Sanjuro

The punch line:

When forced to stand on their own two pili, obligate cheater Myxococcus will sometimes evolve into a non-cheater that outcompetes all of its progenitors and contemporaries.

The details:

Six replicates of an experiment were carried out that combined an obligate cheater of M. Xanthus, called OC, with a non-cheating cousin called GJV2 in a 1% OC/GJV2 mix.  Each replicate was cycled through alternating growth and starvation (called Development for some reason) stages for a total of 6 cycles (i.e. G1-G6 and D1-D6).  Eventually only one of the replicates was followed.  In this replicate, the population had crashed to near unmeasurable levels by D4.  OC was essentially acting as a DI particle, and overwhelming the GJV2 clones until the population crashed.  However, the surviving clones in D4 had apparently evolved the ability to sporulate properly and not only outcompeted GJV2, but were also immune to OC’s efforts to cheat.  This new super Myxo was called Phoenix (PX).

 
The authors found that PX was different from OC by only one nucleotide, in the middle position of a 7 run homopolymer (i.e. a 7 base run of a single type of nucleotide, in this case cytosine).  This run was 128 bases upstream of an acetyltransferase gene, of which there are approximately 30 in the Myxo genome.  It is still not clear exactly what effect the mutation is having.  Acetyltransferase levels are not consistently higher in PX than in other progenitors, though the pattern of expression is certainly different.  It is not clear whether the mutation is only affecting acetyltransferase expression, if it affects a range of other proteins, or if the change in acetyltransferase expression could have knock-on regulatory effects. 

 
An additional experiment was carried out in which the non-cheating parent of OC was transformed to include the PX mutation.  The result was that the OC parent displayed the PX phenotype.  It would really be interesting to see is if the original GJV1 and GJV2 would show the PX phenotype if transformed in the same manner.  There are an additional 14 single nucleotide changes that took place between GJV1 and PX and if they transformed GJV1, they would be able to see if the mutation that created PX was dependent on the other mutations that occurred between GJV1 and PX.  It might also be useful to know what changes might have taken place between GJV1 and GJV2, just to see if any of the same mutations arose separately.