Analyses of random BAC clone sequences of Japanese cedar, Cryptomeria japonica

Conifers have larger genomes than most angiosperms, long generation times, and undergone relatively few chromosome duplications during their evolution. Thus, conifers are interesting targets for molecular evolutionary studies. Despite this, there have been few studies regarding their genome structure, and these studies are mostly limited to the Pinaceae. Our target species, Cryptomeria japonica, belongs to the Cupressaceae family, which is phylogenetically separated from the Pinaceae family by a few hundred million years, and is the most important timber tree in Japan, making investigation of its genome structure both interesting and worthwhile. We analyzed the sequences of eight random bacterial artificial chromosome (BAC) clones from C. japonica and compared them with sequences of comparable size from eight other model plants, including Arabidopsis thaliana and Pinus taeda. From this analysis, we identified several features of the C. japonica genome. First, the genome of C. japonica has many divergent repetitive sequences, similar to those of Physcomitrella patens and P. taeda. Additionally, some C. japonica transposable elements (TEs) seem to have been active until recently, and some might be unidentified novel TEs. We also found a putative protein-coding gene with a very long intron (approximately 70 kb). The three Pinaceae species whose genome sequences have been determined share these features, despite the few hundred million years of independent evolution separating the Pinaceae species from C. japonica.