hckrnws

I am technically an author on this manuscript, if anyone has any specific questions. I probably can't answer them, but I can text the first author. (Was not expecting to see this on HN today)

How do these bacteria compare to hydrozoans like the man o' war? Is the similarity just superficial or do they exhibit some form of differentiation?

> In the new study, scientists have revealed even more complexity in the relationships between MMB cells. First, contrary to long-held assumptions, individual cells within MMB consortia are not genetically identical, they differ slightly in their genetic blueprint. Further, cells within a consortium exhibit different and complementary behavior in terms of their metabolism.

Does that mean there's mutation when they multiply or do these MMBs exhibit some crude form of sexual reproduction - two MMBs with separate genetics merging? What's their life cycle look like when they reproduce?

Have you guys managed to identify any genetic clocks that can help estimate when they broke off from their closest relative? The MMB sounds so much like a hydrazoan (~540 MYa) that I'm curious if there's an evolutionary connection there.

For reproduction of the MMB, cells undergo classical bacterial division where the genome is replicated and the cell divides. The difference is that this is happening in a sphere so the sphere doubles in size and then the entire sphere splits into two. This is commonly considered clonal reproduction (although there is an argument for if "clonal" truly exists...)

Because of this reproduction, it would take only several generations for most genomic heterogeneity to be diluted out and the whole consortium (i.e., single MMB) to become clonal. What this suggests is that MMB are purposefully maintaining there genomic heterogeneity, likely for evolutionary purposes.

As far as a comparison to the Portuguese man o' war, it is similar in many respects. The theory is that MMB have a division of labor, meaning one cells has a job of say, metabolizing acetate, and its neighbor might have the job of storing carbon, and its neighbor has the job of reducing sulfate for energy. This would mean MMB have specialized cells that have a specific function. There is evidence for this but it has yet to be proven. The cyanobacterium Anabaena has specialized cells called heterocysts that fix nitrogen, a difficult job in the presence of oxygen which is a by product of the other cells. This is a clear example of a cellular division of labor in Bacteria. But Anabaena can exist as a single cell, meaning it is not obligately multicellular, which MMB are.

I do not think there is much of a relationship to hydrazoans. MMB likely developed their multicellularity from an incomplete cell division that resulted in the daughter cell staying attached to the mother cell, eventually resulting in the current organism. Multicellularity is not monophyletic and has evolved independently across the whole tree of life. Some research cited within our article shows that a single mutation to a single gene can cause multicellular traits to arise in bacteria.

I have a semi-related question I've always wanted to ask an expert. Is there any evidence that multi-cellular life evolved more than once? If not, do you lean one way or the other on the likelihood of it evolving more than once?

Love the writing as well as the content here.

Especially fascinated by the coordinated replication.

Also curious about magnetotactics - why would such a small scale organism need to orient to such a large scale phenomenon (the earth's magnetic field)? Wouldn't it make more sense for this electromagnetic sense to be used for smaller scale orientation in their environment?

Magnetotaxis in bacteria (and some protist) is passive. The organism will biomineralize a ferromagnetic mineral, such as magnetite or greigite, and in a magnetic field they will passively orientate. This is not active orientation, meaning that even when the organism is dead it will still orientate in the field.

Their movement in the magnetic field is however active. The theory behind the magnetotaxis is that it allows them to know what direction is up. In the northern hemisphere, the magnetic poles come in from above and go down. So to a bacterium, North is down.

Why care what direction North is? if you are sensitive to oxygen, which MMB are, and oxygen diffuses in from the atmosphere above, your magnetotaxis would tell you the direction to swim to get away from toxic levels of oxygen. Wild how evolution works!

Since these bacteria can't live on their own I don't understand what differentiates them from true multicellular organisms.

Hi there, first author of the paper here. We would argue that they are a true multicellular organism, which is rather unconventional for Bacteria. They do indeed have some level of genomic heterogeneity between individual cells within a single consortium but it appears this is actually purposefully maintained by the organism, likely to facilitate their evolution. They have a large genome (8 Mb) compared to E. coli (4 Mb) and have duplicate genes that are under higher rate of evolution (dN/dS) compared to the rest of the genome.

The article says "individual cells within MMB consortia are not genetically identical, they differ slightly in their genetic blueprint."

the next stage on that way i guess is https://en.wikipedia.org/wiki/Portuguese_man_o%27_war - a "colonial organizm"

"a Portuguese man o' war constitutes a single organism from an ecological perspective, but is made up of many individuals from an embryological perspective."