Genomic Duplication, Fractionation and the Origin of Regulatory Novelty

Having diverged 50 MYA, rice remained diploid while the maize lineage became tetraploid and then fractionated by losing genes from one or the other duplicate region. We sequenced and annotated 13 maize genes (counting the duplicate gene as one gene) on one or the other of the pair of homeologous maize regions; 12 genes were present in one cluster in rice. Excellent maize-rice synteny was evident, but only after the fractionated maize regions were condensed onto a finished rice map. Excluding the gene we used to define homeologs, we found zero retention. Once retained, fractionation (loss of functioning DNA sequence) could occur within cis-acting gene space. We chose a retained duplicate basic leucine zipper transcription factor gene because it was well marked with big, exact phylogenetic footprints (CNSs). Detailed alignments of lg2 and retained duplicate lrs1 to their rice ortholog found that fractionation of conserved noncoding sequences (CNSs) was rare, as expected. Of 30 CNSs, 27 were conserved. The 3 unexpected, missing CNSs and a large insertion support subfunctionalization as a reflection of fractionation of cis-acting gene space and the recent evolution of lg2's novel maize leaf and shoot developmental functions. In general, the principles of fractionation and consolidation work well in making sense of maize gene and genomic sequence data.