Guest Blog: Sneaky slime mold
Slime molds allocate less to costly public goods when sharing then when alone.
(Special Guest Blogger: Will Ratcliff )
The slime mold Dictyostelium discoideum ('dicty' for short) spends most of its life alone, hunting soil bacteria and yeast. But when food runs out, tens of thousands of individuals aggregate into a mobile slug (cool youtube video) which crawls to an advantageous place and differentiates into a ball of spores on top of a long stalk. Individual dicty either become a dead stalk cell or a reproductively viable spore.
© Copyright, Mark Grimson and Larry Blanton
What possible incentive could there be for a dicty to sacrifice its life (become a stalk cell) for the benefit of those that become spores? If you answered 'there is no direct advantage to dying for others', you're right! Nonetheless, kin selection can lead to this type of self-sacrifice if a) the dicty in the stalk are highly related to the dicty that form spores (so are highly likely to have the same "unselfish genes"), and b) spores benefit from being higher off the ground (better chance of dispersal?).
But what happens when slugs are composed of more than one genotype? Here stalk formation becomes a 'tragedy of the commons' in which it is in each clone's interest to cheat, letting the other clone form a greater fraction of the stalk. So do dicty cheat? If so, how do they do it?
The short answer, as reported by Buttery et al. in the paper Quantification of Social Behavior in D. discoideum Reveals Complex Fixed and Facultative Strategies recently published in Current Biology is that:
Yes, dicty cheat; they cheat like crazy.
There are two ways in which a dicty clone in a mixed slug could cheat, producing less stalk and leaving more spores than its competitor....
First, the dicty clone could have a higher intrinsic frequency of spore formation. In some clones, most cells become part of the stalk, putting their spores high up in the air, while others favor shorter stalks and thus produce more spores. When mixed together, a clone that forms many spores might automatically cheat a clone that forms a lot of stalk, benefiting from the tall stalk while contributing relatively little to its construction. Second, a dicty clone may sneakily increase its frequency of spore formation when in a mixed slug, relative to what it would do alone, letting the other strain make most of the stalk.
The authors tested 6 natural isolates that varied substantially in their intrinsic frequency of spore formation, and made all possible combinations of two-genotype mixed slugs. What they found is that 5 out of 6 genotypes cheated facultatively, forming more spores when in a mixed slug then when alone. Surprisingly, strains that formed more spores in mixed slugs were also able to keep the competitor strain from doing the same to them . This is a really cool result, demonstrating that evolutionary conflict over stalk production has led to this humble slime mold's ability to measure social context and preferentially cheat nonrelatives.
But here's the catch: while facultative cheating was rampant, it seemed to have little overall impact on fitness (here measured as spore number alone, ignoring the potentially important but unknown effects of stalk size) under test conditions. The fitness rank order of the 6 strains in mixed sporangia was almost completely determined by each strain's intrinsic spore:stalk allocation.
The evolutionary persistence of genes for facultative cheating suggests that it was sometimes important to these strains ancestors in the wild, but that remains to be shown. This is great work, but is hampered by the current black-box status of this microbe's ecology. Stalks are presumed to be beneficial, otherwise natural selection would have long ago eliminated them, but the quantitative benefit of greater stalk height is currently unknown. As a result, the authors had to define fitness only in terms of the number of spores formed. Their 6 strains that varied in spore:stalk ratio therefore differed greatly in 'fitness', but each might have the stalk:spore ratio that maximizes fitness in its source environment, if the benefits of greater stalk size are considered. If this were the case, mixed slugs in the wild would usually form between strains with similar stalk:spore ratios. Once differences in that ratio are taken out of consideration, the fitness consequences of facultative cheating may in fact be quite large.