Multigene evidence for close evolutionary relations between Gromia and Foraminifera

Gromia oviformis is a common marine rhizopod, possessing a large ovoid membraneous theca that resembles the tests of certain monothalamous (single-chambered) foraminifers. In fact, the genus Gromia was initially classified among the Foraminifera, but because of its non-granular, filose pseudopodia it was later included among filopodia-bearing protists (the Filosea). Recent molecular phylogenies suggested that Gromia branches among Cercozoa, a heterogeneous assemblage of mainly amoeboid protists, which show some affinities to Foraminifera. To test how closely related are Gromia and Foraminifera, we have analysed the sequences of actin, large subunit of the RNA polymerase II (RPB 1) and small subunit (SSU) rRNA genes. We also analysed the structure of the polyubiquitin gene of G. oviformis. Our analyses show that Gromia's actin is specifically related to one of the two actin genes families known in Foraminifera. In RPB1-based phylogenies, Gromia appears as the closest relative of Foraminifera, while in the SSU rRNA trees it branches as sister to Foraminifera and Haplosporidia. We identified also a single serine insertion in the polyubiquitin of Gromia, similar to that found in Foraminifera, Plasmodiophorida and some Cercozoa. Altogether, these findings support the hypothesis that the morphological resemblance between Gromia and Foraminifera may be due to a shared common ancestor. If further analyses of protein-coding genes including a more representative sampling of Cercozoa confirm this relationship, then the molecular study of G. oviformis will be of key importance for understanding the origin of Foraminifera.