Development of the exineless pollen wall in Callitriche truncata (Callitrichaceae) and the evolution of underwater pollination

The Callitrichaceae are a monogeneric family of aquatic angiosperms compris ing approximately 50 terrestrial, amphibious, and obligately submersed spec ies. Callitriche is unique in being the only known genus with co-occurring aerial and underwater pollination systems. Mature pollen structure is corre lated with growth habit, pollination biology, and phylogeny within the genu s. In the present study, development of exineless pollen in the obligately submersed species Callitriche truncata was examined, with particular emphas is on the tetrad stage. Pollen ontogeny occurred rapidly and non-synchronou sly; tetrads, free microspores, and two-celled pollen grains were identifie d within the same anthers. Formation of the intine also occurred relatively early, during the tetrad stage. Tetrads were surrounded by a structurally distinct envelope, and its ultrastructure and histochemistry indicate that this callose-like envelope is in a transitional state. Reduction or complet e loss of the exine has evolved at least twice in Callitrichaceae, and the new ontogenetic data indicate that exine loss evolves more quickly than the loss of callose. In addition, developmental information on exineless polle n in C. truncata coupled with other palynological data for the exine-bearin g terrestrial and amphibious growth forms provide support for the hypothesi s that underwater pollination has had a relatively recent origin in the fam ily.