Antennal Transcriptomes Reveal Effect Of Domestication On The Olfactory System Of The Silkworm Bombyx Mori

Insect olfactory system is a highly specific and sensitive chemical detector,which plays important roles in feeding,mating and finding an appropriate oviposition site in adult.Highly developed antennae are served as an important periphery olfactory system contained several sensillum types,such as sensilla trichodea?medium-length and long?,sensilla basiconica and sensilla coeloconica.Insect olfactory recognition can be summarized as follows: firstly,the liposoluble molecular identification was performed by receptor sites on the antennal receptor neurons.Then odorant binding proteins?OBPs?enhanced the solubility of semiochemicals and delivered them to membrane-bound odorant receptors?ORs?located on the olfactory receptor neurons membrane.After activiating of olfactory response,liposoluble molecular was degraded by odorantdegrading enzymes.About 5000 years ago,the domestic silkworm?Bombyx mori?was domesticated from wild silkworm?Bombyx mandarina?.In the course of domestication,several phenotypic characteristics changed,such as color and size of the body.Indeed,except for the obvious phenotypes,domestication process affected the neurophysiological response of the silkworm.It was found that the domestic silkworm's response to environmental odorants was clearly decreased when compared with the wild silkworm.With the release of whole genome sequence,olfactory proteins and odorant degrading enzymes data of the silkworm,we compared the antennal transcriptomes of the domestic and wild silkworms using RNA sequencing?RNA-seq?.And then we used the resequencing genome data of the domestic and wild silkworms to identify the positively selected olfactory genes and perform molecular population analysis.The main results are as follows:1.Comparative analysis of the antennal transcriptomes between the domestic and wild silkworms.We sequenced four transcriptomes derived from female and male antennae of the domestic and wild silkworms using the mRNA deep sequencing.After performing quality control,231428726 paired-end reads were generated,encompassing 61.2 Gb of sequence.We mapped these clean reads to the Bombyx mori reference genome,version₂.0.All of the mapped reads obtained from the four samples were merged and assembled.After assembly,22786 unigenes were generated,in which 74% genes?16904?were FPKM ? 1.In addition,we found that nine genes were extremely highly expressed?FPKM > 10000?in the antennae of the domestic and wild silkworms,which are related with olfactory reception.For a more detailed analysis of the transcriptome,we identified the putative members of olfactory-related genes in antennal transcriptome datasets,We found 5 new IRs,7 new ORs,2 new OBPs,1 new P450 and 1 COE in antennae based on the previous information.Totally,we identified 2109 differentially expressed genes?DEGs?in six comparison of the 4 samples.GO enrichment analysis indicated that amount of DEGs were related to odorant binding,sensory perception of smell,olfactory receptor activity,and sensory perception.In this study,we focused on the DEGs in D_F vs W_F and D_M vs W_M,1173 and 1410 genes were identified,respectively.Furthermore,30 olfactory genes and 19 ODEs were found,in which more olfactory-related DEGs were down regulated in the domestic silkworm.This might be one of the reason for decrease of olfactory sensation in the domestic silkworm.2.Molecular evolution of olfactory related DEGs based on population genomesIn our lab,we have resequenced the genomes of 8 domestic strains and 7 wild silkworms.Population genetics were performed for the differentially expressed OBPs and ORs in D_F vs W_F and D_M vs W_M.Using DnaSP software,???? and Tajima's D of the differentially expressed olfactory genes were caculated.It was indicated that the evolutionary rates of olfactory genes in the domestic silkworms were lower those in the wild silkworms.Using SIFT online software,deleterious mutations were found in OR30 and OR57 of the domestic silkworm.In fact,nonanymous mutations of the domestic silkworms were obviously lower those in the wild silkworms,while deleterious mutations were accumulated in the olfactory genes of the domestic silkworms.This might be one another reason for decrease of olfactory sensation in the domestic silkworm.In summary,we mainly identified the DEGs in D_F vs W_F and D_M vs W_M,30 olfactory genes and 19 ODEs were found.Most of the olfactory-related DEGs were down regulated in the domestic silkworm.This might be one of the reason for decrease of olfactory sensation in the domestic silkworm.In domestication,the silkworm was reared in indoor environment,the dependence of olfactory sensitivity was decreased.Thus,the relaxation of selection might be resulted in accumulation of deleterious mutations in olfactory genes.Our study preliminarily revealed genetic basis of olfactory degradation in Bombyx mori.It provides candidate genes for elucidating the molecular mechanisms of the olfactory differentiation between the domestic and wild silkworms,and has important theoretical and practical values.