GENETIC-ANALYSIS OF THE DWARFING GENE RHT8 IN WHEAT - PART-II - THE DISTRIBUTION AND ADAPTIVE SIGNIFICANCE OF ALLELIC VARIANTS AT THE RHT8 LOCUS OF WHEAT AS REVEALED BY MICROSATELLITE SCREENING

Wheat microsatellite WMS 261 whose 192-bp allele has been shown to be diagnostic for the commercially important dwarfing gene Rht8 was used to screen over 100 wheat varieties to determine the worldwide spread o f Rht8. The results showed Rht8 to be widespread in southern European wheats and to be present in many central European wheats including the Russian varieties 'Avrora', 'Bezostaya' and 'Kavkaz'. Rht8 appears to be of importance to South European wheats as alternative giberellic a cid (GA)-insensitive dwarfing genes do not appear to be adapted to thi s environment. The very successful semi-dwarf varieties bred by CIMMYT , Mexico? for distribution worldwide have been thought to carry Rht8 c ombined with GA-insensitive dwarfing genes. Additional height reductio n would have been obtained from pleiotropic effects of the photoperiod -response gene Ppd1 that is essential to the adaptability of varieties bred for growing under short-winter days in tropical and sub-tropical areas. The microsatellite analysis showed that CIMMYT wheats lack Rht 8 and carry a WMS 261 allelic variant of 165 bp that has been associat ed with promoting height. This presumably has adaptive significance in partly counteracting the effects of other dwarfing genes and preventi ng the plants being too short. Most UK, German and French wheats carry an allelic variant at the WMS 261 locus with 174 bp. This could be se lected because of linkage with the recessive photoperiod-sensitive ppd 1 allele that is thought to offer adaptive significance northern Europ ean wheats.