A Model of F-actin Organization in Granuloreticulopodia in Foraminifera: Morphogenetic and Evolutionary Implications from Novel Fluorescent and Polarised Light Observations

Foraminifera are a group of mostly marine protists that form highly dynamic granular pseudopodia. Recent live experiments on foraminifera labelled with SiR-actin show that F-actin structures are involved in the morphogenesis of granuloreticulopodia and that pseudopodia contain small, motile granules referred to as SiR-actin-labelled granules (ALGs). They may either represent staining artifacts or an unusual form of organisation of actin filaments. To address this problem, we performed double staining of F-actin in fixed specimens of foraminifera using two fluorescent probes (SiR-actin and Phalloidin Atto 488) and analysed the level of co-localisation of their fluorescent signals. Additionally, we performed observations in polarised light to identify birefringence. The images obtained demonstrate similar staining patterns with both probes and birefringence in areas showing intensive fluorescence, thus, ALGs can no longer be considered as staining artifacts. They represent naturally occurring granular structures that contain F-actin and/or are actin-coated. ALGs likely contain F-actin that may play a role in endo-/exocytosis, pseudopodial movement, and/or in intracellular transport. We present a model, explaining their formation and possible functions in relation to other subcellular components. ALGs most likely consist of the adaptation involved in the morphogenesis of granular pseudopodia that predates in phylogeny the occurrence of the shell in foraminifera.