Chapter Nine - Prehistory of the Angiosperms: Characterization of the Ancient Genomes

Abstract

Abstract Angiosperms are by far the largest group of land plants with more than 30,000 living species. Genomics approaches have been most widely applied in the angiosperms, where significant genomics resources, including whole-genome sequences, have been developed. The genomics of land plant lineages that branched off earlier than the angiosperms provide valuable references for comparative evolutionary analyses. In this chapter, we therefore first reviewed the evolutionary history of plant lineages from green algae to angiosperms and current knowledge discovered from the chloroplast genomes in major ancestral groups. We then reviewed the genomic features and resources from three representative outgroup species suitable for comparison to angiosperms (Chlamydomonas reinhardtii, Physcomitrella patens and Selaginella moellendorffii). In ferns, we discussed extensive RNA editing in plastid genomes and the very high chromosome numbers in homosporous ferns. Despite the lack of fully sequenced genomes to date in gymnosperms, rich expressed sequence tag and BAC resources are available, with several genome sequencing projects in progress. We discuss one of the most controversial issues in gymnosperm systematics, which is the phylogenetic position of Gnetales. Finally, we discuss recent studies elucidating two ancestral polyploidy events before the origin of angiosperms—one in the common ancestor of extant seed plants and the other in the common ancestor of angiosperms. These two very ancient polyploidy events could help to provide better insight into the rise and eventual dominance of seed plants and angiosperms.