The Diversity of Slime Moulds

How could you not love an organism that manages to combine both slime and mould? Slime moulds are saprobic organisms (i.e. they gain their nutrients by breaking down dead organic matter) that spend most of their life cycle feeding as separate amoeboid cells or disaggregated plasmodium. However, when conditions become right all the cells or plasmodium near each other will stream together to form a fungus-like fruiting body that releases spores, as shown in the diagram above borrowed from here. Because slime moulds thus resemble protozoa for part of their life cycle but fungi at other times, they were an early protagonist in the destruction of the idea that all organisms could be divided between plants and animals. Slime moulds, it turns out, are mostly not related to plants or animals. As our understanding of organismal phylogeny has progressed, it has become clear that not all slime moulds are even related to other slime moulds. Instead, the term has been used to cover a number of phylogenetically disparate organisms with little in common other than similar life cycles. However, the majority of references to slime moulds out there fail to mention this, focusing on only a small part of "slime mould" diversity, so I thought I'd give a brief overview of the full diversity of organisms with a slime mould-type life cycle.



1 - Myxogastrea: The plasmodial or acellular slime moulds, also known as Myxomycetes. This is the largest group of "slime moulds" - both in terms of number of species and the size reached by some species. While most other groups of slime moulds are fairly microscopic, myxogastreans reach sizes where they can easily be seen with the naked eye, at which point they are usually mistaken for fungi. During the feeding stage of their life cycle, myxogastreans form a plasmodium - a spreading mass that is not divided into individual cells, like threads of jelly or mucus (photo above from here). Recent phylogenetic analyses agree that myxogastreans belong to the Amoebozoa, the clade that includes more familiar amoeboids such as, well Amoeba (Cavalier-Smith et al., 2004). Indeed, the amoeboflagellate genus Hyperamoeba has been shown to represent a polyphyletic assortment of myxogastreans that have dropped the plasmodial habit.



2 - Dictyostelia: While myxogastreans may be the largest group of slime moulds, dictyostelians may be the most famous, because they take the standard coolness of the slime mould life cycle and turn the dial up to eleven. The photo above from here shows the various stages of the life cycle of the most famous dictyostelian, Dictyostelium discoideum. Dictyostelians are cellular slime moulds - while myxogastreans form a plasmodium, dictyostelians spend their nutritive phase as separate individual amoeboids. When the time comes for reproduction, the separate amoeboids swarm together to form a slug-shaped mass that actually moves as one, like something out of a Japanese cartoon. The dictyostelian slug crawls around until it finds a suitable location, at which point it extends outwards to form a sporangium on the end of a long thin stalk. The complexities of Dictyostelium's life cycle have made it a favoured study organism for such topics as kin selection, as researchers attempt to identify what cues incite slug formation, and why some individual amoeboids forming the sporangium stalk are seemingly willing to sacrifice their own reproductive potential in order to promote the reproduction of those cells forming the sporangium.

Phylogenetically, dictyostelians are also amoebozoans, closely related to myxogastreans. However, analyses are currently unable to resolve whether amoebozoan slime moulds share a single origin (forming a clade called Mycetozoa) or whether dictyostelians and myxogastreans independently originated from closely related but separate amoeboid ancestors.



3 - Protostelia: Three small families of slime moulds, the Protosteliidae, Cavosteliidae and Ceratiomyxidae, form a spreading nutritive phase similar to that of the Myxogastrea, and have often been regarded as closely related to the ultrastructurally similar myxogastreans. However, while myxogastreans form a truly acellular plasmodium, different protostelians form a pseudoplasmodium, with cells retaining their individual identity (Protosteliidae and Cavosteliidae), or a plasmodium that breaks up into individual cells before sporangium formation (Ceratiomyxidae). Ceratiomyxidae form small coral-like fruiting bodies - the photo above by Keisotyo shows a Ceratiomyxa species - while Protosteliidae form minute sporangia on slender stalks like dictyostelians. If mycetozoans form a single group, protostelians may represent a morphological connection between the cellular dictyostelians and the acellular myxogastreans. A relationship between protostelians and other mycetozoans was supported by Baldauf (1999), but the group remains little studied. The protostelians themselves are of doubtful monophyly, and some families may be closer to myxogastreans than others.

4 - Buddenbrockia: One parasitic animal was only recently identified as having a slime mould-like life cycle, with disassociated cells in its host aggregating together to give rise to a worm-like reproductive stage. The sordid details were covered in an earlier post.



5 - Acrasea: Acrasids are cellular slime moulds like dictyostelians, and indeed were once united with dictyostelians under the name of Acrasiomycetes. Like dictyostelians, acrasids live as individual amoeboids that aggregate together to form raised sporangia. The photo above from here shows the fruiting bodies of the best-known acrasid, Acrasis. However, acrasids are ultrastructurally distinct from dictyostelians, and are not even amoebozoans - rather, they belong to a protozoan group called Heterolobosea that also includes Naegleria, the organism that causes amoeboid meningitis, and belongs to the Excavata eukaryote superclade. Whether or not it was due to the mistaken assumption that acrasids were closely related to the intensely studied Dictyostelium, or whether it was due to the fact that acrasids seem to be most often found growing on animal poo, studies of acrasids are laughably rare, and only Acrasis rosea appears to have received any recent attention.

6 - Labyrinthulea: The slime nets are members of the Heterokonta, the clade also including brown and golden algae (among others), and have been covered before at this site - twice, in fact.

One protist group, the Phytomyxa or Plasmodiophoromycota, has often been included with the slime moulds due to its formation of a plasmodium for part of its life-cycle. However, phytomyxans, which are parasites of plant roots belonging to the Rhizaria (the eukaryote superclade including foraminiferans and radiolarians), do not seem to have an aggregative phase of the life-cycle comparable to other slime moulds. The best known phytomyxan is Plasmodiophora brassicae, the cause of club root in cabbages and other brassicas.



7 - Myxococcales: Finally comes a group that has never been regarded as slime moulds, but which has a very similar life cycle. The reason why Myxococcales have never been lumped with slime moulds is because they are not eukaryotes of any kind, but bacteria. Myxococcales are saprobic bacteria generally found in soil. They are capable of gliding motility, a form of movement by means other than flagella, though the exact mechanism remains little known. When nutrient supplies run low, some species of Myxococcales are capable of swarming together in a similar manner to cellular slime moulds and releasing dispersive spores. Myxococcales are therefore one of the few groups of bacteria to have developed multicellularity.

REFERENCES

Baldauf, S. L. 1999. A search for the origins of animals and fungi: comparing and combining molecular data. American Naturalist 154 (S4): 178-188.

Cavalier-Smith, T., E. E.-Y. Chao & B. Oates. 2004. Molecular phylogeny of Amoebozoa and the evolutionary significance of the unikont Phalansterium. European Journal of Protistology 40: 21-48.