Mechanism Of Juvenile Hormone Regulation In Elytra Color Plasticity Between Reproductive And Diapause Harmonia Axyridis

Many animals,including insects,exhibit body color plasticity in response to environmental stimuli.Carotenoids are a general term for a group of important natural pigment compounds and varied expression of carotenoids significantly contribute to the body color flexibility.However,it is unclear how environmental cues regulate the type or content of carotenoids on insect body cuticle.Harmonia axyridis is an important biocontrol agent with reproductive diapause characteristics.Long day(LD)14L: 10 D induces H.axyridis into reproduction,while short day(SD)10L: 14 D induces diapause individuals.Preliminary experiments have found that there are significant differences in elytra coloration(the coloration of the carotenoids region)between reproductive and diapause female adults,however the regulatory mechanism behind this phenomenon is not clear.Therefore,this study used H.axyridis as materials to study the photoperiod-mediated regulatory mechanism of elytra color plasticity between reproductive and diapause H.axyridis.The main results are as follows:1.Difference of elytra coloration between reproductive and diapause female adults of H.axyridisIndoor rearing observation showed that elytra coloration of LD-induced reproductive female adults gradually changed from yellow to red,achieving their final hue around 25 days post-eclosion(PE).SD-induced diapause female adults exhibit the same rate of reddening,but the process appears to cease around 15 days PE.In addition,elytra coloration of the reproductive female adults is significantly redder than that of the diapause female adults between 20～30 days PE.There was no significant difference in elytra coloration between reproductive and diapause male adults.The results indicate that elytra coloration of LD-induced reproductive females is significantly redder than that of SD-induced diapause females.2.Difference of carotenoids content between reproductive and diapause female adultsThe distribution patterns of total carotenoids in different body parts of reproductive and diapause female adults revealed that at 20 days PE,there was no apparent difference in the total carotenoids between single reproductive and diapause female adults,although diapause female adults weighed less than reproductive female adults;The mesothorax-metathorax-abdomen complex of reproductive female adults weighed more than those of diapause female adults but contained fewer carotenoids;In the head-prothorax complex,we did not observe obvious differences in both carotenoids content and fresh weight between reproductive and diapause female adults;The total carotenoids found in a single pair of elytra were significantly greater in reproductive female adults compared to diapause female adults.We applied UPLC-MS/MS method and demonstrated that α-carotene and β-carotene were the two main carotenoids in the elytra of female adults,accounting for over 80% in total.The elytra of diapause female adults contained significantly less α-carotene and β-carotene than those of reproductive female adults.The results showed that carotenoids are significantly enriched in the elytra of reproductive female adults,causing the elytra coloration of reproductive female adults significantly redder than that of diapause female adults.3.Mechanism of JH signaling regulating elytra color plasticity in female adultsAfter applying the juvenile hormone(JH)analogs S-Hydroprene to diapause female adults induced ovarian development,decreased fat body accumulation was,changed diapause state to reproductive state,induced significant redder and more total carotenoids content in the elytra.Further studies found that after silencing JH intracellular receptor gene Met and the early responsive gene Kr-h1 to reproductive female adults,it inhibited ovarian development,promoted fat body accumulation,changed reproductive state to diapause state,significantly yellowed and less total carotenoids in the elytra.The results show that JH signaling regulates the carotenoids content of the elytra via Met-Kr-h1 pathway,thereby regulating color plasticity of elytra in female adult H.axyridis.4.Mechanism of the carotenoid transporter participating in elytra color plasticity of female adult H.axyridisFirst,candidate genes for carotenoid transporters were analyzed by transcriptome,with STAR3 and SCRB transcripts presenting high transcriptional levels in reproductive female adults.One STAR3 was identified in H.axyridis,and STAR3 significantly higher expressed in the elytra of the reproductive female at 20 days PE but there was no significant change in elytra color after STAR3 RNAi in reproductive individuals,indicating that STAR3 is not a key gene for regulating the color plasticity of the elytra in H.axyridis.Furthermore,Fourteen SCRB were identified in H.axyridis,of which 6 genes(SCRB1,SCRB3,SCRB10,SCRB11,SCRB12 and SCRB14)were highly expressed in the elytra of reproductive female adults.After silencing these six candidate genes one by one,it was found that the silencing of SCRB10 gene alone led to significant yellowing of elytra color and a significant decrease in total carotenoid content of elytra,but had no significant effect on ovarian development and fat body accumulation.Finally,the expression level of SCRB10 was significantly upregulated after applying JH analogues to diapause female adults,and the expression level of SCRB10 was significantly downregulated by silencing Met and Kr-h1 in reproductive female adults.The results indicate that the carotenoid transporter SCRB10 in response to JH signaling regulates elytra color plasticity of female adult H.axyridis.In conclusion,this study found that there is significant difference in elytra coloration between reproductive and diapause female adult H.axyridis,and revealed that JH signaling regulates expression level of the carotenoid transporter SCRB10 via Met-Kr-h1 pathway,resulting in elytra color plasticity in female adults.The results not only provide a theoretical basis for using elytra color as a diapause discrimination index to guide the factory production of H.axyridis,but also provide a new perspective for understanding the environmental plasticity of the endocrine system regulating animal body color.