[Coral-List] [Cnidarian-dinoflagellate-symbiosis] genetic connectivity of Symbiodinium individuals within a single colony

[Baker, Andrew]

Hi Misha

I’m afraid I have to disagree. We simply do not know how often exogenous symbionts are acquired from the environment. My suspicion is that it happens all the time, just at a very low level, and those symbionts have a very hard time displacing a huge population of resident symbionts. Think of them as mutant alleles in a very large population. Most of them are neutral and come and go all the time and we never notice – in part because we don’t sample an entire colony when we study this. But occasionally some symbionts are acquired that can be positively selected for, and this selection is much more powerful if a bleaching event first removes much of the standing stock of symbionts (thereby reducing the effective population size and allowing the “adaptive mutant” to spread through the population, to continue with the popgen metaphor).

Of course this does not happen to the same degree with all species of coral and symbiont. For example, I think D. trenchii is pretty good at getting into corals, and not just at the larval acquisition stage. But I don’t think it’s the only symbiont that can do this.

There are lines of evidence that point in both directions and of course that means it might be different in different coral species, or under particular circumstances. So I’m just cautioning against making emphatic statements like “no, corals cannot switch symbionts for a new strain as adults” (note: my view above is expressed as a “suspicion” (albeit one based on some experience) hence readers are free to take it or leave it!

I will also note that this same discussion has been playing out for at least 18 year…. Apparently, it has still not yet come of age!

Cheers

Andrew

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From: cnidarian-dinoflagellate-symbiosis-bounces at auburn.edu <cnidarian-dinoflagellate-symbiosis-bounces at auburn.edu> On Behalf Of Mikhail Matz
Sent: Thursday, March 21, 2019 9:54 AM
To: Thomas Krueger <thomas.krueger at epfl.ch>
Cc: cnidarian-dinoflagellate-symbiosis at gump.auburn.edu; coral-list at coral.aoml.noaa.gov
Subject: Re: [Cnidarian-dinoflagellate-symbiosis] genetic connectivity of Symbiodinium individuals within a single colony

you got it, Thomas. No, corals cannot switch symbionts for a new strain as adults. Think of symbionts as efficient parasites that never give up their host. Yes, symbionts can infect only the new generation of corals, so, from popgen point of view, coral generations = symbionts generations.

Misha Matz

On Mar 21, 2019, at 5:34 AM, Thomas Krueger <thomas.krueger at epfl.ch<mailto:thomas.krueger at epfl.ch>> wrote:

Here is a curious question: If the symbiont community in a coral host, as some publications suggest, consists of a single genet (i.e. genetically identical individuals aka clones), how can bleaching ever act as a positive selective force and reshape the surviving residual population towards a more heat resistant one? It literally would require uptake of genetically different individuals (of the same species) from the water column to diversify the genetic pool. Has someone used sequencing data to look at whether it is the residual population that recolonizes a bleached coral or whether it receives new settlers from the water column? If a single genet of Symbiodinium in colonies is really a dominating feature and if it does not change through bleaching events, then horizontal transmission might not really be such a big thing and there is little exchange with environmental Symbiodinium populations in the adult stage (exchange maybe, but not to the point that it reshapes colonies to the point that we can detect an altered genetic pool of the dominating species). This in turn would mean that the coral's larval and juvenile stage is the crucial stage that shapes the holobiont assemblage for the symbiont side. Any thoughts?

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