© 1983 by The Society for Integrative and Comparative Biology
Evolutionary Divergence in the Structure and Function of Pituitary Gonadotropins of Tetrapod Vertebrates1
Department of Zoology, University of California Berkeley, California 94720
Pituitary gonadotropins and thyrotropins have been purified from diverse tetrapod species, representing all four classes. There has been sufficient conservation of structure in these hormones to allow recognition of a separate (homologous) follicle-stimulating hormone, luteinizing hormone and thyrotropin in each class. Thus, the dual gonadotropin system appears to be a common (primitive) feature of the tetrapods. However, variations in the nature of the hormones exist at several levels. Intra-individual heterogeneity in each type of hormone may have important physiological consequences, especially for clearance rates. In at least one group, the squamate reptiles, the gonadotropins may have diverged from the basic tetrapod condition; they possess a relatively unique, and possibly only a single, gonadotropin. Among other species having two gonadotropins, interspecific divergence in the "active sites" of each hormone is evident from the marked species specificity in hormone action. Also, despite considerable conservation in structure, pronounced divergence in the function of each molecule is indicated by interspecific variations in hormonal specificity of certain functions, such as steroid secretion. Such changes in function may be due in large part to evolution in the receptor sites. However, tests with heterologous hormones suggest that there has also been some convergence between the active sites of the different types of glycoprotein hormone: one type of hormone may mimic the action of another when tested in a heterologous species Generalizations about evolution in gonadotropin structure or function should not be made from a limited representation of species, since even related species of hormone (same order or family) may show distinctive properties, and physiological studies based on heterologous hormones must be interpreted cautiously. Surprisingly, neither species or hormonal specificity of the gonadotropins show clear phylogenetic patterns.