Evolution of Placental Calcium Transport in Reptiles

East Tennessee State University, Johnson City TN

Investigators

Abstract

Terrestrial vertebrates exhibit two basic modes of reproduction; viviparity, or the birth of live young, and oviparity, or the laying of eggs in which embryonic development occurs outside the body of the female. All mammals, with the exception of monotremes, are viviparous. Turtles, crocodilians and birds are exclusively oviparous. In contrast, squamate reptiles (snakes and lizards) show a diversity of reproductive patterns with approximately 20% being viviparous and at least three species being reproductively bimodal. Viviparity in squamates has evolved independently more than 100 times and the evolution of viviparity is associated with a transition in the pattern of embryonic nutrition from fortification of energy stores in the egg to placental nutrient exchange. All vertebrate embryos require calcium for successful development. The source and pattern of calcium provision to embryos of oviparous species differs from that of viviparous species. Embryos of oviparous species derive calcium from yolk and eggshell, sources laid down during vitellogenesis and shelling prior to egglaying. Viviparous embryos derive some calcium from yolk but depend also on placental transfer late in development. Does the timing, source and mechanism of calcium transport to embryos influence the potential for evolutionary innovations leading to viviparity? This project will explore the relationship between the pattern of embryonic calcium provision and reproductive mode (egg-laying or live-bearing) in a lineage (squamate reptiles) that exhibits the highest diversity of reproductive patterns among terrestrial vertebrates. The pattern and mechanism of calcium provision to embryos of selected oviparous and viviparous squamates will be compared to develop a model for the evolution of placental calcium transport. The project will compare the timing and mechanism of calcium secretion by the uterus and the timing and mechanism of calcium mobilization by embryos of: 1) two species that have recently evolved viviparity, 2) a viviparous species with little placental calcium transport, and 3) a viviparous species that is heavily dependent on placental calcium. Egg-laying species that are closely related will be studied for comparison. Atomic absorption spectroscopy will be used to define the pattern of calcium uptake by embryos. The timing and mechanism of calcium transport in the uterus and embryonic membranes will be studied by defining the developmental expression of the calcium transporting proteins, calbindin-D28K, Ca2+-ATPase, using immunoblotting and immunohistochemistry. This research will contribute a greater understanding of embryonic development of terrestrial vertebrates through study of a highly conserved mechanism for delivery of nutrients to embryos. The project will integrate research and education by providing opportunities for undergraduate students from predominantly undergraduate departments in the research environment of a Ph.D. granting department. Undergraduate students from underrepresented segments of society will be actively recruited through collaboration with administrators of the Ronald E. McNair Scholars Program on campus. These efforts will target any eligible undergraduate from any institution. These students will receive training with an aim to increase their potential for future success in graduate programs.