Origin and Early Evolution of Angiosperms

Abstract

Contributions from paleobotany, phylogenetics, genomics, developmental biology, and developmental genetics have yielded tremendous insight into Darwin's "abominable mystery"--the origin and rapid diversification of the angiosperms. Analyses of morphological and molecular data reveal a revised "anthophyte clade" consisting of the fossils glossopterids, Pentoxylon, Bennettitales, and Caytonia as sister to angiosperms. Molecular estimates of the age of crown group angiosperms have converged on 140-180 million years ago (Ma), older than the oldest fossils (132 Ma), suggesting that older fossils remain to be discovered. Whether the first angiosperms were forest shrubs (dark-and-disturbed hypothesis) or aquatic herbs (wet-and-wild hypothesis) remains unclear. The near-basal phylogenetic position of Nymphaeales (water lilies), which may include the well-known fossil Archaefructus, certainly indicates that the aquatic habit arose early. After initial, early "experiments," angiosperms radiated rapidly (<or=5 million years [Myr]), yielding the five lineages of Mesangiospermae (magnoliids and Chloranthaceae as sisters to a clade of monocots and eudicots + Ceratophyllaceae). This radiation ultimately produced approximately 97% of all angiosperm species. Updated estimates of divergence times across the angiosperms conducted using nonparametric rate smoothing, with one or multiple fossils, were older than previous reports, whereas estimates using PATHd8 were typically younger. Virtually all angiosperm genomes show evidence of whole-genome duplication, indicating that polyploidy may have been an important catalyst in angiosperm evolution. Although the flower is the central feature of the angiosperms, its origin and subsequent diversification remain major questions. Variation in spatial expression of floral regulators may control major differences in floral morphology between basal angiosperms and eudicot models.