Development of pigment cells and analysis of the expression of related genes in the early embryo of Oryzias curvinotus

In different species of fish, the number and distribution of pigments are specific, which results in the diversity of body colors. The pigments in fish have lots of functions such as avoiding predators, protecting fish from radiation, and changing their body color for courtship. The diversity of pigments can make fish better adapt to the natural environment. According to the theory of natural selection, the process of formation of pigments of different groups may genetically be differentiated due to the selection effect of environment light and so on. Oryzias curvinotus is a kind of small oviparous fish and its body length is about 2-4 cm. O. curvinotus has strong fertility, a short generation cycle, sensitivity to water quality and environmental changes, and wide adaptability to salinity. O. curvinotus were used to living in the layer water, which is widely distributed in Guangdong coastal areas. Because the embryos of O. curvinotus are transparent, which makes them become the ideal materials for pigment research and gene function research. At present, there have been researches on the resources survey, development, and function of O. curvinotus which is expected to be developed as a model species for monitoring coastal water environment in China. To test the above hypothesis, in this study, different latitude groups (Raoping, Gaoqiao and Sanya) of O. curvinotus were bred under the same conditions, and their progenies (F₆) were used as materials. The iridophores and melanophores of embryos of O. curvinotus were observed by stereo fluorescence microscope, in addition, the expression of four main genes (tyr, alk, sox10 and pax3) in the early development of the Gaoqiao population was analyzed by real-time fluorescence quantitative PCR (qRT-PCR), and the observation results were verified. The results of observation showed that the melanophores and iridophores of embryos in high latitude Raoping embryos were concentrated in 8 and 10 somites stages, accounting for 81.9% and 52.1% respectively; Accordingly, the melanophores and iridophores of the embryo in Gaoqiao embryos were concentrated in 7 and 11 somite stages, accounting for 47.7% and 42.1%, respectively; And the melanophores and iridophores of the embryo in Sanya embryos were concentrated in 8 and 12 somites stage, accounting for 44.0% and 62.2%. qRT-PCR analysis of Gaoqiao embryos confirmed that the expression of tyr, the rate-limiting enzyme of melanin formation, was up-regulated from the 7 to 9 somites stage, and then tended to be stable; And the sox10 and pax3 genes, which inhibit the formation pathway of melanin and initiate the differentiation of iridescent cells, showed the single peak expression pattern from the 10 to 14 somites stage as the downstream alk genes. In conclusion, this study supports that the pattern of melanin and iridescent pigments in the early development stage of O. curvinotus is consistent with the classical pathway of fish, which is regulated by the orderly expression of tyr, alk, sox10, pax3 genes, and the iridescent cells in the low latitude population generally occur later, which supports the existence of population genetic differentiation, but the specific mechanism remains to be studied in the future.