Ezra writes:

Dear TWiM team,

On episode 183, a listener named Triet asked for an example of a microbial symbiont that dictates the relationship with its host. I thought I’d expand on the topic and provide an interesting example.

As part of their publication, Kevin Foster et al. discuss different microbiome models (1). One of these is the “host-control model”, in which a host “tightly regulates microbial phenotypes”. The bobtail squid, highlighted by Triet, falls into this category. The “symbiont-control model”, in which “a microbe alters global phenotypes – such as reproduction, survival or behaviour – in order to increase its own fitness” relates to Triet’s question. In both models, low microbial diversity within the host is required, which is perhaps why it is hard to think of many examples.

One example of symbiont control is the symbiont Wolbachia, which is found in many different insect species (2). By living intracellularly, it avoids competition from other microbes. In Drosophila simulans, Wolbachia strain wRi can increase host survival from viral infection (3). The same strain also manipulates host reproduction, causing embryos originating from an uninfected mother and an infected father to be non-viable (4). Since wRi is only maternally transmitted, it hereby increases its transmission. Thus, wRi can alter host survival and reproduction such that infected mothers have a higher fitness than non-infected ones, resulting in an increased fitness of the microbe. A fascinating example that fits the symbiont control model.

Thank you for hosting such an interesting and thought-provoking show! Writing this email was a lot of fun.


Ezra Herman (Statistics MSc student @ Uni of Glasgow)

  1. Foster KR, Schluter J, Coyte KZ, Rakoff-Nahoum S (2017) The evolution of the host microbiome as an ecosystem on a leash. Nature 548(7665):43–51.
  2. Correa CC, Ballard JWO (2016) Wolbachia Associations with Insects: Winning or Losing Against a Master Manipulator. Front Ecol Evol 3:153.
  3. Osborne SE, Leong YS, O’Neill SL, Johnson KN (2009) Variation in Antiviral Protection Mediated by Different Wolbachia Strains in Drosophila simulans. PLoS Pathog 5(11):e1000656.
  4. Kriesner P, Hoffmann AA, Lee SF, Turelli M, Weeks AR (2013) Rapid Sequential Spread of Two Wolbachia Variants in Drosophila simulans. PLoS Pathog 9(9):e1003607.

Larry writes:

Dr Racaniello et alia,

I was having trouble visualizing the CRISPR and Cas activity in your & Dr. Sternberg’s excellent discussion from September, so I paused and looked on YouTube and found what seems to be a perfect 5-minute video accompaniment for neophytes such as myself:


“What is the PAM? – A CRISPR whiteboard lesson”.

Thanks again for the great shows,


San Diego

Sarah writes:

Hi TWIM team!

I’ve had a lot of conversations recently about failed experiments. In an epidemiology lecture, we discussed how journals have a bias towards publishing papers that reject the null hypothesis—in other words, papers that seemingly find something new. In relation to this, while discussing evidence based intervention development, we lamented that because the failures of some interventions are never reported or published, those interventions continue being practiced (and then continue to fail).

I’m sure that this type of bias exists in many fields. I certainly remember times in the virology lab where students stressed over poster presentations because they could not prove their hypothesis with the evidence they had. However, finding out that something does not fulfill your expectations is just as valid as finding something that does. Edison found hundreds of ways not to make a lightbulb, after all.

I wanted to hear your opinions on this with respect to your academic experiences. How do you deal with interpreting research that doesn’t seem to find anything new? Do you think there is a space for a journal of “failed” experiments, if it doesn’t exist already?

Thank you,