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Brief overview of the foraminiferan Reticulomyxa filosa

Reticulomyxa filosa is a foraminiferan that lives in a diversity of terrestrial freshwater habitats, ranging from damp leaf litter in close proximity to tidal wetlands (Nauss 1949), to fresh water lakes (Hülsmann 2006). Unlike the majority of other foraminiferans, R. filosa does not possess an external test (or shell), but is instead characterized by a “naked” globose cell body that is covered by a glycocalyx and surrounded by an extensive network of anastomosing pseudopodia (or reticulopodia) (Koonce and Schliwa 1985, Pawlowski et al. 1999). The pseudopodial array can extend for several centimeters out from and around the periphery of the central mass of the cell.

The reticulopodia of R. filosa exhibit bidirectional streaming, a phenomenon in which organelles and cytoplasmic vesicles are simultaneously transported away from and in towards the central cell body (Bowser and Travis 2002). Particles, such as food items, can also be transported on the surface of the reticulopodia. The reticulopodia are underlain by a complex network of branched and anastomosed microtubules (Chen and Schliwa 1990). 

The multinucleate cytoplasm of R. filosa contains hundreds to thousands of haploid nuclei that are 5 µm in diameter. In laboratory cultures, only asexual mitotic multiple fission (plasmotomy) of the multinucleate cytoplasm has been observed; however, numerous genes that code for meiosis and the potential flagellar apparatus of gametes are present in the genome (Glöckner et al. 2014). In response to deteriorating environmental conditions, vegetative individuals may undergo multiple fission and form spherical-to-ovoid resting stages that can temporarily survive unfavorable conditions for a few weeks at a time. When conditions improve, the resting stages recover and reassemble their pseudopodial networks (Nauss 1949). During the summer months, walled cysts may form in response to adverse environmental conditions, such as extreme temperature and extended periods of desiccation (Gothe et al. 1997). 

The genome of Reticulomyxa filosa is the first foraminiferal genome to have been sequenced (Glöckner et al. 2014). The genome is approximately 320 Mb in size, highly repetitive, and contains 40,443 predicted protein-coding genes (with a mean of length of 957 bases). Of these genes, 29,352 are unique to R. filosa. Genes introduced by lateral gene transfer from bacteria appear to be relatively rare. 

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