The Basis Of Wing Morphology And Potential Molecular Mechanisms Of Flight Loss In The Silkworm,Bombyx Mori

Insects are the most diverse and abundant animals on Earth.Acquisition of flight are enables them to search for mates,food,disperse widely and avoid enemies.Due to habitat changes,very few species are lost their flight ability during adaptive evolution.Studies were found that derangement of flight muscle,morphological change and degeneration of wing or were important factors affecting flight ability in insects.Bombyx mori was domesticated from B.mandarina.B.mori has lost flight ability during long-term domestication,while the mechanism is still unknown.We compared the wing morphology and transcriptomes of wing and thorax between the Bombyx mori and B.mandarina.In addition,we used resequenced genome data of the domestic and wild silkworms to detect the selection signals for the DEGs.These studies can find some clues for flight loss in domestic silkworm.Main results were as follows:(1)Comparative analysis of wing morphology between Bombyx mori and B.mandarinaIn total,we collected 820 adults,including 350 individuals in domestic Dazao,245 in domestic Xiafang,and 225 in wild silkworms.We investigated the length,width and area of the right forewing of each individual,measured weight,area and length of body,and calculated the wing loading,aspect ratio and circularity.We analyzed the correlation and principal components of these parameters.Three principal components(PC)were obtained,such as PC-Size,PC-Heft and PC-Shape.According to the percent of component,we found that PC-Size and PC-Heft contribute more to flight ability.By using linear regression analysis for PC-Size and PC-Heft,we found that the slopes of Dazao(Slope = 0.65)and Xiafang(Slope = 0.68)were similar,while the linear regression slope of B.mandarina was reached to 1.72,which was about 2.5 times higher than that of B.mori.This result indicated that wing size of domestic silkworm increased slowly with increase of body weight,which was resulted in increase of wing loading to2-3 times of the wild silkworm.Although the wing loading of domestic silkworm has increased,it is still in the range of Lepidoptera insects with flight ability.Therefore,the increase of wing loading of domestic silkworm may reduce the flight ability,but it is not the only factor lead to flight ability lost.(2)Molecular mechanisms of flight loss in the silkworm,Bombyx moriTranscriptomes of wing and thorax was sequenced in the domestic and wild silkworms.After performing quality control,the number of clean base in all transcriptome libraries was 54.14 Gb.After assembly,29603 unigenes were generated,we found that 21,262 genes were expressed(FPKM ≥ 1)in the wing and 18,258 genes were expressed(FPKM ≥ 1)in the thorax.In addition,we found that 323 genes were highly expressed(FPKM > 1000)in the wing and thorax,including 11 flight muscle protein genes,34 energy metabolism related genes and 16 cuticular protein genes.Meanwhile,In addition,we found that 28 genes were extremely highly expressed(FPKM > 10000)in the wing and thorax of the domestic and wild silkworms,which are related with energy metabolism and flight muscle.Using edge R and DESeq,2018 and1537 differentially expressed genes(DEGs)were identified in the wing and thorax between the domestic and wild silkworms,respectivley.Through functional annotations,we found that 87 DEGs were related to energy metabolism.Population genetics were performed for the differentially expressed genes.It was revealed that 13 DEGs involved in carbohydrate and lipid metabolism had artificial selection signals in the silkworm,and most of them were down-regulated in silkworm.By comparing the ultrastructure of flight muscles of B.mori and B.mandarina,we found that the content of mitochondria was significantly decreased in the domestic silkworm.And,we also found that the average oviposition number per female of the domestic silkworm was significantly higher than wild silkworm.Therefore,there is a trade-off of energy metabolism between flight and reproduction in the domestic and wild silkworms.The decrease of energy consumption in flight-related tissues might be one of the reasons for flight loss in domesitc silkworm.In addition,it was showed that there was no obvious disturbance of flight muscles filaments in the domestic silkworm,while the length of sarcomeres significantly increased.In our study,only three DEGs related flight muscle were found in thorax.Flightin plays an important role in structure and contraction of normal sarcomeres.The decrease of the expression level of flightin may lead to increase of sarcomere length in domestic silkworms.Therefore,the increase of sarcomere length might be another reason for the deterioration of flight ability of silkworm.Compared with wing morphology,transcriptome of wing and thorax,and ultrastructure of flight muscles between the domestic and wild silkworms,we found that significant increase of wing loading,trade-off relationship between flight and reproductive,and increase of sarcomere length might have cumulative effect on flight loss in domestic silkworms reason for the flight loss of silkworm.The genes related to sarcomere length and differentially expressed energy metabolism genes under artificial selection will be the important candidates for discovering molecular mechanisms of flight loss in domestic silkworms.