Introduction

The order Primates is one of the most speciose placental orders. According to the tabulation of the living mammalian species by Wilson and Reeder (1993), there are 233 primate species. Only four other orders: Rodentia, Chiroptera, Carnivora, and Eulipotyphla consist of more species than primates. There are also roughly twice as many fossil species of primates. Therefore, a large number of speciation events occurred during the course of primate evolution since the initial radiation of plesiadapiforms in the Paleocene (Fleagle, 1999). Primates exhibit a diverse array of evolutionary tempos. For instance, the genera Aotus, Tarsius, and Macaca seem to have stayed morphologically the same for tens of millions of years, whereas some genera show remarkable rates of evolution within a relatively short time period (Fleagle, 1999). A good

Soojin Yi and Wen-Hsiung Li • Department of Ecology and Evolution, University of Chicago, Chicago, IL 60637

example of the latter situation is Homo—the very genus that includes our own species. Currently, primates occupy many different types of habitats and show a great diversity in their adaptive traits such as behavior, diet, and locomotion (Fleagle, 1999). Understanding the bases of such adaptive evolution is one of the ultimate goals of the study of evolution.

Protein hormones might have played an important role in primate adaptation because of their essential role in physiology. In the last several decades, much data has accumulated on the structure and function of protein hormones. In parallel to this, the molecular evolutionary patterns of some protein hormones have been investigated in depth, especially in primates, due to their implications for medicine. This chapter focuses on several cases of rapid evolution in protein hormones that might have caused significant physiological changes in some primate lineages.

The study of physiological mechanisms that are controlled by protein hormones has a long history. Early development of this field depended heavily on animal models. For example, in the 1920s, the discovery of insulin was largely based on studies with dogs. From the 1930s to the present, all hormonal substitution therapies have benefited from pharmacological trials on a variety of mammalian species. For some hormone therapies, purified animal products, such as porcine insulin, have been the choice before the advent of genetic engineering (for references, see De Pablo, 1993). In view of the fact that nonprimate hormones usually worked on humans, it was not surprising to find that the amino acid sequences of hormone proteins have been well conserved among species (see Li, 1997). However, when the molecular evolutionary features of some protein hormones were investigated in detail, many cases of "episodic" evolution were found. Episodic evolution refers to the situation in which the rate of evolution of a biomolecule changes dramatically in a short time period (Li, 1997; Wallis, 1994, 1996). It has been shown, and is described in a later section, that such dramatic acceleration is usually confined to a few lineages, while the "basal" rate of evolution in the majority of lineages remains approximately constant. Episodic evolution is often considered to be the signature of adaptive evolution. However, it may also occur by relaxation of functional constraints; so determining the cause of an episodic event can be difficult. Interestingly, to date some of the best-characterized examples of episodic evolution occurred in primate lineages. In this chapter, we describe some of the best-studied cases.

To determine whether some molecular changes in evolution are due to positive selection requires some statistical methods. In this chapter, we first explain some current statistical tools for this purpose. Then, we use the example of the molecular evolution of lysozyme in some primate lineages to demonstrate one such method. The reason for using lysozyme is twofold. First, even though lysozyme itself is not a protein hormone, it is also a secretive protein. Second, some molecular changes of this protein have been linked to the adaptive evolution of a physiological trait in some primate lineages (Stewart and Wilson, 1987). This example will help readers understand how positive selection of amino acid substitutions can be investigated using statistical tools. We then describe in detail the molecular evolution of growth hormone (GH) and growth hormone receptor (GHR) in primates. The GH was the first protein hormone noticed to exhibit an episodic mode of molecular evolution (Wallis, 1994). This was possible mostly because of the abundant data on this protein, reflecting the long interests of both evolutionary biologists and biochemists. Together GH and GHR provide a rare opportunity to investigate the functional basis of molecular evolutionary changes underlying the coevolution of two proteins.

The GH in higher primates demonstrates another means of adaptive evolution, namely, gene duplication. A gene duplication initially increases the protein production, but later, the two genes may diverge in tissue expression and become specialized in different tissues. As will be described, the GH gene has been duplicated to multiple copies in higher primates. While one copy is still expressed only in the pituitary, the other copies are expressed in a new tissue, the placenta.

Next, we describe the evolution of Chorionic Gonadotropin (CG) in primates. The CG is a member of a tightly regulated network of reproductive hormones that establish and maintain pregnancy. The CG hormone arose from a gene duplication but has acquired the specialized role of keeping the pregnancy immediately after fertilization. Therefore, it is another example for the evolutionary significance of gene duplication. The usage of CG for this purpose, as well as the presence of this hormone itself, is confined to some lineages of primates. We will examine the evolution of CG in these lineages. This example shows two essential steps in the evolution of a new protein hormone following a gene duplication: first, a new expression pattern is established; and second, a novel protein coding sequence evolves by adaptive evolution.

Finally, we describe the episodic modes of molecular evolution of several protein hormones in other mammalian species. This section provides a glimpse of the extent of the phenomenon of episodic molecular evolution of hormones in mammals.

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