Hundertmark, Antje
(2019)
Endosymbionts in spiders: driving force or hitchhikers?
PhD thesis, University of Nottingham.
Abstract
Endosymbionts such as Wolbachia and Rickettsia and their effects on their hosts have been extensively studied. In recent years these endosymbionts have gained attention as potential drivers of speciation by inducing reproductive isolation in differentially infected host populations. Research in this area has so far provided support for, as well as against, this theory. Whilst a plethora of studies exist on endosymbiont infections in insects, the knowledge about the effect of endosymbionts on spiders is still limited. Only a few studies so far could attribute observed sex ratio distortions and incompatibilities to the presence of one or more endosymbiont species. The work presented in this thesis aimed to further our understanding of endosymbiont infections in spiders and the role of endosymbionts in the speciation process.
The first part of the thesis is dedicated to Wolbachia infections in Nephila senegalensis. A strong female bias has been found in a population from KwaZulu-Natal. However, so far it remains unknown if Wolbachia is involved in the sex ratio distortion. The finding that Wolbachia infects N. senegalensis was a serendipitous discovery which inspired the following studies on Linyphia hortensis and Linyphia triangularis. These two linyphiid species were found to carry the endosymbionts Wolbachia, Rickettsia and Spiroplasma. Furthermore, a reduced diversity in mtDNA haplotypes was detected which is consistent with endosymbiont-induced selective sweeps. All populations showed a female biased sex ratio, but since no matrilines could be successfully reared in the lab, it is not clear whether this sex ratio distortion is the result of the used sampling method, whether there is a skewed operational sex ratio or whether endosymbionts cause the observed bias. Potentially a combination of all three factors could be responsible.
The study could not confirm whether the endosymbiont infections cause incompatibilities between differently infected individuals in L. hortensis or L. triangularis. No assumptions can be made at this point on whether endosymbiont infected males prefer to mate with infected compared to uninfected females. The spiders, which were collected from the United Kingdom, Germany, Denmark and Sweden, did not show any differences in mating behaviour in the few mating experiments that were conducted in the lab. Nevertheless, the sample size was not sufficient to be entirely sure that there are no differences and whether endosymbionts play a role in the mating system of these spiders.
All three spider species were infected with Wolbachia strains, which carry the temperate bacteriophage WO. This phage has been shown to influence the level of cytoplasmic incompatibility (CI) in insect populations. It is possible that the phage is not only able to disrupt the capability of Wolbachia to induce CI, but that it can also have an effect on the expression of one or more of the sex ratio distorting phenotypes. At this point the role of the phage in spider host systems remains a mystery.
The theoretical work presented in this thesis indicates that the endosymbiont infections found in the two Linyphia species do not show a high enough transmission fidelity to be able to successfully spread through the host populations. Choosy males, which discriminate against mating with uninfected females, would therefore not have a fitness advantage. Under these conditions a preference mutation would not be able to spread through a population either.
This thesis, though it cannot answer all the questions that were asked in the beginning, presents a good foundation for further research in the area of endosymbiont infections in spiders. The knowledge acquired about the three study species N. senegalensis, L. hortensis and L. triangularis will be useful in future studies, investigating the effects of endosymbionts on their spider hosts and their potential of driving speciation processes.
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