When someone looks at a spider, I’m sure most of people don’t immediately think “I wonder what bacteria live inside that spider?”  

This blog post is a confession: I do. This is what graduate school does to you.  

My work in graduate school focused on bacteria that live inside arthropods called endosymbionts. These endosymbionts can influence their arthropod hosts in a number of ways and can range from beneficial, detrimental, or neutral. Most of these endosymbionts in arthropods are passed on through the mom- meaning that whatever she happens to be infected with, her offspring will be infected with too. Some endosymbionts have a particular talent for ensuring that they get passed on to the next generation by manipulating their hosts reproductive systems.  

There are a couple of ways that endosymbionts have been documented doing these reproductive manipulations through a wide variety of arthropods. One of these ways is a process called cytoplasmic incompatibility, or CI. By using this cytoplasmic incompatibility, the bacteria are able to pick and choose when a host is able to successfully reproduce. The bacteria do so by manipulating the sperm in the infected male. If that infected male happens to mate with an uninfected female, the sperm is unable to be corrected, and the eggs in the female fail to be fertilized. Remember that these bacteria are passed on through the mom, so if the mother arthropod is not infected with the bacteria, they won’t make it to the next generation. Therefore, these bacteria are often the puppeteers pulling the strings on whether or not offspring will be produced from a mating.

 

 

It turns out that some of the arthropods with the highest diversity of these endosymbionts that have the potential to reproductively manipulate their hosts are spiders (White et al. 2020). In particular, I studied these endosymbionts in a sheet-weaving spider known as Mermessus fradeorum. These spiders are found everywhere, but are commonly found in agricultural areas.  

 

 

I collected these spiders from an alfalfa field and brought them into the lab. We first asked what they happened to be infected with and determined that they were able to be infected with up to five different potential reproductive manipulators, and every spider was infected with at least one of these endosymbionts. Specifically, we found that these spiders were infected with a strain (or variety) of Rickettsia, three strains of Wolbachia, and a strain of Rickettsiella. This strain of Rickettsiella infected nearly every single spider in our sampled population, so we decided to see if it was causing a reproductive manipulation in M. fradeorum.  

To do so, we first took some of our spiders and treated them with antibiotics to clear them of their endosymbionts. We used the offspring of those antibiotically treated spiders to account for any effects that may have occurred through the antibiotic treatment. Using these endosymbiont-free spiders, we then created the crosses that we would need to document the occurrence of cytoplasmic incompatibility being caused by Rickettsiella. 

Following a lot of romancing by the spiders, we finally had the results of our crosses. Overall, there was no difference in the number of eggs laid across the four crosses, meaning that all the females were still producing the same number of eggs despite their infection status. We found that infected females mating with infected males had 99.7% of their eggs hatch, while uninfected females mating with uninfected males had 83.9% of their eggs hatch. Infected females mating with uninfected males had 91.3% of their eggs hatch. However, uninfected females mating with infected males only had 13.2% of their eggs hatch. This meant that Rickettsiella caused cytoplasmic incompatibility within Mermessus fradeorum.  

This documentation of Rickettsiella causing cytoplasmic incompatibility in M. fradeorum emphasizes the importance of endosymbionts and their relevance to their arthropod hosts. By examining a small, brown, agricultural spider, we were able to find a bacteria that is capable of doing manipulations that we could potentially use for pest management. Currently, there are a few companies that are using Wolbachia and its cytoplasmic incompatibility for local population control in mosquitoes. Our study emphasizes that these potential tools may be much more widespread in arthropods than previously appreciated, and that they could potentially be used in a broader spectrum than just mosquitoes.

Maybe next time you take a look at a spider, you’ll also think about the bacteria hiding away inside. I saved you from the struggles of graduate school. You’re welcome.  

Article by Laura Rosenwald, BCE

References 

Rosenwald L.C., Sitvarin M.I., White J.A. 2020 Endosymbiotic Rickettsiella causes cytoplasmic incompatibility in a spider host. Proc. R. Soc. B 287: 20201107. http://dx.doi.org/10.1098/rspb.2020.1107 

White, J.A., Styer, A., Rosenwald, L.C. et al. Endosymbiotic Bacteria Are Prevalent and Diverse in Agricultural Spiders. Microb Ecol 79, 472–481 (2020). https://doi.org/10.1007/s00248-019-01411-w 

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