Adaptation Of Two Whitefly Species, Bemisia Tabaci (Gennadius) B-biotype And Trialeurodes Vaporariorum (Westwood) (Homoptera: Aleyrodidae), To Thermal Stress: Temperature Responses And Molecular Mechanisms

Bemisia tabaci (Gennadius) B-biotype and Trialeurodes vaporariorum Westwood are both important invasive alien pests in China. These two sympatric whiteflies species diverged in regional population predominance in the process of habitat adaptation: population predominance is not decided by time order of their first invasion under condition that suitable host plants are available in their habitat; B. tabaci B-Biotype is tending to displace T. vaporariorum in many regions or on many host plants; T. vaporariorum causes extensive damage in the northern China, while B. tabaci B-Biotype explosive outbreak almost throughout the country from the south to north areas; population of B. tabaci B-Biotype is generally predominant in hot summer while T. vaporariorum toward the cool autumn or spring. So we hypothesize the adaptability of tolerance to thermal stress may be one of the important reasons that leads to two whitefly species interspecific differentiation in seasonal dynamics and distributions. The association between interspecific differentiation in thermal adaptation and the interspecific displacement was revealed in the level of biology and molecular biology. The main results of this study are as follows.1. The survivals of three developmental stages of two whitefly species decreased with the increase of temperature and the prolonging of heating period. Further, the survival rates of eggs, red-eyed nymphs and adults of B. tabaci B-biotype were higher than those of T. vaporariorum. There were significant differences when heat tolerance between these two whitefly species was compared. The survival rates of B. tabaci B-biotype adults were not significantly affected by the temperature varied from 37 to 41℃, however, the adult had the weakest tolerant ability to extremely high temperature, 43 and 45℃. In the case of T. vaporariorum, the egg had the highest heat tolerance among the three tested stages.2. In both whitefly species, the decreased survival due to the brief exposure of high temperature represented significant difference between female and male adults and females appeared higher survival than that of males. In addition, B-Biotype B. tabaci female adults had higher survival than that of T. vaporariorum at the same conditions. The similar phenomenon was also observed in male adults of two whitefly species.3. Acclimation at non- lethal high temperature significantly influenced heat tolerance of B. tabaci B-biotype, which can enhance its survival as high as 63.9% at 45℃. However, heat tolerance of T. vaporariorum adults were not significantly enhanced after acclimation at 37℃.4. Shorter period of exposure to high temperature had no influence on pre-oviposition period of two whitefly species and female adults began to lay eggs around 2 days. The female fecundities were not significantly different when B. tabaci biotype B adults were exposed to all temperatures. In contrast, the fecundities of T. vaporariorum declined with the increase of temperature, and only a few eggs were oviposited at 43℃. Survivals or hatch rates of the F1 nymphs of both whitefly species declined as the increase of temperature, and no T. vaporariorum nymphs were hatched at 43℃. Similarly, percentages of F1 offspring developed to adults for both whitefly species also declined as the increase of temperature. Sex ratios of the F1 offspring were not significantly affected for T. vaporariorum but were slightly affected for B. tabaci biotype B at the heat shock temperatures, 43 and 45℃.5. Heat shock protein 70 (hsp70) gene functioned as molecular chaperones was cloned, with a result of 3 partial cDNA sequences for B. tabaci biotype B and 2 partial cDNA sequences for T. vaporariorum. This showed that hsp70 gene family is evolutionarily conservative.6. The plasmid with hsp70 cDNA of B. tabaci B-biotype adults was used as a template. Based on GenBank, one TaqMan-MGB probe and the corresponding primers were designed to construct a series of standards for real-time fluorescence quantitative Real-time-PCR to detect the expression of hsp70 gene of adults B. tabaci B-biotype. High temperature can induce the expression of hsp70 gene in B. tabaci B-biotype adults. Within the range of 37 to 41℃, the expression level of hsp70 gene of B-Biotype B. tabaci adults rised with the increase of temperature. However, when temperature increased to 43℃and 45℃, the hsp70 gene expression level decreased sharply. The hsp70 gene expression level of B. tabaci B-biotype adults were changed in response to the diurnal temperature change. When air temperature increased from 34℃to 41℃, the hsp70 gene expression level increased dramatically. And the hsp70 gene expression level decreased when air temperature decreased to 33℃at nightfall.Our research indicated that the differences of thermal stress tolerance between B. tabaci B-biotype and T. vaporariorum was one of the main reasons that lead to the interspecific differentiation of geographic variation in distribution and seasonal dynamics. The molecular mechanism of B. tabaci B-biotype adaptation to thermal stress was revealed. The significance of heat tolerance was elucidated in insect evolutionary adaptation and distribution. The results also have theoretical and practical significance in explaining the population expanding mechanisms of biological invasion and prediction of population dynamics.