Nephromyces, a beneficial apicomplexan symbiont in marine animals.

Nephromyces, a beneficial apicomplexan symbiont in marine animals. (SAFFO et al. 2010)

Nephromyces is an very unusual marine endosymbiotic protist. Its peculiar habitat, paired with unusual structural and developmental features has caused a great deal of debate over where exactly it belongs in the phylogenetic tree of life. For a while it was classified as a chytrid fungus due to several shared features with the fungi - such as chitinous walls and the absence of chloroplasts, however several features of this organism like its posteriorly biflagellate cell stage are atypical for fungi.

In this paper, SSU rDNA (short-sub unit ribosomal DNA) sequenced from Nephromyces cells isolated from four species of Molgula species: M.occidentalis, M.citrina, M. manhattensis and M. retortiformis, have given the evidence required to finally place this bacterium into the phylum Apicomplexa where it belongs. This analysis was confirmed via FISH (fluorescence in siti hybridization) which tested for both apicomplexan-specific and Nephromyces-species SSU rRNA oligonucleotide probes. Toxoplasma gondii was used a non-Nephromyces apicomplexan control.

Phenotypic features of the Nephromycesis also support this phylogenetic conclusion. For example characteristics such as  the non-flagellate, motile infective stage found in the blood and similarities between the only stage of Nephromyces known to cross epithelial boundaries strongly resembling the infective stages (sporozoits) of apicomplexans such as Toxoplasma. In addition, Nephromyces spores (the host-transfer stages, which give rise to the infective cells) contain inclusions reminiscent of rhoptries of the apical complex, a key structure in host-cell invasion among apicomplexa, and the ultrastructural hallmark of the apicomplexa and sister lineages

The most exciting point that arises from putting Nephromyces into the apicomplexan genus is the fact that this protist is currently (at the time of this paper) the only known beneficial apicomplexan – a stark constant from the other phylum members, which are either parasites or pathogens!

Nephromycesis has been documented in every adult individual of Molgula species (a highly derived group of ascidians)  surveyed, regardless of population, geographic location, environmental conditions, season or year of collection. This obligate symbiont has (in return) only even been discovered in molgulids, with most of its life history constrained to the renal sac lumen. The renal sac lumen is a “storage tissue” rich in calcium, oxalate, nitrogen and urate, and these characteristics provide a specific physiological niche for Nephromycesis. The high urate oxidase activity of Nephromyces, along with its urate-rich host habitat, suggests that Nephromyces may share, alongside other apicomplexans, a metabolic dependence on host purines

Nephtomycesis  also contains hereditarily transmitted intra-cellular bacteria. These bacteria are a symbiont to our original symbiont! It is hypothesised that they have important metabolic effects with regards to Nephromycesis, including the possibility that these bacteria are the source of urate oxidase activity found in their peroxisome-free apicomplexan hosts. 

There have only been two other descried apicomplexans with bacterial endosymbionts, and the sequence analysis reveals that at least one (Cardiosporidum cionae) is a close relative of Nephromyces.

Saffo, M. B., McCoy, A. M., Rieken, C., & Slamovits, C. H. (2010). Nephromyces, a beneficial apicomplexan symbiont in marine animals.Proceedings of the National Academy of Sciences, 107(37), 16190-16195.