Turk Rhen

PHENOTYPIC PLASTICITY FOR GROWTH IN THE COMMON SNAPPING TURTLE: EFFECTS OF INCUBATION TEMPERATURE, CLUTCH, AND THEIR INTERACTION

We examined a critical component of the Charnov-Bull hypothesis of temperature-dependent sex determination (TSD) by determining the reaction norms of hatchling growth to embryonic incubation temperature in the common snapping turtle, Chelydra serpentina Hormone manipulations of eggs produced females at male temperatures and vice versa, which thereby permitted same-sex comparisons of hatchling growth across a range of incubation temperatures. In this way, the normally confounded effects of incubation temperature and sex were dissociated experimentally. The resultant hatchlings, including controls and experimentals, exhibited normal gonadal structure and sex steroid profiles. The subsequent growth of hatchlings monitored for 6 mo was strongly affected by embryonic incubation temperature but not by sex As predicted, growth was enhanced at incubation temperatures that produced males. Clutch effects and interaction effects (clutch by incubation temperature) on growth were significant. In addition, there was a positive genetic covariance among incubation temperatures, but incubation temperature effects varied among clutches. The variation in growth plasticity among clutches was consistent with Charnov-Bull predictions. In this TSD species, incubation temperature is likely to have differential fitness effects on the sexes mediated via differences in growth.

AMONG-FAMILY VARIATION FOR ENVIRONMENTAL SEX DETERMINATION IN REPTILES

Unlike birds and mammals, in many reptiles the temperature experienced by a developing embryo determines its gonadal sex. To understand how temperature‐dependent sex determination (TSD) evolves, we must first determine the nature of genetic variation for sex ratio. Here, we analyze among‐family variation for sex ratio in three TSD species: the American alligator (Alligator mississipiensis), the common snapping turtle (Chelydra serpentina) and the painted turtle (Chrysemys picta). Significant family effects and significant temperature effects were detected in all three species. In addition, family‐by‐temperature interactions were evident in the alligator and the snapping turtle, but not in the painted turtle. Overall, the among‐family variation detected in this study indicates potential for sex‐ratio evolution in at least three reptiles with TSD. Consequently, climate change scenarios that are posited on the presumption that sex‐ratio evolution in TSD reptiles is genetically constrained may require reevaluation.

Expression of putative sex-determining genes during the thermosensitive period of gonad development in the snapping turtle, Chelydra serpentina.

Modes of sex determination are quite variable in vertebrates. The developmental decision to form a testis or an ovary can be influenced by one gene, several genes, environmental variables, or a combination of these factors. Nevertheless, certain morphogenetic aspects of sex determination appear to be conserved in amniotes. Here we clone fragments of nine candidate sex-determining genes from the snapping turtle Chelydra serpentina, a species with temperature-dependent sex determination (TSD). We then analyze expression of these genes during the thermosensitive period of gonad development. In particular, we compare gene expression profiles in gonads from embryos incubated at a male-producing temperature to those from embryos at a female-producing temperature. Expression of Dmrt1 and Sox9 mRNA increased gradually at the male-producing temperature, but was suppressed at the female-producing temperature. This finding suggests that Dmrt1 and Sox9 play a role in testis development. In contrast, expression of aromatase, androgen receptor (Ar), and Foxl2 mRNA was constant at the male-producing temperature, but increased several-fold in embryos at the female-producing temperature. Aromatase, Ar, and Foxl2 may therefore play a role in ovary development. In addition, there was a small temperature effect on ERα expression with lower mRNA levels found in embryos at the female-producing temperature. Finally, Dax1, Fgf9, and SF-1 were not differentially expressed during the sex-determining period, suggesting these genes are not involved in sex determination in the snapping turtle. Comparison of gene expression profiles among amniotes indicates that Dmrt1 and Sox9 are part of a core testis-determining pathway and that Ar, aromatase, ERα, and Foxl2 are part of a core ovary-determining pathway.

