Male reproductive structures of Araucaria araucana and Wollemia nobilis (Araucariaceae, Coniferales) in light of conifer evolution

Abstract

Pollen cones of Araucaria araucana and Wollemia nobilis were investigated. They consist of hundreds of hyposporangiate microsporangiophores with numerous microsporangia, arranged in up to three abaxial rows. Convincing evidence was found that excludes the widely accepted sporophyll concept of coniferous microsporangiophores. It is shown that the units treated generally as sporophylls in fact represent a complex fusion product of numerous microsporangiophores (= synangia). The vasculature in the microsporangiophore may indicate that the hyposporangiate shape is derived from an ancestral perisporangiate condition. In particular, the bundle strands ending blindly in the tissue in distal parts of the scutellum can only be understood as belonging to aborted microsporangia. Transferring this concept to other conifers with hyposporangiate microsporangiophores a perisporangiate origin seems also highly likely for them. This is well supported by the fact that in usually hyposporangiate taxa occasionally perisporangiate microsporangiophores are developed. Thus, the gained results clearly indicate that each microsporangiophore corresponds to a lateral cone (= flower equivalent structure), and the entire pollen cone to a compound structure (= inflorescence equivalent structure). Thus, the evolutionary pathway of araucariaceous pollen cones fits well to the pseudanthial origin suggested previously for the male reproductive structures in other coniferous groups, e.g., Pinaceae or Taxaceae.