Petal development and elaboration.

Petals can be simple or elaborate, depending on whether they have complex basic structures and/or highly specialized epidermal modifications. It has been proposed that the independent origin and diversification of elaborate petals have promoted plant-animal interactions and, therefore, the evolutionary radiation of corresponding plant groups. Recent advances in floral development and evolution have greatly improved our understanding of the processes, patterns, and mechanisms underlying petal elaboration. In this review, we compare the developmental processes of simple and elaborate petals, concluding that elaborate petals can be achieved through four main paths of modifications (i.e. marginal elaboration, ventral elaboration, dorsal elaboration, and surface elaboration). Although different types of elaborate petals were formed through different types of modifications, they are all results of changes in the expression patterns of genes involved in organ polarity establishment and/or the proliferation, expansion, and differentiation of cells. The deployment of existing genetic materials to perform a new function was also shown to be a key to making elaborate petals during evolution.