Effects of developmental temperature on the biology of Aedes albopictus: a strain comparison

Mosquitoes are poikilotherm animals with a holometabolous life cycle. Thermal conditions they experience during development have a great impact on both juvenile and adult stages. For instance, cold thermal regimes are expected to cause mosquitoes to grow slower but larger and to live longer and to have a higher fecundity compared to mosquitoes developing at warmer temperatures. The potential transmission of pathogens is directly related to developmental speed, adult longevity and fecundity because a faster development, a longer lifespan and a higher fecundity increase the vector population density and the number of vectors which have been exposed to pathogens through a blood meal on an infected host and have become infectious. Despite the known importance of ambient temperature during the mosquito development, we lack data on the extent of its impact on both juvenile and adult stages of the mosquito life. Especially in the case of invasive species unravelling the relationship between developmental temperature and the mosquito performance at the various life stages and identifying possible differences across populations might highlight the most vulnerable life stages for control strategies and identify the populations with the highest invasion potential in face of current climatic changes. The Asian tiger mosquito Aedes albopictus is a highly invasive species which vector health-threatening arboviruses such as Zika, Dengue and Chikungunya and is currently present in every continent of the World except Antarctica. For this species, we lack precise information on traits which are highly influenced by developmental temperature and are also important for pathogen transmission, such as developmental speed, longevity and fecundity. Within this context, during my PhD I investigated the effect of different developmental temperatures on the performance of juvenile stages and their further effects on adults of Ae. albopictus mosquitoes of a tropical (Foshan, China) and a temperate (Crema, Italy) origin. Furthermore, I extended my study to the transcriptome, the physiology, thermal traits and the density of Wolbachia, a thermosensitive bacterium of great importance for the mosquito fitness. The temperatures I decided to investigate include 18°C, which represents the average daytime temperature registered during spring 2021 in Northern Italy, 32°C, which was the peak of warm temperatures registered in the summer 2021 in the same region, and 28°C, which is the standard rearing temperature for Ae. albopictus. My results show strong strain-specific effects on both fitness and energy reserve in mosquitoes reared under different thermal regimes, whereas I measured few thermal-related changes in adult thermal preference and knock-down temperature (i.e. heat resistance). Overall, the thermal condition of 32°C was the one mostly reducing the mosquito fitness at the adult level, whereas the temperature of 18°C drastically slowed down development of early stages. At both 18°C and 32°C I recorded responses linked to stress (such as cytochrome p450 and heat shock proteins) and the lowest densities of Wolbachia. I further proved that mosquitoes’ wing length and body mass follow the Bergmann’s rule and found that only the tropical Foshan strain follows developmental isomorphy, suggesting that depending on their geographical origin different strains might have different adaptability potential of early life stages in response to climate change.