An Investigation On The Interactions Of Bemisia Tabaci-TYLCCNV-plant And The Underlying Nutritional Mechanisms

The B biotype of Bemisia tabaci has spread widely to much of the rest of the world from its origin in the Middle East-Asia Minor region in the past 20 years. Recently, this event has been followed by the global spread of a second invasive B. tabaci, the Q biotype which has spread from its origin in Iberian Peninsula to countries bordering the Mediterranean Basin and some regions in Asia and America. The two biotypes coexist in some areas, and caused great loss to plant production. The widespread invasions of the B and Q biotypes of B. tabaci have often been followed by epidemics of geminiviruses, resulting in huge decrease in crop yield. However, the role of whitefly-virus-plant interactions on the invasion of alien biotypes of whitefly has been rarely investigated. In this thesis, the alien B biotype and Q biotype of B. tabaci, the indigenous ZHJ1 biotype and ZHJ2 biotype of B. tabaci, and tomato yellow leaf curl virus (TYLCCNV) were used to study the triple-interaction effects on fitness of whitefly in two model systems: B/ZHJ1-TYLCCNV-tomato system and Q/ZHJ2-TYLCCNV-tobacco system. The amino acid composition in the phloem of healthy or virus-infected tobacco plants, as well as the free amino acid composition in the body of and honeydew excreted by whiteflies feeding on TYLCCNV-infected or healthy tobacco plants were examined. Our objectives were to reveal the influence of whitefly-geminivirus-plant interactions on the invasion of alien biotypes of the whitefly and to explore the underlying nutritional mechanisms from the aspect of amino acids. The results are summarized as follows:(1) Comparison of performance between B biotype and ZHJ1 biotype whiteflies on TYLCCNV-infected tomato plantsWhen tomato cv. HEZUO-903 was used as the host plant, the fitness of ZHJ1 biotype was decreased on tomato plants infected by TYLCCNV. The cohorts of ZHJ1 biotype could not be established on TYLCCNV-infected plants. In contrast, the performance of B biotype whitefly on healthy and virus-infected tomato plants did not differ significantly. The cohorts of B biotype increased rapidly either on healthy or TYLCCNV-infected plants. When the variety of tomato plants were changed to FA-516 or FA-189, the fecundity and longevity of female adults of both biotypes decreased on TYLCCNV-infected plants compared to those on healthy plants; the survival of immature stage of the B biotype did not change on TYLCCNV-infected tomato plants while that of ZHJ1 biotype decreased greatly compared to that on healthy plants.(2) Comparison of performance between Q biotype and ZHJ2 biotype whiteflies on TYLCCNV-infected tobacco plantsCompared to those on healthy tobacco plants, the fecundity and longevity of female adults of Q biotype increased significantly when they were transferred to TYLCCNV-infected tobacco plants upon emergence on cotton plants. When the Q biotype fed on TYLCCNV-infected plants for a whole generation, its fecundity increased by 7 times and longevity became doubled. For the ZHJ2 biotype, the fecundity and longevity of female adults somewhat increased whether they were transferred to TYLCCNV-infected tobacco after emergence on cotton plants or developed on TYLCCNV-infected tobacco plants and the difference was not significant. These results indicated that a significant mutualism developed between TYLCCNV and the Q biotype whitefly but not so obvious between TYLCCNV and the ZHJ2 biotype whitefly.(3) Influence of TYLCCNV infection on amino acid composition of tobacco plants and whiteflyThe concentration of amino acid in phloem sap in healthy and TYLCCNV-infected tobacco plants increased as plants grew. The composition of some amino acids was significantly higher in TYLCCNV-infected plants than in healthy plants. Some essential amino acids seemed to be more abundant in TYLCCNV-infected plants compared to healthy plants. When the two biotypes of whiteflies fed on TYLCCNV-infected tobacco plants, most of the amino acids in their honeydew decreased dramatically, especially in the Q biotype. These preliminary results indicated that, amino acid composition was altered by TYLCCNV infection to tobacco plants, thus bringing benefits to whiteflies. The Q biotype seemed more efficient in utilizing amino acids than the ZHJ2 biotype, hence conferring more benefits to its fitness. These changes in the plants and differences between the whitefly biotypes may represent some important mechanisms responsible for the obvious mutualism between TYLCCNV and the Q biotype whitefly.An integrated analysis of the results of this study and those from the literature indicate that the relationships between whitefly, geminivirus and host plant would differ depending on the species involved. On host plants of low suitability, virus-infection of plants increased the fitness of invasive whiteflies dramatically but had no such effects on indigenous whiteflies. In contrast, on plants that show high suitability to invasive whiteflies but low suitability to indigenous whiteflies, virus-infection of plants had no effects or only minor negative effects on the fitness of invasive whiteflies, but caused significant decreases to the fitness of the indigenous whiteflies. Taken together, these results indicated that, compared to the indigenous whiteflies, the invasive whiteflies are often more likely to take advantages from their interactions with viruses and host plants in the processes of their invasion and displacement of indigenous competitors.