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Ovule Function and the Evolution of Angiosperm Reproductive Innovations

A broad perspective of seed plant reproductive biology, including information from both living and fossil gymnosperm groups, is crucial in order to understand the evolution of angiosperm reproductive innovations. Within this context, angiosperms are unique in the degree to which they have shifted reproductive functions away from their ovules to associated tissues and organs. This is particularly true in regard to the carpel, which has assumed roles in pollination, seed protection, and seed dispersal previously restricted to the ovule itself. Perhaps as a consequence, angiosperm ovule development is more efficient than in nearly all other groups of living and extinct seed plants, where reproductive functions often constrain both patterns of development and the allocation of resources. For example, many living gymnosperm species are as efficient as angiosperms in minimizing ovule development prior to pollination, but they require a megagametophyte for embryo provisioning that constrains their efficiency in minimizing investment prior to fertilization. By decoupling ovules from pollination, the carpel also fundamentally altered the way in which pollination occurs in angiosperms relative to other seed plants. In particular, this change gave angiosperms greater control over the spatial positioning of their reproductive organs and may have therefore expanded the types of reproductive structures angiosperms could develop and the specificity with which these structures could interact with pollinators.