How does an obligately intracellular symbiont maintain genetic diversity? The Wolbachia story

I recently had the pleasure of finally sitting down to read some publications (both open access!) on my favorite bacterium, Wolbachia pipientis.  These recent pubs interested me because they focused on the population genetics of Wolbachia within individual hosts, upon host transfer, and after many generations.  The BIG question that comes out of this body of work, in my mind, is how are low-titer strains in the maternally transmitted population maintained!  (We can discuss ongoing hypotheses at the end of this post)

The first paper I'll tackle (Schneider et al) asks if Wolbachia strains exist as diverse quasi-species within a host and reveals that diversity using host transfer techniques.  In "Uncovering Wolbachia Diversity upon Artificial Host Transfer" by Schneider et al., the authors use the cherry fruit fly Wolbachia (wCer strains) as the inoculum for injection of two new hosts: Drosophila simulans or Ceratitis capitata.  For those unfamiliar with the technique, what it comes down to is harvesting many many embryos from your D. simulans, using differential centrifugation techniques to concentrate the Wolbachia fraction and using that, as you would in microinjection of a construct to make transgenic flies.

The cool thing about this paper is that they see cryptic polymorphisms rise after host transfer.   They looked at 150 generations after microinjection and saw a low titer variant increase in frequency such that it was detectable via PCR.  Now, the data in this paper is entirely PCR based -- they sequenced amplifed fragments and used them to detect SNPs.  That said, if found to be true, it suggests that the host and symbiont evolve really rapidly and that Wolbachia maintains diversity, even under conditions when it should be primarily maternally transmitted (lab stocks).

The second paper I'm highlighting (Symula et al., 2013) investigated the diversity of Wolbachia in tsetse fly populations and correlated Wolbachia haplotypes with specific host mtDNA haplotypes.  Their result = LOTS of Wolbachia diversity and evidence that these infections happened independently, multiple times.  The authors collected tsetse flies across a region in Africa and did an analysis of the Wolbachia MLST genes and groEL - they also looked at host mtDNA haplotypes. Again, they used PCR amplification and sequencing but were VERY conservative in their sequence post-processing (removing all recombinants, for example).  So, the data they present are potentially a lower bound estimate of Wolbachia diversity.  The number of haplotypes found within each host was astounding (see Table 1).  In some cases, 6 different haplotypes found within just 2 hosts!

Mechanisms for maintaining genetic diversity in a maternally transmitted symbiont? 

1) Bend the rules:
During my doctoral work, a lab member discovered that there was cryptic diversity within the maternally transmitted endosymbionts of the deep sea clams. In that work, they discovered that a low frequency (0.02) symbiont haplotype existed in a population of clams that were geographically localized.  It was hypothesized there that the trick to maintaining diversity in this maternally transmitted symbiont was to basically bend the rules: occassionally, transmit your symbiont horizontally.  Since we find evidence of horizontal transmission in Wolbachia, this is one mechanism that genetic diversity could be maintained in the population.

2) Increase mutation rates:
It would be theoretically possible for an endosymbiont to have such rapid rates of mutation that individual populations within a single host would exhibit variability detectable by the methods employed by Schneider et al. and Symula et al.  Evolutionary rates are elevated in endosymbionts, so this is a potential source of new genetic diversity for Wolbachia.

It will be quite interesting to see which (or if both?) of these scenarios play a role in Wolbachia genomic evolution.  These changes in symbiont population dynamics and densities could potentially allow Wolbachia to colonize new hosts, potentially acting as a quasi-species (as seen in virus systems).


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