Sex Determination in Reptiles
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Chapter 1 Sex Determination in Reptiles Daniel A. Warner Iowa State University, Ames, IA, USA of most organisms is determined by genetic factors located SUMMARY on sex-specific chromosomes. The sex-determining mechanisms (SDMs) of reptiles are remarkably diverse, ranging from systems that are under complete Research has since revealed far greater diversity in sex- genetic control to those that are highly dependent upon temper- determining mechanisms (SDMs). Karyological studies atures that embryos experience during development. Because have revealed a variety of sex-specific chromosomal reptiles exhibit a remarkable diversity of SDMs, this group arrangements (e.g., male vs. female heterogamety (Mittwoch, provides excellent models for addressing critical questions about 1996)). In many species, heteromorphic sex chromosomes the proximate mechanisms of sex determination, as well as its do not exist, but instead sex-determining factors lie on ecology and evolution. The goal of this chapter is to integrate autosomes. In other organisms, sex is determined by the studies from different disciplines (e.g., ecology, evolution, phys- ratio of X chromosomes to autosomes or the ploidy level iology, molecular biology) to broadly summarize the current of the zygote (Cook, 2002). During the latter half of the understanding of reptilian sex determination. This chapter is 20th century, the ubiquity of these genetic systems was divided into six topics that cover the (1) diversity of reptilian challenged by studies that demonstrate a role of environ- SDMs, (2) taxonomic distribution of SDMs, (3) molecular and physiological mechanisms underlying sex determination, mental factors in the sex-determination process. Indeed, in (4) timing of sex determination during embryogenesis, (5) many species, environmental conditions experienced ecology and evolution of reptilian SDMs, and (6) current gaps in during embryogenesis (rather than genotypic factors) our understanding of this field and where future research should be trigger the developmental cascade that eventually leads to directed. the male or female phenotype. This environmental sex determination (ESD) has since been shown to exhibit remarkable diversity in itself (Bull, 1983). It is now well established that SDMs range from those under complete 1. INTRODUCTION genetic control to those that are highly dependent upon In sexually reproducing species, the sex of an individual is environmental parameters. arguably one of the most important aspects of its pheno- The diversity in SDMs is intriguing since the division type. Whether an embryo develops as a male or a female of the sexes is so similar throughout most animals. Indeed, will have profound consequences on its life history, without knowledge of such diversity, one might expect behavior, physiology, morphology, and ultimately its that a single mechanism would have been stabilized at an fitness. Indeed, differences between the sexes are among early stage of evolution. Why, then, have so many the most spectacular sources of phenotypic variation within different mechanisms evolved as a means to produce populations. It is not surprising, then, that factors governing males and females? How do the proximate mechanisms of sexual development are of primary interest to scientists and different SDMs vary? Which SDMs are ancestral and how have been debated for over three millennia. Over this have new mechanisms arisen? Answers to these and many expanse of time, our understanding of sex determination related questions are continuously sought after by biolo- has transformed from mythological speculations to expla- gists in many disciplines. The ideal approach for tackling nations based on hard-won scientific evidence (reviewed in such questions requires an integrative examination of Mittwoch, 2000). The end of the 19th century marked closely related organisms that vary in their SDMs. The a critical point at which advances in cytogenetics enabled class Reptilia is a group that satisfies this requirement. the discovery of sex chromosomes and their relationship to Reptiles show spectacular diversity in SDMs, ranging sex determination (reviewed in Brown, 2003). After this from systems that have a strong genetic basis to those that discovery, it was widely accepted that the sexual phenotype are under almost complete environmental influence Hormones and Reproduction of Vertebrates, Volume 3dReptiles 1 Copyright Ó 2011 Elsevier Inc. All rights reserved. 