The Morphology and Evolution of Male Reproductive Structures of Gnetales

Male reproductive structures among extant Gnetales display a high level of morphological correspondence. The two primary architectural elements in the male reproductive structures are cones and flowers. Cones consist of an axis with bracts at each of the nodes and flowers that are positioned in the axils of the bracts or on an annular mound located above the bracts at each of the nodes. Flowers of the extant genera have both a perianth and androecium. The perianth of all three genera has bracts in anterior and posterior positions, but Welwitschia has an additional pair of lateral bracts. The androecium of all three extant genera appears to be structurally homologous with a pair of highly synorganized microsporophylls. Diversity is present among the extant genera at both the cone and flower levels of organization. Some aspects of diversification, such as the introduction of sterile ovules into male reproductive structures, have affected both the cone and flower levels of organization. Extant Gnetales appear to have radiated into most of the basic "morphological spaces" available to them for the placement of ovules. This may reflect enhancements for reproductive ecology, especially the attraction of insect pollinators, that are available when sterile ovules are present. Diversity among the extant Gnetales is strongly biased in two aspects of development: phyllotaxy and synorganization. All male reproductive structures appear to have the phyllotactic constraint of forming more than one appendage per node (or forming a concrescent unit derived from the unification of structures that were more or less separate in an ancestor), and those of most species have a decussate arrangement of appendages along the reproductive axes. Appendages at nearly all nodes of male reproductive structures are synorganized, but within this bias they display various degrees of connation. The complete unification of nodal appendages has created distinctive forms in Gnetum relative to Ephedra and Welwitschia. The arguments presented for recognizing structural homologies among the features of male reproductive structures of Gnetales should provide a stronger framework for coding characters and their states for phylogenetic analyses and for the discussion of morphological diversity among seed plants.