Structure and evolution of metareticulate pollen

In a palynological study of the Amaranthaceae, a peculiar type of reticulate pollen was found that is characterized by the presence of a porate aperture in each of the meshes of the reticulum. Previously, this type of pollen has been described as "reticulate". However, closer investigations show that the reticulum in pollen of Amaranthaceae is composed of mesoporia and pores. Consequently, this kind of reticulum represents a fundamentally different type, and is not homologous to the well known examples of pollen grains with a true reticulum (e.g. in Bromeliaceae, Lamiaceae). Therefore, the term "metareticulate" is proposed (i.e., pantoporate pollen with a reticulum-like structure of mesoporia and pores). The new term allows to distinguish between metareticulate and truely reticulate pollen, what is important in phylogenetic studies. Metareticulate pollen occurs only within lineages characterized by pantoporate pollen, and is found to be derived from pantoporate pollen in a cladistic analysis. Apart from the Amaranthaceae, metareticulate pollen evolved parallel in Vivianiaceae and Zygophyllaceae. In Caryophyllaceae and Convolvulaceae only a trend towards a metareticulation is observed. Metareticulate pollen is suggested as representing the highest developmental level in successiformy, which is one of the major patterns in pollen evolution leading from tricolpate to pantoporate grains.