Talmudic Question #93

by Paul Evans

Which  bacteria besides the rhizobia undergo terminal (irreversible) differentiation?  (no fair saying 'spores')

Comments

[Nathan Myers]

If they cannot replicate, they must be doing something that benefits their sisters but not even near-relatives. The only way I can see that occurring is in a monoculture. I will speculate from the position of complete ignorance that these rhizobia live in a colony that originates from a single individual, segregated from other colonies, and that some members do retain the ability to procreate and form a new colony when they leave or the one they are in dissolves.

[paul evans]

Anabena heterocysts may meet the criteria.
Is it known that a preparation of pure heterocysts
(prepared from density centrifugation, for instance)
does NOT form fresh replicating photosynthetic cells?
If so, then I'll concede they meet the requirement.

The un-cultivatible problem, is as Dr. Martin suggests,
a can of worms. While some (Buchnera in insects) are
currently un-cultivatible, I don't see them as differentiated.
They are what they are - alive, monomaniacally fastidious,
and certainly capable of replication.
For those symbionts acquired new from the environment,
they are likewise capable of replication (presumably in the environment - from which we can't culture them,
and inside the cells - where they are similarly un-cultivated).
So.... I don't think either make cut for terminal differentiation; an intractable symbiosis seems a better description.

As Dr. Martin suggests "What does uncultivable MEAN?"
is certainly a question of Talmudic subtlety.

Caulobacter. Those rascals. The stalked cell is certainly terminally differentiated, but one still capable of replication - of new swarmer cells. So, in the manner of a TQ, and for the sake of argument, I'll say it's NOT terminal differentiation
if replication remains possible.

The Rhizobia seem to have sold themselves down the river
into leguministic slavery, from which they can never replicate,
nor return.

[barry]

i seem to recall leo buss in his book 'evolution of individuality' that the fact that metazoan cells can't swim and divide at the same time drove the evolution of germline/soma split in animals. they have to pull in their undulipodia to use the microtubule organizing centers for mitosis.

is that true?

Elio replies:
I'm not a maven on eukaryotes, leave alone metazoans. Anyone care to answer?

[Mike jones]

Once a Caulobacter crescentus cell differentiates into its sessile, stalked form, it never again knows the joy of motility. It is not a unique event within a population but an inevitable end point of its lifestyle - but terminal none-the-less.

I find it interesting that these cells separate motility from cell division. Could it be utilizing scarce resources for just one activity at a time, instead of trying to do both at the same time?

[Mike Rust]

Caulobacter doesn't count?

[Mark O. Martin]

I like the Anabaena idea, though heterocysts are not separate cells from the trichome; it's still a good idea.

I wondered about the Riftia symbiont---it clearly colonizes the worm from an environmental niche. It cannot be cultivated once inside the trophosome...but on the other hand, it cannot be cultivated from the environment? So perhaps the entire "cultivability" meme needs to be overlain on this Talmudic Question?

I remain fascinated by the changes among the rhizobia during root nodule colonization. Wouldn't it be great to learn (though there is no evidence for) that DNA rearrangements are part of the bacteria-bacteroid transition, much as they are during heterocyst differentiation (Jim Golden's work in 1998; a nice diagram here: http://biology.kenyon.edu/courses/biol114/Chap12/Chapter_11B.html)?

[Tim]

Anabaena forms terminally differentiated heterocysts, which are seemingly devoted to nitrogen fixing in times of nitrogen depleted conditions.