2 Hormones and Reproduction of Vertebrates (Bull, 1980). In many cases, closely related species differ 2. WHAT MECHANISMS OCCUR in fundamental aspects of their SDMs, making these taxa IN REPTILES? DIVERSITY OF well-suited for comparative analyses. Many characteristics SEX-DETERMINING MECHANISMS (SDMs) of reptiles make them especially amenable to experimental manipulation, which facilitates tests of theoretical AND PATTERNS predictions about the evolution of alternative SDMs Reptilian SDMs are traditionally placed into two main (Janzen & Paukstis, 1991a). Not surprisingly, reptiles have categories: one in which sex is determined solely by genetic received considerable research attention, and have factors (i.e., GSD) and the other a form of ESD in which provided critical insights toward our understanding of sex sex is determined primarily by the temperature that determination. Our understanding, however, is far from embryos experience during development (i.e., TSD). complete, and reptiles will undoubtedly continue to serve Intriguingly, within both of these sex-determining systems, as excellent models for addressing fundamental issues in we see remarkable diversity in patterns. For example, under this field. GSD, both male and female heterogamety evolved inde- The primary objective of this chapter is to provide an pendently multiple times within reptiles. Under TSD, overview of our current understanding of sex determina- shapes of reaction norms that describe the relationship tion in reptiles. Given that the vast literature on this topic between developmental temperature and sex determination spans several decades, specific details on all aspects of vary considerably among taxa (Figure 1.1). Moreover, such a broad topic cannot be covered in a single review. evidence is accumulating that elements of both TSD and The author’s hope is that this review will provide a sense GSD may co-occur in some species, even within single of the current state of the field and a framework that will populations (e.g., Shine, Elphick, & Donnellan, 2002). In guide research in specific directions that warrant further this section, the current knowledge on the diversity of sex- study. An additional goal of this review is to provide an determining patterns of reptiles is reviewed. To ease awareness of different perspectives on reptilian sex discussion, GSD and TSD are treated independently in determination (e.g., viewing SDMs as a dichotomy vs. many parts of this review, but recent arguments suggest that a continuum), as well as to identify gaps in our knowl- this dichotomy may oversimplify the complexity of SDMs edge and finally to provide suggestions for future and that TSD and GSD may not be alternative mechanisms research. but instead represent endpoints of a continuum (Sarre, This chapter is divided into six major sections that Georges, & Quinn, 2004). focus on the ‘what,’ ‘who,’ ‘how,’ ‘when,’ ‘why,’ and ‘where’ questions of reptilian sex determination. The first section begins by introducing the diversity of SDMs 2.1. Patterns of Genotypic Sex Determination (the ‘what’ question) found within reptiles by detailing the Genotypic sex determination is a system in which offspring specifics of genotypic sex determination (GSD) and the sex is irreversibly fixed by its own (or its parent’s) genotype variations in reaction norms that describe temperature- (Bull, 1980; Janzen & Paukstis, 1991a). That is, genetic dependent sex determination (TSD). Secondly, a brief factors inherited from the parents determine the sex of the overview of the taxonomic and phylogenetic distribution of offspring. These genetic factors may reside on sex chro- alternative SDMs (the ‘who’ question) is provided, and the mosomes, which differ from autosomes in their size, evolutionary transitions between systems discussed. The number, and gene content, and are elements of the genome third section addresses the proximate mechanisms involved that segregate during meiosis. Importantly, sex-determining in reptilian sex determination (the ‘how’ question). This genes located on sex chromosomes direct the pathways that section discusses the molecular and hormonal aspects of lead to male or female development. The two most sex determination and evaluates commonalities and common types of GSD are male and female heterogamety. differences among mechanisms. The fourth section Under male heterogamety, offspring that inherit the explores the timing of sexual lability during embryonic Y chromosome from the father develop into males (XY) development (the ‘when’ question), and compares different and those that inherit the father’s X chromosome develop methods for addressing this issue. The fifth section into females (XX). In many species, this system is reversed explores the ecology and evolution of reptile SDMs (the and females are the heterogametic sex; sex chromosomes in ‘why’ question). This section establishes the existence of this system are referred to as Z and W (i.e., males are ZZ, TSD in nature and evaluates hypotheses of adaptive and females ZW). significance and micro-evolutionary potential