Isozymes of Eleusine (Gramineae) and the origin of finger millet

The predominantly African grass genus Eleusine comprises nine species, including diploids and tetraploids based on n = 8, 9, and 10. Among the polyploids are the important crop finger millet, Eleusine coracana subsp. coracana, and its putative wild ancestor, E. coracana subsp. africana. Eleusine coracana is believed to be an allotetraploid derived by hybridization between E. indica and an unknown diploid. To evaluate this hypothesis, 16 isozyme loci coding nine enzymes were compared among seven of the nine Eleusine species (E. intermedia and E. semisterilis were unavailable). Genetic variability differed substantially among diploid species, ranging from P = 0.563, A = 1.6, H = 0.208 in E. indica to P = 0.188, A = 1.2, H = 0.042 in E. jaegeri. The diploids tended to be genetically distinct, with values of Rogers' Similarity ranging from S = 0.294 (E. jaegeri/floccifolia) to S = 0.794 (E. indica/tristachya). Both subspecies of the tetraploid E. coracana exhibited fixed heterozygosity at several loci, verifying their hypothesized allotetraploid status. Both tetraploids also possessed E. indica marker alleles at all loci, corroborating ancestry by this taxon. Genotypes of the non-indica ancestor, inferred separately for each tetraploid, differed substantially from all candidate diploids and also from each other. These data indicate that 1) none of the candidate diploids investigated is likely to have been the non-indica ancestor of E. coracana, and 2) the non-indica ancestor of the wild tetraploid may differ from that of the crop. The latter conclusion is inconsistent with the complete chromosomal homology exhibited between the two tetraploid subspecies, indicating the need for additional evidence bearing on their relationships.