Pollen-cones of Pinus bungeana Zucc. ex Endl. (Pinaceae, Coniferales): Do they indicate a pseudanthial origin?

Abstract

Even today, the evolutionary origin of coniferous pollen-cones is still controversial and conflicting theories about the identity of their microsporangiophores exist. Previous studies strongly suggest that the simple pollen-cone structure of some Taxaceae s.l. is most likely derived from a compound structure and each microsporangiophore represents a lateral, however markedly reduced flower. To test if a similar evolutionary pathway remembering the pseudathium concept sensu Wettstein applies also for Pinaceae, normal shaped and abnormal pollen-cones of Pinus bungeana (Pinaceae) are investigated with SEM and paraffin microtome technique. Pinus sylvestris, which is the type species of the genus, is used as another example. The early development of bisporangiate microsporangiophores starts with two distinct primordia. They fuse in basal parts and form a common stalk. The distal parts remain free and each develops a stalk, a distinct phyllom-like scutellum and one abaxial microsporangium. In some bisporangiate microsporangiophores two monosporangiate microsporangiophores are inserted laterally at the common stalk, which forms a distinct terminal apex. Other microsporangiophores bear a subunit in form of a second, however, aborted microsporangiophore in a lateral position at the base of the common stalk. It is suggested that the bisporangiate microsporangiophore is not a staminate leaf in the sense of a microsporophyll, but a dorsiventral synangium consisting at least of two fused microsporangiophores. In this case each microsporangiophore corresponds to a markedly reduced cone (= flower) which however has lost its pherophyll (= subtending leaf). Similar as shown for Torreya the simple pollen-cone structure in Pinaceae is thus derived from a pseudanthial (= compound) origin.