ALLOZYME VARIATION IN OLD WORLD RACES OF SORGHUM BICOLOR (POACEAE)

A survey of allozyme variation in cultivated races of sorghum (Sorghum bicolor) was undertaken. Eight plants each of 83 accessions representing the five primary races and five of the intermediate races of sorghum were analyzed for 15 enzyme systems encoded by 27 loci. Low levels of variation were found within accessions, which is typical of self-pollinating species. Little variation was also found among accessions. Compared with other cereals, S. bicolor is depauperate in allozyme variation. We found an average of 1.81 alleles per locus with a mean expected heterozygosity of 0.008 for the accessions and total panmictic heterozygosity of 0.093. Only 9% of the variation present was found within accessions, while 91 % was among accessions. Most of the variation present is attributable to differences in geographic origin of the accessions rather than racial differences. Westem and eastem Africa have the highest levels of total heterozygosity (0.108 and 0.088, respectively), whilesouthem Africa has the lowest (0.008). Principal component analysis revealed continuous variation among races and geographic regions with the accessions failing to segregate into discrete racial or geographic clusters. IN THE STUDY of crops and their wild progenitors, isozyme analysis can give a critical assessment of how much genetic diversity is present and how this diversity is distributed among populations. This information provides insights into the evolution of the crop and can also be useful for identifying sources of genetic diversity. Isozyme analysis is well suited to the study of genetic diversity because 1) isozymes normally show high levels of variation within species, 2) they are encoded by numerous loci scattered over the genome, and 3) the analyses are relatively easy to apply to large numbers of individuals. Isozyme analysis may also be used to trace the dispersal of crops that results from human migration or trade. Sorghum [Sorghum bicolor (L.) Moench] is the fourth most important cereal crop worldwide and has been the principal grain crop throughout much of Africa. In addition to use as a cereal, it is cultivated as a fodder, and used for building materials and making brooms. I Received for publication 12 February 1988; revision accepted 21 June 1988. We wish to thank Dr. Terrence Walters for technical assistance in producing the principal component graphics and Drs. Gary Hart, Jim Price and Hugh Wilson for critically reviewing this manuscript. This research was supported in part by a grant from Pioneer Hi-Bred Intemational of Johnston, Iowa, and by the U.S. Department of Agriculture Grant No. 86-CRCR1-2161. All correspondence and reprint requests should be addressed to JFD. Sorghums with sweet and juicy stems (sorgos) are used for making syrup. It is estimated that the domestication of wild sorghum began in the third or fourth millennium BC (Doggett, 1970). The oldest known substantiated archeological sorghum samples were discovered in northeastern Tanzania and were dated to the fourth century AD (Doggett, 1970). It has been suggested that the domestication of sorghum first occurred in subSaharan savannah between present-day Nigeria and Sudan (deWet, 1978), and this crop subsequently spread to tropical West Africa, Southeast Central Africa and Northeast Africa. An exception to this may be race Kafir, for which evidence indicates a possible independent domestication from the wild race verticilliflorum (deWet, 1978). Introgression from local wild sorghums may have been a factor in the evolution of sorghum throughout Africa. The movement of sorghum to India, and eventually China, is believed to have occurred between 1,500 and 1,000 years BC via Arabian trading vessels (Doggett, 1970). Sorghum landraces are cultivated in most countries of Africa and parts of Saudi Arabia, India, and China. The geographical landscape, across which sorghum traverses, varies from tropical rain forests in West Africa to arid deserts in Ethiopia and Sudan. Because of the selection pressures applied by its cultivators and the environmental parameters under which it grew, sorghum has diversified into a mul-