Flowering plants (angiosperms) are the dominant land plants of today, forming an integral part of the carbon, oxygen and water cycles and are thus essential to the climate stability of the planet. Flowering plants are also essential for human existence in providing food and medicines, as well as resources for building, clothing, manufacturing, and many other uses. Angiosperms have been proposed to have arisen ~120 to 170 million years ago (MYA), between the Cretaceous period and end of the Jurassic period –– according to molecular estimates, as well as fossil records. They then rapidly diversified into >350,000 species, with monocot and eudicot groups accounting for 20% and 75%, respectively, of modern angiosperm species. The forces driving such evolutionary success, and rich biological diversification, remain unclear.
Evolutionary relationships among flowering plants have long been investigated, with most studies focusing on visible traits, such as flower and pollen grain morphologies. Advances in DNA sequencing technologies led to studies based on molecular markers in the 1990s. Since the turn of the century, there has been an explosion of publications reporting plant genome sequences, the blueprint of all aspects of an organism’s history; these provide new opportunities to decipher the critical events underlying the evolutionary success of angiosperms. Recently, there has been a shift toward studies of angiosperm evolution –– based on reconstructed ancestral genomes that no longer exist rather than low-resolution (gene-based) pairwise comparisons of extant genomes.
This new approach provides an integrated framework in which evolutionary forces can be accurately deciphered by comparing inferred extinct and derived extant genomes. Reconstructed paleogenomes may not be absolutely identical to the true ancestral genome, but they almost certainly have a very similar structure. Thus, they constitute an elegant means of bridging the gap between ancestral and extant genomes to infer the evolutionary history of modern angiosperms. This ultimately leads to identification of conserved compartments (stable regions under purifying selection) and lineage- or species-specific compartments (plastic regions under accelerated evolution).+
Authors in the attached article show that reconstruction of the genome of the most recent common ancestor (MRCA) of modern monocots and eudicots –– accounting for 95% of extant angiosperms –– with its potential repertoire of 22,899 ancestral genes conserved in present-day crops. This MRCA genome provides a starting point for deciphering the reticulated evolutionary plasticity between species (rapidly versus slowly evolving lineages), subgenomes (pre- versus post-duplication blocks), genomic compartments (stable versus labile loci), genes (ancestral versus species-specific genes), and functions (gained versus lost ontologies), which are the key mutational forces driving the success of polyploidy in crops. Estimation of the timing of angiosperm evolution, based on MRCA genes, suggests that this group emerged ~214 MYA during the late Triassic era, before the oldest recorded fossil. This MRCA genome should constitute a unique resource for scientists to dissect major agronomic traits in translational genomics studies –– extending from model species to crops.
Nature Genet April 2o17; 49: 490–496