Remember the tricky Mystery Micrograph #10? Probably not, since my MM write-ups are ridiculously overdue even by undergrad standards. But nevertheless, we had these fine specimens to stare at:
SEMs of the fruiting bodies (sorocarps) at various stages of development. (Olive & Blanton 1980 J Protozool)
Upon seeing these, you'd probably be reminded of something like a slime mould. The following image sequence heavily implies this thing might be some modified Dicty -- this:
Times (min): A - 0 ; B - 20; C - 30 (cell adhesion becomes apparent); D- 50; E - 90 (shiny mucous is secreted); F - 100; G,H - 120min (rising aggregate); I,J - 160min (mature sorocarp) Scalebar = 100um (Sugimoto & Endoh 2006 JEM)is not supposed to be done by this:
Colpodean ciliate, Sorogena, responsible for the fruiting bodies. (Bardele et al. 1991 J Protozool)
The fruiting bodies are aggregations of ciliates, of all things -- Sorogena. That's right -- a swarm of ciliates! Just as in the cellular slime mould Dictyostelium, these aggregations are of genetically unrelated (to an extent, of course) organisms for the purpose of dispersal upon food shortage, not for reproduction. However, unlike Dicty, there are no stalk or base cells in Sorogena -- instead, a sheath of mucus is secreted by the clump of cells as they move upwards. Thus, all the cells get to benefit by dispersal, so the coordinated behaviour is not as puzzling as in Dicty.
Sorogena life cycle. The ciliates prey on other ciliates, of genus Colpoda, and form fruiting bodies when food gets scarce. (Blanton & Olive 1983a Protoplasma)
Curiously, experiments involving placing pollen grains on different regions of the stalk show no movement of each part of the stalk; rather, the sorogen (clump of ciliates) pushes through the sheath of mucus as it secretes it (Blanton & Olive 1983b Protoplasma). Subsequently, the ciliates form spores and disperse. Such aggregate-formed fruiting bodies are typically expected in amoeboid things like Dictyostelids (amoebozoans), the nucleariid Fonticula (opisthokonts), and heterolobosean Acrasids (excavates). (Protostelids, a polyphyletic group of amoebozoans, do a similar thing, but as unicells)
Flagellates aren't particularly known for their desire to clump up, and this may have something to do with amoebae frequently inhabiting terrestrial environments (where being flagellated isn't particularly useful; soil-dwelling hypotrich ciliates prefer to walk on their cirri while many Cercozoans are expert gliders; that said, there's quite a variety of truly flagellated stuff in damp soil, of course, like bodonids and euglenids -- since on that scale, damp soil is sort of aquatic-like and swimmable). Dispersal is generally not too much of a problem in water, but can be quite useful in dry terrestrial environments, where food is scarce and motility slow and inefficient. Thus, most instances of fruiting body formation are found in terrestrial organisms, like Dicty and, as expected, Sorogena -- first isolated from dead plant parts (Bradbury & Olive 1980 J Protozool).
So now we have slime mould fruiting bodies of swarming ciliates. What next, multicellular dinoflagellates? Why not, anyway? I'll stop with the spoilers now... =P
BARDELE, C., FOISSNER, W., & BLANTON, R. (1991). Morphology, Morphogenesis and Systematic Position of the Sorocarp Forming Ciliate Sorogena Stoianovitchae Bradbury & Olive, 1980 The Journal of Eukaryotic Microbiology, 38 (1), 7-17 DOI: 10.1111/j.1550-7408.1991.tb04785.x
Blanton, R., & Olive, L. (1983). Ultrastructure of aerial stalk formation by the ciliated protozoanSorogena stoianovitchae Protoplasma, 116 (2-3), 125-135 DOI: 10.1007/BF01279829
Blanton, R., & Olive, L. (1983). Stalk function during sorogenesis by the ciliated protozoanSorogena stoianovitchae Protoplasma, 116 (2-3), 136-144 DOI: 10.1007/BF01279830
OLIVE, L., & BLANTON, R. (1980). Aerial Sorocarp Development By the Aggregative Ciliate, Sorogena Stoianovitchae. The Journal of Eukaryotic Microbiology, 27 (3), 293-299 DOI: 10.1111/j.1550-7408.1980.tb04260.x
SUGIMOTO, H., & ENDOH, H. (2006). Analysis of Fruiting Body Development in the Aggregative Ciliate Sorogena stoianovitchae (Ciliophora, Colpodea) The Journal of Eukaryotic Microbiology, 53 (2), 96-102 DOI: 10.1111/j.1550-7408.2005.00077.x