Gene expression patterns in seed plant shoot meristems and leaves: homoplasy or homology?

The fossil record reveals that seed plant leaves evolved from ancestral lateral branch systems. Over time, the lateral branch systems evolved to become determinate, planar and eventually laminar. Considering their evolutionary histories, it is instructive to compare the developmental genetics of shoot apical meristems (SAMs) and leaves in extant seed plants. Genetic experiments in model angiosperm species have assigned functions of meristem maintenance, specification of stem cell identity, boundary formation, polarity establishment and primordium initiation to specific genes. Investigation of roles of the same or homologous genes during leaf development has revealed strikingly similar functions in leaves compared to SAMs. Specifically, the marginal blastozone that characterizes many angiosperm leaves appears to function in a manner mechanistically similar to the SAM. We argue here that the similarities may be homologous due to descent from ancestral roles in an ancestral shoot system. Molecular aspects of SAM and leaf development in gymnosperms is largely neglected and could provide insight into seed plant leaf evolution.