Evolutionary fate of rhizome-specific genes in a non-rhizomatous Sorghum genotype

What is the fate of organ-specific genes after the organ is lost? For Sorghum propinquum and Sorghum halepense genes that were previously shown to have rhizome-enriched expression, we have conducted comparative analysis of both coding regions and regulatory sequences in Sorghum bicolor (non-rhizomatousness) and S. propinquum (rhizomatousness). Most genes with rhizome-enriched expression appear to have similar numbers of paralogous copies in both genotypes, with only three of 24 genes studied showing significant differences in copy numbers. We detected no greater propensity for mutation in S. bicolor than in S. propinquum of genes with rhizome-enriched expression in the latter. Several cis-acting regulatory elements, particularly an Myb-binding core (AACGG) that is involved in the regulation of the mitotic cyclin, were more abundant in promoters of S. propinquum than in non-rhizomatous S. bicolor or Oryza sativa (rice). We suggest that many genes with rhizome-enriched expression in S. propinquum may serve multiple functions, with partial loss of some of these functions in S. bicolor but ongoing purifying selection acting to preserve the remaining functions. Expressed genes in polyploid S. halepense rhizomes appeared to be more frequently derived from the S. propinquum than the S. bicolor progenitor, but there was some evidence of formation of novel alleles and 'recruitment' of S. bicolor genes to rhizome-enriched expression in S. halepense, suggesting that polyploidy may have offered new evolutionary potential to S. halepense.