63 thoughts on “Are You Human?”

1. In fact, common cause is the reason why multicellular bodies are sustainable. If genes are focussed upon getting out in gametes, there is no advantage to individual diploid cells replicating asexually for reproductive (as opposed to somatic) purposes.

2. Allan,

   If a cheat in that inoculum leaves a disproportionate number of descendants, the light will be feeble, their host loses its selective edge, and hence so do they.

   That’s what I was getting at here:

   Of course, if too many individuals take that approach, the light is too weak and the benefit is lost.

3. Gotta know when to hold/fold ’em.

4. keiths,

   Yes, but 1 individual is too many in such a bottlenecked population. It’s no good saying ‘cheating is favoured’ when it isn’t!

5. walto,

   Gotta know when to hold/fold ’em.

   You’re better off bluffing (that is, freeloading) if you can get away with it until bluffers form a certain percentage of the population. Beyond that point, you’re better off being honest. There’s an equilibrium cheater population.

   Interestingly, in the case of V. fischeri, the bluffers may not be able to get away with it because the squid themselves may enforce honesty:

   Cheryl Whistler at the University of New Hampshire and Margaret McFall-Ngai propose that cryptochrome photoreceptors allow squid to detect bioluminescent symbionts and impose sanctions on dark bacteria. This model has evolutionary appeal in that it explains how the host ensures that it receives light from the symbiosis. How animals could distinguish and selectively sanction dark bacterial cells that are mixed with bright ones is not known, but finding cryptochromes in the light organ merits further investigation.

6. Allan Miller:

   What I don’t see is any input from the supposed field of biosemiotics. It seems a bit niche, and what little I’ve read seems mostly about telling people what a great and promising new field it is, rather than actually gaining insight into any process.

   I cannot argue with that. It seems mostly philosophical elaborations of a straightforward idea. Based on the little I know, it does not attract many scientists for research.

   I only commented to point out that it was scientifically innocuous and not some deep challenge to standard biology. I don’t think it is particularly useful or that there is an ongoing dialog between the philosophers and many scientists.

7. Allan,

   Your assumption is that cheating is self-defeating because it adversely affects the viability of the host, but that isn’t necessarily true. Many parasites adversely affect their hosts. The question is whether the advantage gained by cheating is, or isn’t, more than offset by the disadvantage due to the host’s reduced viability.

8. keiths,

   Your assumption is that cheating is self-defeating because it adversely affects the viability of the host, but that isn’t necessarily true.

   No, of course it isn’t necessarily true. But in this particular instance, if the advantage of bioluminescence is what it brings to eukaryote hosts, it is true.

   Many parasites adversely affect their hosts. The question is whether the advantage gained by cheating is, or isn’t, more than offset by the disadvantage due to the host’s reduced viability.

   And it clearly isn’t in this case. Hosts experience a fitness gain, not loss, through the bioluminescence pathway, and so the bacteria do too, given nutrient and protection. It is not in the interests of the bacteria to diminish that by local ‘cheating’ competition with rival bacterial cells. If they don’t bring the benefit, they don’t reap the reward.

   In other scenarios, different dynamics may apply. Disease typically reduces viability, though most bacteria-eukaryote interactions are not diseases. Disease organisms can be selected to reduce or increase virulence in a dynamic tension between bacterial multiplication and that of the host. This happened with myxomatosis for example, which diminished in virulence as it killed more hosts. Not sure where ‘cheating’ comes in there though.

9. Allan,

   A longer answer later, but for now I’ll note that if Whistler and McFall-Ngai are right, then the squid evolved cryptochrome receptors in their light organs as a defense against cheating by “dark bacteria”. If cheating were “self-defeating”, why would the squid need to evolve a defense against it?

10. keiths:
    Allan,

    A longer answer later, but for now I’ll note that if Whistler and McFall-Ngai are right, then the squid evolved cryptochrome receptors in their light organs as a defense against cheating by “dark bacteria”.If cheating were “self-defeating”, why would the squid need to evolve a defense against it?

    Is cancer self-defeating, and do we have defenses against it?

    Not a statement about the squid, which I wouldn’t pretend to know about, it’s just about the argument.

    Glen Davidson

11. keiths:

    Your assumption is that cheating is self-defeating because it adversely affects the viability of the host, but that isn’t necessarily true.

    Allan:

    No, of course it isn’t necessarily true. But in this particular instance, if the advantage of bioluminescence is what it brings to eukaryote hosts, it is true.

    The relative effect sizes matter. As I wrote:

    Many parasites adversely affect their hosts. The question is whether the advantage gained by cheating is, or isn’t, more than offset by the disadvantage due to the host’s reduced viability.

    Allan:

    And it clearly isn’t in this case. Hosts experience a fitness gain, not loss, through the bioluminescence pathway, and so the bacteria do too, given nutrient and protection. It is not in the interests of the bacteria to diminish that by local ‘cheating’ competition with rival bacterial cells. If they don’t bring the benefit, they don’t reap the reward.

    That doesn’t make sense. Imagine a single cheater mutant in a population of cooperators. The cooperators all happily produce light, and the squid rewards them with nutrients and protection. The cheater cell saves 20% of its energy budget by cheating, but it reaps all of the same rewards as the cooperators. If it can channel that energy into reproduction, it will increase its prevalence in the population. Cheating is very much in the interests of that mutant. It’s a winning strategy.

    Now take an extreme scenario in which 100% of the bacteria were “dark”. Would that be “self-defeating”? In the very long term, it probably would be, if the bacteria didn’t continue evolving, because the squid would eventually evolve countermeasures, as Whistler and McFall-Ngai suggest has actually happened.

    But in the shorter run, even an all-dark population would not necessarily be “self-defeating”. The dark bacteria would still be getting the benefits of protection and nutrients, despite their failure to provide luminescence to the host squid. The only way they could lose would be if the lightless squid were so much more susceptible to predation that it more than offset the reproductive advantage gained due to the energy savings of the dark bacteria.

12. Glen,

    “Dark” V. fischeri are found in nature, so the mutation clearly isn’t “self-defeating”. But if it were, then it would quickly go extinct whenever it arose. The window within which it could exert a selective pressure on the squid would be small.

    Under those circumstances, it seems to me that the mutation would have to be extremely frequent in order to have an impact on the squid’s evolution.

    Compare antibiotic resistance. The problem isn’t that wild-type microbes keep evolving resistance over and over again within particular humans, then going extinct — it’s that resistant strains get passed around from human to human.

13. How to culture V. fischeri at home:

    http://www.instructables.com/id/Bioluminescent-Bacterial-Lightbulb-Water-Polluti/