Comparative sequence analysis between orthologous regions of the Arabidopsis and Populus genomes reveals substantial synteny and microcollinearity

More than 300 kb of DNA sequence from five Populus bacterial artificial chromosome (BAC) clones was compared with the complete sequence of the Arabidopsis genome to search for collinearity between the genomes of these two plant genera. Approximately 27% of the DNA sequences from the Populus genome were homologous to protein-coding regions in the Arabidopsis genome. BLAST scores and synteny were used to infer orthologous relationships between the Populus and Arabidopsis homologs. The probability that any pair of genes on a single Populus BAC will have orthologs on the same Arabidopsis chromosome is 46%-58%, substantially greater than the 20% expectation if there is no conservation of synteny between the Populus and Arabidopsis genomes. Likewise, the probability that any pair of genes on a single Populus BAC will have orthologs on a single Arabidopsis BAC is 19%-25%, much higher than the 0.1% expected if the orthologs are randomly distributed. These results provide evidence for substantial "pockets" of conserved microcollinearity between regions of the Populus and Arabidopsis genomes as well as for conservation of synteny even when local gene collinearity is not preserved during genome evolution.