Evidence of Multicellularity in Single Cell Organism | Hacker News Reader

Evidence of Multicellularity in Single Cell Organism

research.gatech.edu

129 pointsgmays18 comments

For those interested in the fuzzy boundaries of life, the siphonophores [1] are worth reading about. The most well-known (and intimidating) species is probably the Portuguese Man o' War but most are rather less imposing. They're in the phylum Cnidaria, which includes the likes of jellyfish and anenomes. They are colonial organisms with extensive specialization of the individuals. Made out of tiny multicellular organisms known as zooids, each individually budded by the root/origin zooid. The zooids cluster around a digestive zooid, and the cluster has reproductive, structural, locomotive, etc. zooids, and is more or less self-sustaining, and acts semi-independently. Yet those clusters, like the zooids that make them up, are also part of the larger organism. Not quite organs, not quite cells. Siphonophores are usually predatory and mobile, living in the deep ocean. Many species are long (tens of metres), thin repeating chains. They are quite beautiful, in their alien way.

[1] https://en.wikipedia.org/wiki/Siphonophorae

it's the zerg!

Siphonophorae are colonies composed of members that all belong to the same species right?

Do the zooids each do their own reproduction, or do they just have one reproductive member, or both?

The members of one individual are all genetically identical, produced by budding/cloning, from a single zooid. Only the pro-bud, the first zooid of an individual organism, buds new zooids. But that's not exactly reproduction, it's more like cellular growth within a multicellular organism, or growing new organs maybe. Reproduction is the creation of a new colony. Most species reproduce sexually. There are specialised reproductive zooids. They produce a larval stage which floats free and then fertilizes or is fertilized (they have both eggs and sperm), and then it starts budding to form a new colony.

How is this different from a true multicellular organism? From your description they sound the same modulo terminology.

I don't think they are any different. Humans are also colonies of individual organisms.

Each zooid grows from an individual embryo, one cell, in the same kind of process of division and cell specialization all multicellular animals go through. For humans, or sven jellyfish, the embryo develops into an independent adult form. But in siphonophores, these individuals are completely specialised to one role within a colony, and can't survive without the colony.

> in siphonophores, these individuals are completely specialised to one role within a colony, and can't survive without the colony

Again, I fail to see the distinction between what you are describing here and true multicellular organisms. My body can be viewed as a colony of individual cells. That colony consists almost exclusively of cells that are completely specialized to one role within the colony, and which cannot survive outside without the colony. (But there are exceptions, like HeLa cells.)

Because you start from only one embryo. Almost all animals start from one embryo. These animals are formed from thousands to millions of embryos. From a feature categorization perspective, I guess you have a point. They look a lot like non-clonal jellyfish in practice!

But the implied underlying question is: how did that mechanism evolve? Two organisms may look similar but have completely different evolutionary histories and mechanisms to arrive at that appearance. Like eyes, which seem to have evolved independently multiple times.

This is a completely different mechanism to construct a multicellular organism. Not stem cells, but embryos, cloning a specialized twin, each of which undergoes embryonic development into a sort-of-organ. In almost all other animals, all genetically identical specialized cells come from a single embryo, which splits into stem cells, which then specialize. That's why it's interesting and stands out, at least to me.

The BBC’s Octonaughts taught me about these!

This was my first thought as well, I know them because the Octonauts met them in an episode.

Similarly odd but perhaps in the other direction, I appreciate the Chlorophyta for pushing the bounds of a single cell. E.g., Valonia ventricosa 5 cm cells, or the Bryopsidales genera like Codium or Caulerpa with large, articulating macroalgae plants that are somehow also single, multinucleated cells. You've even got Halimeda in there that is a single cell plant with a calcium carbonate skeleton.

There's also the xenophyophores, which are (multinucleated) cells that get pretty huge. https://en.wikipedia.org/wiki/Xenophyophorea

It seems kind of like an ant colony, with lots of specialized ants that all belong to the same species, except the ants are all glued to each other.

In a nutshell:

> [Researchers] discovered something new – a multicellular self-assembly process in E. coli. Researchers observed unattached, single-celled organisms combining into four-cell rosettes, a natural multicellular formation thought to be uncommon in bacteria.

The press release could use some work, that could have been in the first or second paragraph rather than the 12th.

It's amazing to see that we still have things to discover on Escherichia coli despite it being a reference organism which was researched so much.

Paper (full text with interesting media): https://www.cell.com/iscience/fulltext/S2589-0042(22)02068-5

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