| Summary: | <p>Abstract</p> <p>Background</p> <p>The benzoxazinoids 2,4-dihydroxy-1,4-benzoxazin-3-one (DIBOA) and 2,4-dihydroxy-7- methoxy-1,4-benzoxazin-3-one (DIMBOA), are key defense compounds present in major agricultural crops such as maize and wheat. Their biosynthesis involves nine enzymes thought to form a linear pathway leading to the storage of DI(M)BOA as glucoside conjugates. Seven of the genes (<it>Bx1-Bx6</it> and <it>Bx8</it>) form a cluster at the tip of the short arm of maize chromosome 4 that includes four P450 genes (<it>Bx2-5</it>) belonging to the same <it>CYP71C</it> subfamily. The origin of this cluster is unknown.</p> <p>Results</p> <p>We show that the pathway appeared following several duplications of the <it>TSA</it> gene (α-<it>subunit of tryptophan synthase</it>) and of a Bx2-like ancestral <it>CYP71C</it> gene and the recruitment of <it>Bx8</it> before the radiation of Poaceae. The origins of <it>Bx6</it> and <it>Bx7</it> remain unclear. We demonstrate that the Bx2-like <it>CYP71C</it> ancestor was not committed to the benzoxazinoid pathway and that after duplications the <it>Bx2-Bx5</it> genes were under positive selection on a few sites and underwent functional divergence, leading to the current specific biochemical properties of the enzymes. The absence of synteny between available Poaceae genomes involving the <it>Bx</it> gene regions is in contrast with the conserved synteny in the <it>TSA</it> gene region.</p> <p>Conclusions</p> <p>These results demonstrate that rearrangements following duplications of an <it>IGL/TSA</it> gene and of a <it>CYP71C</it> gene probably resulted in the clustering of the new copies (<it>Bx1</it> and <it>Bx2</it>) at the tip of a chromosome in an ancestor of grasses. Clustering favored cosegregation and tip chromosomal location favored gene rearrangements that allowed the further recruitment of genes to the pathway. These events, a founding event and elongation events, may have been the key to the subsequent evolution of the benzoxazinoid biosynthetic cluster.</p>
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