Body Color Characteristics And Comparative Transcriptome Analysis Of Three Different Phenotypes Of Pristella Maxillaris

As one of the most diverse phenotypic traits under strong selection pressure in many organisms,coloration plays numerous adaptive functions,such as predator deterrence,species recognition and even protecting the organism from solar ultraviolet radiation damage.Skin coloration can be influenced by many factors.Nevertheless,genetics are still the major determining factor.Pristella maxillaris,known as the X-ray fish,is a small translucent fish with a definite ornamental value.The degree of transparency of the body is one of the important indexes to determine its ornamental value.At present,it is found that there are a variety of mutant types such as gray-white-transparent in the breeding process.However,the morphological characteristics and the molecular regulatory mechanisms of these phenotypic mutations are still unknown.In this study,three typical phenotypes of Pristella maxillaris were selected,namely wild type(WT),mutant I(MU1),and mutant II(MU2).Phenotypic morphology methods,histological methods and transcriptome analysis were performed to explore the differences in the chromatophores types and pigment-related genes among the different phenotypes.The purpose of this study was to provide a valuable theoretical foundation for the molecular regulation of the transparent phenotype and the function of subsequent genes.The main results are as follows:1.The operculum lining,skin and peritoneum tissues of the three phenotypic Pristella maxillaris were observed by stereoscopic microscope.The results revealed that three types of chromatophores(melanophores,iridophores and xanthophores)were observed in WT.Compared with WT,there were two types of chromatophores(iridophores and xanthophores)in MU1 and.MU2 fish only had xanthophores.The skin and peritoneum tissues of the three phenotypes of Pristella maxillaris were used as a comparative observation by histological methods.and the results showed that numerous melanophores and iridophores were observed in the skin and peritoneum tissues of WT fish.Compared with WT,there were a large number of iridophores in the skin and peritoneum tissues of MU1.However,there were no chromatophores in the skin and peritoneum tissues of MU2 fish.The difference in the transparency of the three phenotypic Pristella maxillaris was mainly caused by the different types of pigment cells.2.Transcriptome sequencing generated 72.49 million,71.70 million and 74.89 million clean reads from the skin and peritoneum tissues of WT,MU1 and MU2 fish,respectively.Clean reads were spliced with Trinity software,and a total of 282,320 Transcripts and 166,089 Unigenes were obtained.After comparing the intergroup gene expression levels,more than 3000 unigenes with significantly different expression levels were found.Through Gene Ontology(GO)and Kyoto Encyclopedia of Genes and Genomes(KEGG)pathway analysis of the differentially expressed genes(DEGs),a few genes(wnt8,asip,map2k2,cam,cerb and gst)involved in the signaling pathway of melanophore development were downregulated in the skin and peritoneum of MU1 fish compared with their expression in WT fish.Many genes downregulated in MU2 fish compared with their expression in the skin and peritoneum of WT and MU1 fish(hprt,pfka,eno,pgam1,ndk and gart etc.)were annotated to pathways related to the formation of iridophores.This results indicated that these genes might be key to the proliferation and differentiation of melanophores and iridophores.However,it needs to be further verified whether the differential expression of genes in these pathways is directly related to the formation of the transparency phenotype.