A 192bp allele at the Xgwm261 locus is not always associated with the Rht8 dwarfing gene in wheat (Triticum aestivum L.)

The height-reducing gene Rht8 was introduced into Italian wheats by breeder Nazareno Strampelli from the Japanese landrace Akakomugi, and has been widely used in wheats adapted to southern and eastern European conditions. Following identification of a close linkage to Rht8, microsatellite marker Gwm261 has been used extensively to screen large numbers of diverse international germplasm. A 192bp allele at this locus has been taken as "diagnostic" for Rht8 and used to infer the international distribution of Rht8. In this paper, we report several instances of cultivars and mapping populations that vary for the presence of the 192bp allele at the Xgwm261 locus (Xgwm261 ^sub 192^), but with no associated reduction in height, suggesting a lack of association with Rht8. For instance, in the population derived from a cross between Sunco (Rht-B1b, Xgwm261 ^sub 165^) and Tasman (Rht-D1b, Xgwm261 ^sub 192^), there were significant height differences associated with the segregation of Rht-B1b and Rht-D1b, but no height differences between Xgwm261 genotypes. Similar results were obtained in a population derived from the cross between Molineux (Rht-B1b, Xgwm261 ^sub 192^) and Trident (Rht-D1b Xgwm261 ^sub 208^). In contrast, the cross between Trident and Chuanmai 18 (Xgwm261 ^sub 192^) gave significant height effects at both the Rht-D1 and Xgwm261 loci, with no epistatic interaction between loci. Chuanmai 18 is closely related to the Strampelli wheat Mara (ancestrally derived from Akakomugi) and is therefore likely to carry Rht8. The old Japanese cultivar Norin 10, used by Norman Borlaug to introduce Rht-B1b and Rht-D1b into Mexican wheats, also has a 192bp allele at the Xgwm261 locus, and the sequence of the amplified product is identical to that of Akakomugi. We suggest that the widespread use of Norin 10-derived germplasm during and after the Green Revolution introduced a second haplotype into international germplasm, in which Xgwm261 ^sub 192^ has no association with Rht8. Therefore, the presence of Xgwm261 ^sub 192^ is only indicative of Rht8 in wheat cultivars that have inherited this allele from Akakomugi or a Strampelli wheat ancestor.[PUBLICATION ABSTRACT]