Phylogenetic analysis of the acetyl-CoA carboxylase and 3-phosphoglycerate kinase loci in wheat and other grasses

We have applied a two-gene system based on the sequences of nuclear genes encoding multi-domain plastid acetyl-CoA carboxylase (ACCase) and plastid 3-phosphoglycerate kinase (PGK) to study grass evolution. Our analysis revealed that these genes are single-copy in most of the grass species studied, allowing the establishment of orthologous relationships between them. These relationships are consistent with the known facts of their evolution: the eukaryotic origin of the plastid ACCase, created by duplication of a gene encoding the cytosolic multi-domain ACCase gene early in grass evolution, and the prokaryotic (endosymbiont) origin of the plastid PGK. The major phylogenetic relationships among grasses deduced from the nucleotide sequence comparisons of ACCase and PGK genes are consistent with each other and with the milestones of grass evolution revealed by other methods. Nucleotide substitution rates were calculated based on multiple pairwise sequence comparisons. On a relative basis, with the divergence of the Pooideae and Panicoideae subfamilies set at 60 million years ago (MYA), events leading to the Triticum/Aegilops complex occurred at the following intervals: divergence of Lolium (Lolium rigidum) at 35 MYA, divergence of Hordeum (Hordeum vulgare) at 11 MYA and divergence of Secale (Secale cereale) at 7 MYA. On the same scale, gene duplication leading to the multi-domain plastid ACCase in grasses occurred at 129 MYA, divergence of grass and dicot plastid PGK genes at 137 MYA, and divergence of grass and dicot cytosolic PGK genes at 155 MYA. The ACCase and PGK genes provide a well-understood two-locus system to study grass phylogeny, evolution and systematics.