| Duplicated genes created during polyploid formation ('homoeologues') may experience a variety of fates depending upon the evolutionary forces operating on these loci. Homoeologue divergence may be limited if selection operates to maintain duplicate gene function, or divergence may be permitted if selective pressure on a functionally redundant locus is relaxed. In an attempt to determine the fate of duplicate loci in a polyploid genome, I have isolated and described sequence evolution at 15 sets of homoeologous loci from allotetraploid cotton (Gossypium L.) and the corresponding orthologues from its progenitor diploid genomes. Homoeology and orthology relationships of these loci have been demonstrated by in-situ hybridization for the 5S rDNA array, and by comparative linkage mapping for 12 low-copy anonymous loci and two known cellulose synthase genes, CelA1 and CelA2. In combination, these results demonstrate that relaxation of selective pressure (as indicated by an increase in the substitution rate) at duplicate loci subsequent to polyploidization may be minimal across the majority of loci in this duplicated genome. Ten of these loci (which correspond to mapped anonymous PstI-genomic probes) show rate equivalency between polyploid subgenomes and between subgenomes and their progenitor diploid genomes, indicating that selection continues to limit divergence at these loci. In contrast, the remaining five loci (5SrDNA, A1550, A1713, CelA1 and CelA2) show significant rate differences among the genomes tested. Two of these loci (CelA1 and CelA2) are known to be preferentially expressed in developing cotton fiber, and they show significantly elevated substitution rates in the D- and A-subgenome lineages, respectively. In addition, CelA2 from the A-subgenome of G. hirsutum has experienced a marked rate acceleration since polyploidization, and has accumulated a greater than expected number of non-synonymous substitutions without exhibiting the hallmarks of pseudogenization. These results indicate that cellulose synthase A2 may be responding to directional or diversifying selection, perhaps as a consequence of human-mediated selection upon fiber quality attributes. |
