Comparative pollen structure and pollination biology in the Callitrichaceae

Abstract

The Callitrichaceae is a widespread, monotypic (Callitriche) family, consisting of approximately 50 terrestrial, amphibious, and obligately submersed species. Several pollination syndromes have been suggested to occur within the genus, including anemophily, epihydrophily (pollen transfer at the water surface), and hypohydrophily (entirely submersed pollination). However, cross-pollination has only recently been unequivocally documented in the family. Terrestrial species appear to be geitonogamous, and produce distinctly tricolpate pollen, with a well-defined, intectate exine. In the various amphibious species, fertilization can occur in two different ways: “typical geitonogamy”, as in terrestrial species, or by “internal geitonogamy”, whereby pollen grains precociously germinate within indehiscent anthers and pollen tubes subsequently grow through nodal and pistillate tissues directly to the ovules. Anemophily is also likely in these amphibious species. Pollen grains have weakly differentiated apertures, but well-developed, intectate exines. Molecular data confirm that the obligately submersed C. hermaphroditica is hypohydrophilous. The actual mechanism of pollen transfer has not yet been observed, but several features suggest that pollen grains are effectively dispersed by the water. Pollen of C. hermaphroditica is exineless in perennial forms of the species, and has only a rudimentary exine in annual forms. Callitriche represents the only infrageneric system in which both aerial and underwater pollination co-occur, and provides an unprecedented opportunity to investigate the evolution of hypohydrophily from aerial systems. © Société botanique de France 1994.