FUNCTION, CONSTRAINT, AND THE EVOLUTION OF SEED PLANT REPRODUCTIVE ALLOCATION

Because the functions that plant organs perform can be well constrained, even in ancient groups, the fossil record of terrestrial plants offers an excellent opportunity to explore how function can influence broad patterns of morphological evolution. In this study, we relate patterns of resource allocation in ovules, vital components of seed plant reproductive structures that ultimately develop into the seeds, to the functions that they perform during reproduction by comparing ovule sizes at several important stages in development. The earliest seed plants invested most of their reproductive resources in ovules prior to the reception of any pollen and therefore prior to the start of the reproductive process. In contrast, all living seed plant groups delay at least some aspects of resource investment until after pollination occurs, and some reproductively advanced groups such as flowering plants delay any significant investment until after pollen has been transferred and the embryo has been formed. While delaying investment is a more efficient use of resources, the various functions that ovules perform constrain patterns of resource allocation. For example, some living seed plant groups are very efficient in minimizing investment prior to pollination, but they require pre-existing tissue for embryo provisioning that limits their efficiency in minimizing investment prior to fertilization. Many living seed plant groups have shifted at least some ovule functions to associated tissues, which may relax these constraints, and reproductively advanced groups like flowering plants appear to have shifted nearly all functions away from the ovules, perhaps underlying their more efficient reproductive cycles. Patterns of ovule development in seed plants as a whole suggest consistent modification of the reproductive cycle towards increased efficiency within bounds set by functional constraints. However, very long time periods were apparently required for these changes; fossil evidence suggests that more than 200 million years separates the appearance of the seed habit and the evolution of the highly efficient reproductive cycles of flowering plants.