Rhizarian taxonomy (nitpicky details alert)
Rhizaria is a very morphologically diverse group held together mainly by molecular data. It has everything from amoeboid Chlorarachniophytes, 'famous' for secondary endosymbiosis of a green algal plastid; seashell-like Foraminifera; flagellates like Heteromita, scaly-tested Euglyphids; to spiny Radiolaria, so wondefully illustrated in Haeckel's Kunstformen der Natur (pdf of original available on that page). It's a tremendously understudied group...
So where do Rhizaria fit in the tree of life? From the same year: Moreira et al. 2007 Mol Phylogenet & Evol: basal to Chromalveolates and Archaeplastids; Hackett et al. 2007 Mol Biol & Evol: basal to stramenopiles+alveolates. Shit. Conundrum. Knowing nothing about trees, I could resort to Proof by Impact Factor:
MPE: 3.871Alternatively, we could check later sources: Baldauf 2008 J Systemat & Evol and Keeling 2009 J Euk Microbiol place the Rhizarians basal to stramenopiles+alveolates, although not fully certain, whereas TC-S (sen. author of Moreira et al. paper) insists on sticking it basal to chromalv+archaeplastids in his latest megaphylogenetic megatree of megaeukaryotic megaevolution (TC-S 2009 J Euk Microbiol). Time for Proof by Democracy...
Branching with stramenopiles+alveolates it is. QED.
And suddenly I realise probably no one here (except the taxonomist =P) really cares where Rhizaria roots, so let's move on.
Another taxonomic troublespot is Radiolaria. Originally they included Acantharia, Spumellaria, Nasselaria and Phaeodaria, but it turns out Acantharians are quite unique due to their strontium(!) sulfate skeletons (as oposed to silica in Nasselaria and Spumellaria), while Phaeodarians are a whole other thing altogether, quite distant from the rest of the 'radiolaria'. Nowadays, Nasselaria and Spumellaria are clumped into Polycystea, with Acantharians forming their sister clade. Like this:
(Pawlowki & Burki 2009 J Euk Microbiol)
(Phaeodaria are in Cercozoa)
Ah, so many more new obscure organisms to read about! *drools*
Now for some Coelodiceras spinosum from Paterson et al. 2007 Deep Sea Research Part II: Topical Studies in Oceanography:
Phaeodarians are unicellular organisms composed of a fine cytoplasmic mesh surrounding a delicate silica skeleton. They contain a central capsule with a large opening leading to a phaeodium, a congregation of food and waste vacuoles. They are fierce predators devouring anything from bacteria to dinoflagellates and diatoms by catching them with rhizopodia, or fine thread-like extensions of the cytoplasm. Some have a single large polyploid nucleus (Dogiel' 1965 General Protozoology). Would be pretty interesting if this one also has only a single large nucleus, since cells this big tend to be multinucleate.
The most is known about their skeletal structure, since that is what is easiest to preserve and deal with. They fossilise well, and are used as indicators in the oil industry. Sadly, almost nothing is known about the genetics and developmental biology of these organisms. Many of them inhabit the deep sea (eg. Coelogiceras), and are difficult or simply impossible to culture. As a rule of thumb, predators are complicated to deal with, especially if their lifestyle is unknown - sometimes they may require auxiliary organisms to be present to process their prey in such a way that they can make use of it, for example. It's a pain.
Some more details of the skeleton: (again, from Paterson et al. 2007)
It would be fascinating to know the function and development of these intricate structures, but the paper is mostly descriptive (the specimen was already dead before collection), and finding further information on their cell structure is incredibly difficult. This is probably the most difficult post I've yet written, with about two hours of literature research going into each paragraph on average. But I hope this inspired some appreciation of how complex a unicellular organism can get!
Dogiel, V. A., (1965) General Protozoology. Revised by J. L. Poljanskij and E. M. Chejsin (II. Auflage) VII und 747 S., 326 Abb. Oxford: Clarendon Press
Hackett, J., Yoon, H., Li, S., Reyes-Prieto, A., Rummele, S., & Bhattacharya, D. (2007). Phylogenomic Analysis Supports the Monophyly of Cryptophytes and Haptophytes and the Association of Rhizaria with Chromalveolates Molecular Biology and Evolution, 24 (8), 1702-1713 DOI: 10.1093/molbev/msm089
KEELING, P. (2009). Chromalveolates and the Evolution of Plastids by Secondary Endosymbiosis Journal of Eukaryotic Microbiology, 56 (1), 1-8 DOI: 10.1111/j.1550-7408.2008.00371.x
Moreira, D., von der Heyden, S., Bass, D., López-García, P., Chao, E., & Cavalier-Smith, T. (2007). Global eukaryote phylogeny: Combined small- and large-subunit ribosomal DNA trees support monophyly of Rhizaria, Retaria and Excavata Molecular Phylogenetics and Evolution, 44 (1), 255-266 DOI: 10.1016/j.ympev.2006.11.001
PATERSON, H., PESANT, S., CLODE, P., KNOTT, B., & WAITE, A. (2007). Systematics of a rare radiolarian—Coelodiceras spinosum Haecker (Sarcodina: Actinopoda: Phaeodaria: Coelodendridae) Deep Sea Research Part II: Topical Studies in Oceanography, 54 (8-10), 1094-1102 DOI: 10.1016/j.dsr2.2006.05.046
PAWLOWSKI, J., & BURKI, F. (2009). Untangling the Phylogeny of Amoeboid Protists Journal of Eukaryotic Microbiology, 56 (1), 16-25 DOI: 10.1111/j.1550-7408.2008.00379.x