Another Story of O: On the Origin of the Ovule

Explaining the mechanisms by which fundamental novelties originated in the history of life is a key goal of evolutionary developmental biology (evo-devo). Several attractive systems for study are provided by land plants (embryophytes), a monophyletic lineage that generated numerous novel reproductive structures within the last 450 million years, such as archegonia, carpels, stamens, and flowers. An especially intriguing case in point is the ovule, yielding a seed after fertilization and thus leading to the seed habit. Ovules and seeds have significantly contributed to the evolutionary "success" of the seed plants (spermatophytes) in terms of species number, biomass, and land coverage. The evolutionary origin of the ovule remains poorly understood, however. One reason is the huge morphological gap between the ovule and the reproductive structures of the closest extant relatives of seed plants, monilophytes (including ferns). So far, the fossil record does not provide an unequivocal and detailed scenario of ovule origin. Here we review what is currently known about the origin of ovules from both paleobotanical and neobotanical points of view. We outline how novel approaches such as phylogenomics and comparative transcriptomics of extant seed plants and ferns currently help to answer a long-standing question: how did the ovule originate?