Evolutionary and Functional Aspects of Pituitary Gonadotropins in the Green Turtle,CheloniaMydas

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Abstract
Information on the pituitary gonadotropins in Chelonia mydas represents some of the most complete data available for any reptile and thus provides an important basis for evaluating evolutionary processes in tetrapod endocrine physiology. The two gonadotropins isolated from Chelonia pituitary glands show clear chemical and immunological homologies to mammalian follicle-stimulating hormone (FSH) and luteinizing hormone (LH). However, receptor studies and biological tests indicate that the functions of these hormones may be different in turtles and mammals. In particular, Chelonia LH shows an unusual ability to interact with FSH receptor sites and to stimulate physiological functions normally attributed to FSH. Results with Chelonia LH demonstrate that errors may arise from using mammalian hormones to investigate reproduction in turtles. Measurements of endogenous gonadotropin levels in the plasma of breeding and nesting Chelonia provide a different perspective of the potential roles of the FSH and LH from that obtained in physiological tests. In particular, FSH and LH secretion are markedly dissociated during the nesting cycle; FSH has only a transient peak during oviposition, whereas LH, along with progesterone, displays a pronounced “ovulatory” surge in the day following nesting. Preliminary studies with synthetic gonadotropic releasing factors in Chelonia suggest that these may be useful in inducing reproductive changes.

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