DEMARAIS, A.A.*; OLDIS, D.L; Univ. of Puget Sound; Univ. of Puget Sound: Visualization of Mother-to-Embryo Transfer in Livebearing Fishes

Fishes of the family Poeciliidae are livebearers: eggs are retained in the ovary where they are fertilized and embryonic development ensues. At parturition, larval fish are released. Although the eggs of most Poeciliid fishes contain large yolk stores, additional maternal transfer of nutrients to the embryos (matrotrophy) occurs in some species. The degree of matrotrophy varies widely among species. In some, matrotrophy is required (obligate matrotrophs) whereas others show little to no matrotophy (lecithotrophs). Other species appear to vary the degree of nutrient contribution to their embryos (facultative matrotrophs). We have begun to investigate the mechanism(s) of matrotrophic transfer in the ovaries of four Poeciliid fishes: Heterandria formosa, Poecilia reticulata, Gambusia affinis, and Gambusia geiseri. We have developed a protocol for the direct visualization of matrotrophic transfer using small, fluorescent microspheres (FluoSpheres^®; Molecular Probes, Eugene, OR). We inject microspheres (40 nm) intramuscularly into the caudal peduncle of pregnant females. After an 8 to 12 hour incubation period, the embryos are removed, fixed, and visualized using a Leica confocal microscope. The degree of fluorescent labeling of embryos parallels the matrotrophic strategy predicted for each species: microsphere transfer is seen in all embryos of each brood and at all stages of development in H. formosa (an obligate matrotroph). Reduced microsphere transfer is observed in embryos of the lecithotrophic P. reticulata. Variable degrees of microsphere transfer are seen in embryos of G. affinis and G. geiseri, species that are facultative matrotrophs; transfer variability is seen both within broods and at different stages of embryonic development.