| To study the regulation of individual development by force from environment is an unfailing topic. Among the multiple factors, climate changing and temperature variation are omnipresent, they influnence animals’ environmental development and even guide the orientation of evolution.In the 19th century, the Germany biologist Carl Bergmann propose that animals’body sizes tend to increase with latitude, in the higher latitude with colder climate, animals’s body size tend to be larger while in the lower latitude with warmer climate, animal’s body size tend to be smaller. This is the well known Bergmann’s rule which summarize the general rule for organism size with temperature based on intraspecific comparisions of size among endotherms. This relationship of body size to temperature is more profound in ectotherm and is usually called Temperature-Size Rule(TSR). People study the phenomenon of this biological rule since the early 19th century, however, at cellular and molecular levels, the underlying mechanisms are still not clear. Model organism Drosophila is a well supporter to explore the mystheries of TSR, as early as 1940, people begin to use Drosophila to study the mechanism behind TSR. Within the suitable temperature to survive, body size of fly adult has strong temperature plasticity, for example, pupal size of fly rearing at 18℃is approximately ten percent larger than the one rearing at 25℃. The larval stage determines the final adult size, so we focus on the development mechanism of larvae. Fly larvae pocesses mutiple sensation as the adult, including chemical sensation, mechanical sensation and temperature sensation. The larval peripheral nervous system reponsible for the multiple primary sense perceptions, they innervate to brain, the central nervous system, to form a relatively simple neural network with complete function. Animals sense the enviroment clue through primary sensation neurons, so we try to study the mechanism of TSR focousing on the nervous system of larvae by the methods of artificially control the excitability of neurons and basic cellular and molecular detection.Drosophila insulin also named dilps (Drosophila-insulin-like-peptides) is one of the key components that regulate development of larvae, and are envolutionaliy conserved. The insulin producing cells (IPCs) synethsis and secret several main kinds of insulin protein as secretory neurons in the fly. We found that, inhibiting IPCs eliminates the normally increasing of pupal size in colder temperature while activation IPCs leads to bigger pupal size. By the experiment of in vivo calcium imaging, we found that cold temperature can stimulate IPCs. Moreover cold temperature promotes synthesis and secretion of Dilp2 protein. This part of data account for the importance of IPCs as key regulatory cells and cold temperature responsible neurons.Additionally, based on screening, we found a Ga14 line named 11216 which restrictly labels a group of neurons in larval head. By the experiment of in vivo calcium imaging, we found that cold temperature can stimulate 11216-Gal4 labled in the larval head, we confirmed this group of neruons to be primary cold sensing neurons by further indetification of expression parttern and morphology of these neurons. More importantly, we found that, this group of cold sensing neurons project into larvae brain to form direct contact with IPCs by the method of GRASP(GFP reconstitution across synaptic partners). We further established the neuronal circuit from cold sensing neurons to IPCs by taking advantages of calcium imaging, optogenetics technique. Activating these cold sensing neurons was sufficient to promote the synthesis and secretion of dilps in IPCs and finally increasing the pupal size, just as cold temperature does.Taken together, these findings not only reveal a neuronal circuit that mediates the effects of low temperature on fly growth but also discloses a new mechanism to explain TSR and Bergmann’s rule. |
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