SignificanceViviparity is a type of reproductive system in which the embryo utilizes a maternal nutrient supply until birth. Viviparous species occur in many taxa including bony fish. The mother fish raises her offspring in the ovarian lumen or oviduct during gestation. Embryos of the viviparous bony fish Xenotoca eiseni (family Goodeidae) utilize nutrients secreted into the ovarian lumen. However, the source of the maternal nutrients and their mother-to-embryo transport have not been experimentally demonstrated. In this study, we focus on the yolk nutrient protein vitellogenin (Vtg) as a matrotrophic factor. Our results are of fundamental importance to the investigation of viviparous systems in teleosts and in other vertebrates and invertebrates.

Vitellogenin (Vtg), a yolk nutrient protein that is synthesized in the livers of female animals, and subsequently carried into the ovary, contributes to vitellogenesis in oviparous animals. Thus, Vtg levels are elevated during oogenesis. In contrast, Vtg have been genetically lost in viviparous mammals, thus the yolk protein is not involved in their oogenesis and embryonic development. In this study, we identified Vtg protein in the livers of females during the gestation of the viviparous teleost, Xenotoca eiseni.Although vitellogenesis is arrested during gestation, biochemical assays revealed that Vtg protein was present in ovarian tissues and lumen fluid. The Vtg protein was also detected in the trophotaenia of the intraovarian embryo. Immunoelectron microscopy revealed that Vtg protein is absorbed into intracellular vesicles in the epithelial cells of the trophotaenia. Furthermore, extraneous Vtg protein injected into the abdominal cavity of a pregnant female was subsequently detected in the trophotaenia of the intraovarian embryo. Our data suggest that the yolk protein is one of the matrotrophic factors supplied from the mother to the intraovarian embryo during gestation in X. eiseni. To our knowledge, this is the first report of the experimental verification of mother-to-embryo substance transport in a viviparous teleost.Breeding and experiments were performed in the laboratory of Dr. Atsuko Sehara-Fujisawa. We thank Yoshiko Aihara for helpful discussion. Ryo Kurokawa helped with the sample collection. This work was supported by crowdfunding via The Academist, Inc. The crowdfunding investors for this project were