Molecular Mechanism Of Ovarian Development Regulated By Photoperiod Based On Multi-Omic Analysis In Nile Tilapia

Tilapia is one of the most important commercial fished in the world.In2018,the export was 8.4% among the general aquatic trade product exports in China,which were exported to America,Europe,Africa,Mexico and so on.The volume and value of exports were increased by 9.31% and 11.64%,respectively.The export volume of tilapia ranked top three.In the early spring,low temperature of water and short sunshine time leads to immature ovary,low yield and higher production cost.Therefore,it is necessary to understanding the role of photoperiod modulating gonadal development of tilapia.In current research,we screened out the best photoperiod based on the preliminary knowledge of the role of photoperiod in promoting ovarian development in nile tilapia.The purpose of this study was thus to unravel key candidate genes and further investigate the molecular mechanism involved in ovarian development under different photoperiod through transcriptome,Genome-wide DNA methylation and ATAC-seq.The main results were as follows:1.Photoperiod regulates ovarian development in nile tilapia.Transcriptome was applied to analyze the expression of gene involved in ovarian development of nile tilapia under different photoperiodic conditions.The photoperiods tested were as follows: LD(12 h light: 12 h dark),LL(24 h light: 0 h dark)and DD(0 h light: 24 h dark).The results revealed that long photoperiod significantly promoted ovarian development.These data indicated that 988,992 and 1,036 differentially expressed genes(DEGs)were detected between the groups of LD–LL,LD–DD,and LL–DD,respectively.2.Effects of different photoperiod regimes on the growth and gonadal development of nile tilapiaSample fishes were fed under 4 different photoperiod regimes including12L/12 D,14L/10 D,16L/8D and 18L/6D,which were fed for 30 days.Subsequently,growth indicators and reproductive hormones(Gn RH,E2 and MT)in serum were measured,Hematoxylin-eosin(HE)staining of ovaries was conducted.In summary,the contents of gonadotropin-releasing hormone(Gn RH),estradiol(E2)and melatonin(MT)in serum were increased continuously with the extension of photoperiod.Moreover,the hormone contents were highest under 18L/6D condition,and the ovarian maturity was the best under the photoperiod of 16L/8D.Taken together,the current study investigated that the photoperiod of 16L/8D was the best condition for nile tilapia growth and gonadal development.3.Transcriptome analysis of photoperiod affecting ovarian developmentTranscriptome was applied to analyze the ovarian differences of nile tilapia between treatment group(16L/8D)and control group(12L/12D).Totally,3,171 DEGs were detected,including 2,858 up-regulated genes and 313down-regulated genes.GO enrichment and KEGG analysis showed that some signaling pathways were significantly involved in the photoperiod regulation of ovarian development in nile tilapia.The signaling pathways were as follows:ovarian steroidogenesis,MAPK signaling pathway,c AMP signaling pathway.Several candidate genes including adcy6,cyp1b1,cyp1c1,gth-rii and pla2g4 a were selected.4.Genome-wide DNA methylation analysis of photoperiod affecting ovarian development in nile tilapiaComparative methylome analyses identified 1,833 differentially methylated regions(DMGs),of which 892 were hypermethylated(Hyper)regions and 941 were hypomethylated(Hypo)regions between treatment group(16L/8D)and control group(12L/12D).Enrichment analysis found that DMGs were significantly enriched in glutamatergic synapse,dopaminergic synapse and circadian entrainment etc.Integrative analyses of DNA methylation and gene expression data showed that 96 genes were differentially methylated and expressed,but with different patterns of change.A list of candidate genes was screened,which included ryr3,drd3,grik4,grin2 c,grin2b,cacna1 d,cacna1a and so on.5.ATAC-seq analysis for the differences of photoperiod affecting ovarian development in nile tilapiaATAC-seq was adopted to analyze the chromatin open areas in ovaries between the treatment group(16L/8D)and the control group(12L/12D).The number of unique peaks was 149,720,which were involved in 236 gene regions and 102 genes in the control group(12L/12D).Whilst,45,017 unique peaks including385 gene regions and 149 genes were identified in the treatment group(16L/8D).According to the results of combined analysis of ATAC-seq and transcriptome,10 genes were found to have significant changes in chromosome level and m RNA level.Fifty unique motifs were detected in the treatment group(16L/8D),among them,which 22 motifs were known compared to the Motif databases,and 160 transcription factors were predicted.Hence,a cluster of transcription factors were revealed,including ryr3,drd3,grik4,grin2 c,grin2b,cacna1 d and cacna1 a.In brief,the study found that photoperiod improved ovarian development in nile tilapia.We had screened out optimal photoperiod of gonadal development as16 L / 8D.This study investigated the molecular mechanisms of photoperiod regulation of ovarian development in nile tilapia on m RNA,DNA,chromosome and obtained many importance candidate genes.The results was help to analyze the relationship between photoperiod and gonadal development in nile tilapia.It is important to promote the balanced production of tilapia fry.