Wolbachia is a genus of bacteria that has become fairly well known in recent years due to its effects on disease transmission by mosquitoes. Since there are plenty of articles about Wolbachia and disease control out there, I’ll keep my explanation brief. Initial work with Wolbachia was focused around infecting vectors such as Aedes aegypti (yellow fever, dengue, chikungunya, and most recently Zika) and Anopheles mosquitoes (malaria) with the bacteria when it was discovered that certain strains reduced the life spans of their hosts and could prevent mosquito eggs from hatching. What scientists found was that in addition to reduced life spans, Wolbachia prevented the replication of the dengue virus within mosquitoes, meaning that Wolbachia infected mosquitoes were incapable of transferring the disease. This discovery has led to a massive push towards the use of Wolbachia as a method of curbing the spread of disease in areas where other mosquito control methods are infeasible. But I’m not writing this to talk about vector control, I want to talk about spiders.
About a week ago, while doing preliminary research for my master’s thesis, I stumbled upon an article from 2011 by Vanthournout et al. titled “Spiders do not escape reproductive manipulations by Wolbachia“. This made me very interested, as up until this point I had only ever heard about Wolbachia in the context of disease control. As I learned reading this article, Wolbachia is an endosymbiotic bacteria that is transferred maternally within arthropods, meaning that only females are capable of transferring Wolbachia between generations.
Since this method of transference means that Wolbachia that happens to be transferred to male offspring are evolutionary “dead ends” (since the bacteria is unable to transfer itself to another host and will die with the infected individual), Wolbachia and several other endosymbiotic bacteria like it will actually alter the reproductive behavior of their host to increase the number of female offspring, increasing the likelihood for the bacteria to propagate in a population. These methods include inducing parthenogenesis (the production of female offspring without the need for fertilization), killing male offspring during development or early infancy, and even feminizing males (the altering of development which leads to males developing as females). In this article, scientists looked at the effects of Wolbachia on the sex ratios of the dwarf spider Oedothorax gibbosus from the family Linyphiidae.
O. gibbosus was chosen for this study for two reasons: 1) this species has a significant degree of sexual dimorphism (the variation between males and females) that makes identifying the sex of an individual simple, and 2) the ratio of males to females in wild populations is significantly skewed towards females, a difference that could possibly be explained by the presence of reproduction-altering endosymbionts.
The first part of this study was focused around determining whether bacteria capable of altering reproductive behavior were present in the population. To this end, the researchers collected spiders from two locations within Belgium: Walenbos and Damvallei. Polymerase Chain Reaction (PCR) was used to test for the presence of four different endosymbiotic bacteria known to alter the reproductive behavior of their hosts in other species: Wolbachia, Cardinium, Rickettsia, and Spiroplasma. Of these, Spiroplasma was the only endosymbiont not found within the spiders tested.
The next step was to determine whether the bacteria was causing differences in the overall sex ratio of the two populations. By comparing the number of male and female offspring produced by infected and uninfected females, the researchers were able to determine that only Wolbachia had a significant impact on the sex ratio, with only 36% of the offspring of infected females being male, compared to 47% of the offspring produced by uninfected females being male.
Finally, the scientists took their work from an observational level to an experimental level. They took group of infected females (all of which were related, to reduce the genetic variability of the females which could skew the results) and separated them into three groups: 1) a group that was treated with tetracycline hydrochloride, 2) a group that was treated with penicillin, and 3) a group that received no treatment. Tetracycline and penicillin are both antibiotics, and were selected due to their differing effects on the types of bacteria present in the experiment. Tetracycline is capable of eliminating Wolbachia, Cardinium, and Rickettsia whereas penicillin can only kill Cardinium.
As you can see in the figure above, the treatment of females with antibiotics had a major impact on the proportion of male offspring. More specifically, tetracycline significantly normalized the sex ratio relative to the control group. While penicillin does show a shift towards an even ratio of male to female offspring, statistically speaking there was no significant difference between the penicillin group and the control (meaning that while there is a difference, when you account for the range of results there is enough overlap between the two groups that it is likely that differences between the two groups is a result of random chance).
So there it is. Experimental evidence that Wolbachia is altering the ratio of males to females in O. gibbosus. I found this paper to be fascinating, because not only did it introduce me to an aspect of Wolbachia that I wasn’t even aware of, but it also went through both the observational and experimental studies in one paper, something that I often see split between two separate papers. If you found this work as interesting as I did, I would recommend reading the full article.