Ecology and evolution of psyllid-bacterial endosymbiont symbioses: Glycaspis brimblecombei-C. Arsenophonus species and Bactericera cockerelli-C. Liberibacter psyllaurous

In this dissertation, two psyllid study systems (Glycaspis brimblecombei Moore and Bactericera cockerelli Sulc.) were explored for facultative bacterial endosymbionts. Once facultative (secondary) bacterial endosymbionts were found in these psyllid systems their potential role and maintenance in their insect host was determined. In the first psyllid study system I found that G. brimblecombei possesses one type of secondary endosymbiont in California, which is a Candidatus Arsenophonus species in the phylum gamma- Proteobacteria. Across 19 psyllid populations studied, Arsenophonus-infection frequencies varied dramatically from 0-75%. Moreover, Arsenophonus infection frequencies of the G. brimblecombei populations were positively related to parasitism pressure on a geographic level in field populations. My study is the first to find this relationship (between parasitism and infection frequency) in the field. Resistance of an infected secondary endosymbiont psyllid to a parasitoid may be an important parameter to test in future manipulated trials.;In the second psyllid study system, I discovered and genetically characterized a novel bacterial symbiont, Candidatus Liberibacter psyllaurous, which is associated with B. cockerelli. My results from insect-plant transmission trials revealed that L. psyllaurous does indeed infect phloem of potato and tomato plants and furthermore is associated with the disease psyllid yellows. The cause of this yellowing disease has been a mystery for over 100 years. I also found that any psyllid lifestage can transmit L. psyllaurous into tomato phloem tissue within 24hrs of inoculation. Moreover, when an infected L. psyllaurous tomato plant is grafted to a healthy tomato plant, L. psyllaurous can infect the healthy plant within 24hrs and cause psyllid yellows symptoms within two weeks, similar to infected psyllid inoculated plants. Consequently, other potential symptom-inducing effects of the psyllid's feeding (e.g. salivary toxins) can be ruled out and the disease historically known as psyllid yellows can be associated with the bacterium L. psyllaurous. Additionally, I found that this bacterium most likely is maintained and spread in psyllid populations because it is transmitted both maternally and horizontally. Moreover, L. psyllaurous can be transmitted to citrus by the tomato psyllid and may be present in the phloem tissue of field populations of citrus in California.