Chromosome mapping and phylogenetic analysis of the cytosolic acetyl-CoA carboxylase loci in wheat

The cytosolic isoform of plant acetyl-CoA carboxylase is a multidomain enzy me involved in the synthesis of very-long-chain fatty acids and in secondar y metabolism. Chromosome mapping of wheat identified one locus containing c ytosolic acetyl-CoA carboxylase genes (Acc-2) and a related partially proce ssed pseudogene (psi -Acc-2) in the distal region of the long arm of wheat homoeologous group 3 chromosomes. multiple copies or the Acc-2 genes, whose presence was suggested by sequence analysis, are likely to be arranged in tandem repeats. At least three out of five genes cloned from hexaploid whea t map to this locus. Another locus containing Acc-2-related sequences is pr esent in the distal region of the long ann of chromosome 5D. The identity o f the hybridizing DNA present at this locus remains unknown. A system based on PCR-cloning and DNA sequence analysis of acetyl-CoA carboxylase genes w as developed to address various phylogenetic and systematics questions in g rasses. It was applied to reconstruct the phylogeny of the Acc-2 genes from D- and S-genome Aegilops and A-genome Triticum diploid species, AABB- and AAGG-genome tetraploid wheat, and AABBDD-genome hexaploid wheat, as well as from rye and barley. The combined cytogenetic and molecular evolution appr oach allowed assignment of gene sequences included in phylogenetic analysis to specific loci on homoeologous chromosomes. Recurring gene duplication f ollowed by chromosome translocation and/or possible loss of some gene copie s, as well as loss of introns, occurred in the gene family in different pla nt lineages. Two major Acc-2 clades appeared before the divergence of barle y and rye. Nucleotide substitution rates in different parts of the Acc-2 ge ne were assessed. This analysis of the Acc-2 loci provides detailed informa tion regarding evolutionary events at a low-copy-number locus containing im portant functional genes. These events are likely to be common and to play a significant role in shaping grass genomes.