"Retaria" hypothesis, a sister relationship between Foraminifera and Radiolaria

Abstract

Foraminifera and Radiolaria have calcareous or siliceous shell, which have been preserved in a long fossil record from the Cambrian. Their sensitivity to environmental changes and morphological variation of the shells in their evolutionary histories allow them to use as good environmental indicator and key fossils through geological time. However, these fossilized organisms are the least studied group regarding molecular and cell-ultrastructural studies, making their phylogenetic position unclear. Recent molecular phylogenetic studies along with thorough cell-ultrastructural studies have greatly changed our understanding of systematics of the eukaryotic "domain". In 1990's, molecular phylogenetic studies of Foraminifera and Radiolaria have been started, and grouped them into the eukaryotic supergroup "Rhizaria". However, the phylogenetic positions of Foraminifera and Radiolaria within Rhizaria were controversial until very recently, when many molecular data of Radiolaria have been accumulated. Based on these data, the "Retaria" hypothesis uniting Foraminifera and Radiolaria, as originally proposed by Cavalier-Smith (1999), was verified. Here, we review the molecular phylogenetic studies of Foraminifera and Radiolaria, and discuss their possible evolutionary scenario from the view of paleontology.