Meeting Abstract
88.4 Friday, Jan. 7 Calcium Placentotrophy in a Lizard with Prolonged Uterine Egg Retention STEWART, J.R.*; ECAY, T.W.; THOMPSON, M.B.; East TN State Univ, Johnson City; East TN State Univ, Johnson City; Univ Sydney stewarjr@etsu.edu
The eggshell of oviparous lizards consists of a thick matrix of organic fibers, overlain by calcium carbonate, secreted by the oviduct soon after ovulation. Variation in eggshell thickness reflects a tradeoff between factors promoting protection of the embryo and those providing exchange with the environment. Thus, evolution of prolonged uterine egg retention, which results in reduction in the efficiency of gas exchange, should favor reduction in eggshell thickness. Consequent loss of the calcium layer deprives the embryo of an important source of nutrition. Saiphos equalis is an Australian scincid lizard with geographic variation in length of uterine egg retention. We compared structure and histochemistry of the uterus and eggshell and calcium content of egg compartments of two populations to test two hypotheses: 1) eggshell thickness is inversely correlated with length of uterine egg retention and 2) eggshell thickness is positively correlated with size of uterine shell glands. We found support for the first hypothesis but we did not find a correlation between size of uterine shell glands and eggshell thickness. Eggshell calcium content was correlated with eggshell thickness and thus was lower in eggs retained for longer intervals. A surprising finding was that prolonged uterine calcium secretion occurred in both populations. Variation in eggshell structure in this species is consistent with the hypothesis that selection promotes thinner eggshells in association with uterine egg retention but also reveals a novel relationship between uterine calcium secretion and egg retention. Placental calcium transport reduces the nutritional cost of loss of eggshell calcium and reveals a mechanism for the concomitant evolution of reproductive mode and calcium placentotrophy.
