Observation and analysis of morphology abnormalities in development of Oryzias melastigma embryos

Fish embryos are widely used as models in toxicology, drug development, and human disease research because of their high sensitivity, observability, and operability, providing the basis for an in-depth understanding of the embryogenesis. Increasing studies have indicated that birth defects are hereditary. In this study, we used Oryzias melastigma as a model to conduct a study of 185-day embryogenesis and observed self-induced non-pathological abnormal embryogenesis. O. melastigma experienced pre-puberty stage, adolescence stage, and senescence stage, and individuals produced up to 102 eggs per day. However, the fecundity was markedly reduced during the senescent stage. During the active egg and blastodisc stages, pseudo-fertilization and pseudo-blastocysts were observed. During cleavage at the 4- to 32-cell stages, we observed blastomeres separation or dislocation. Excessively separated blastomeres formed double blastoderms, eventually resulting in conjoined twins. During the blastula stage, we observed abnormally increased cell volume, narrowed and elongated blastocysts, and abnormally coated blastoderms. At the organogenesis stage, we observed abnormal numbers of Kupffer’s vesicles and conjoined twins. Abnormality in the location and number of oil droplets were observed in various development stages. Abnormal development was more commonly observed in fertilized eggs produced by broodstock in prepuberty or senescence stages, which is probably related to the age of fish and the egg quality. This study can provide the materials for comparative analysis in toxicological and molecular studies of O. melastigma, and may provide evidence for other economic fish that produce sticky eggs.