, the Pooideae, as exemplifiedby oats and wheat (and more recently the forage grasses, O.-A. Rognli, personalcommunication), and the Panicoideae, as exemplified by pearl and foxtail millet, sorghum,maize, and sugarcane. These translocations are shown in red in Figure 1. Forexample, in the Pooideae, insertion of present-day rice chromosome 10 (R10) into R5represents the structural organization of the Triticeae group 1 chromosomes. The samesituation exists for oat chromosome A (Van Deynze et al 1995a). Interestingly, in bothwheat and oats, the region corresponding to rice chromosome 10, which displaysnormal recombination with a map length of 150 cM in rice, is highly compressed tojust a few map units in the low recombinogenic centromeric regions of the cerealswith larger genomes.A question of interest to cereal taxonomists that will probably be hotly debatedfor many years to come is the nature of the primeval grass genome. One small steptoward resolving the structure of the ancestral genome may be provided by the com-82 Gale et al