Pristella maxillaris is known as the X-ray fish based on its translucent body. However, the morphological characteristics and the molecular regulatory mechanisms of these translucent bodies are still unknown. In this study, the following three phenotypes, a black-and-gray body color or wild-type (WT), a silvery-white body color defined as mutant I (MU1), and a fully transparent body with a visible visceral mass named as mutant II (MU2), were investigated to analyze their chromatophores and molecular mechanisms. The variety and distribution of pigment cells in the three phenotypes of P. maxillaris significantly differed by histological assessment. Three types of chromatophores (melanophores, iridophores, and xanthophores) were observed in the WT, whereas MU1 fish were deficient in melanophores, and MU2 fish lacked melanophores and iridophores. Transcriptome sequencing of the skin and peritoneal tissues of P. maxillaris identified a total of 166,089 unigenes. After comparing intergroup gene expression levels, more than 3,000 unigenes with significantly differential expression levels were identified among three strains. Functional annotation and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses of the differentially expressed genes (DEGs) identified a number of candidates melanophores and iridophores genes that influence body color. Some DEGs that were identified using transcriptome analysis were confirmed by quantitative real-time PCR. This study serves as a global survey of the morphological characteristics and molecular mechanism of different body colors observed in P. maxillaris and thus provides a valuable theoretical foundation for the molecular regulation of the transparent phenotype.

Relationships of two species of Channa argus (northern snakeheads) named "Bicolor type" and "White type", as well as other six Channa species were investigated based on their partial sequence of mitochondrial DNA 16S rRNA genes in the present study. For the Channa family, the average genetic distance was 0.0863 with the inter-species genetic distance ranged from 0.0173 to 0.1384, and the average intra-species genetic distance in the genus channa was estimated as 0.00273 (range: 0.0019 to 0.0038). For the two C. argus species, the mean pair-wise genetic distance between "Bicolor type" and "White type" northern snakehead was estimated as 0.00218, which was within the intra-species genetic distance interval for Channa species, indicating that they belonged to the same species at molecular level. Moreover, these snakeheads can be divided into two distinct groups via 16S rRNA, which might be more accurate for Channa classification than traditional method based on the absence or presence of the pelvic fins of the fish.

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