Molecular Mechanisms of Sex Determination in Reptiles

Charles Darwin first provided a lucid explanation of how gender differences evolve nearly 140 years ago. Yet, a disconnect remains between his theory of sexual selection and the mechanisms that underlie the development of males and females. In particular, comparisons between representatives of different phyla (i.e., flies and mice) reveal distinct genetic mechanisms for sexual differentiation. Such differences are hard to comprehend unless we study organisms that bridge the phylogenetic gap. Analysis of variation within monophyletic groups (i.e., amniotes) is just as important if we hope to elucidate the evolution of mechanisms underlying sexual differentiation. Here we review the molecular, cellular, morphological, and physiological changes associated with sex determination in reptiles. Most research on the molecular biology of sex determination in reptiles describes expression patterns for orthologs of mammalian sex-determining genes. Many of these genes have evolutionarily conserved expression profiles (i.e., DMRT1 and SOX9 are expressed at a higher level in developing testes vs. developing ovaries in all species), which suggests functional conservation. However, expression profiling alone does not test gene function and will not identify novel sex-determining genes or gene interactions. For that reason, we provide a prospectus on various techniques that promise to reveal new sex-determining genes and regulatory interactions among these genes. We offer specific examples of novel candidate genes and a new signaling pathway in support of these techniques.

SEX-LIMITED MUTATIONS AND THE EVOLUTION OF SEXUAL DIMORPHISM

Abstract.— Although the developmental and genetic mechanisms underlying sex differences are being elucidated in great detail in a number of species, there remains a breach between proximate and evolutionary studies of sexual dimorphism. More precisely, the evolution of sex‐limited gene expression at autosomal loci has not been well reasoned using either theoretical or empirical methods. Here, I show that a Mendelian genetic model including elementary details of sexual differentiation provides novel insight into the evolution of sex differences via sex limitation. This model indicates that the nature of allelic effects and the pattern of selection must be known in both sexes to predict the evolution of sex differences. That is, selection interacts with genetic variation for sexual dimorphism to produce unanticipated patterns of trait divergence or convergence between the sexes. Ultimately, this model may explain why previous models for the evolution of sexual dimorphism do not predict the erratic behavior of the sex difference during artificial selection experiments.

Embryonic Temperature and Gonadal Sex Organize Male-Typical Sexual and Aggressive Behavior in a Lizard with Temperature-Dependent Sex Determination*

Temperature during embryonic development determines gonadal sex in the leopard gecko, Eublepharis macularius. Moreover, both embryonic temperature and gonadal sex influence adult behavior. Yet it remains unclear whether the effects of embryonic temperature and gonadal sex on behavior are irreversibly organized during development. To address this question, we gonadectomized adult females and males generated from a temperature that produces mostly females (30 C) and a temperature that produces mostly males (32.5 C). Females and males from both temperatures were then treated with equivalent levels of various sex steroids. We found that both embryonic temperature and gonadal sex had persistent effects on the expression of male-typical sexual and aggressive behaviors. For example, adult females do not scent mark and display very little courtship and mounting behavior even when treated with levels of hormones (primarily androgens) that activate these behaviors in males. In contrast, species-typical aggressive displays were less sex specific and were activated by both dihydrotestosterone and testosterone (T) in males and by T in females. Nevertheless, the average duration of aggressive displays was significantly shorter in T-treated females than that in T-treated males. With regard to submissive behavior, androgens decreased flight behavior in males, but had no effect in females. Embryonic temperature had enduring effects on certain behaviors in males. For instance, males from a male-biased embryonic temperature scent-marked more than males from a female-biased embryonic temperature when treated with dihydrotestosterone or T. Conversely, and across hormone treatments, males from a female-biased embryonic temperature mounted more than males from a male-biased embryonic temperature. Finally, treatment with 17b-estradiol decreased submissive behavior in males from a male-biased embryonic temperature compared with that in males from a female-biased embryonic temperature. Courtship and aggressive behavior were not influenced by temperature. These results strongly suggest that male-typical behaviors in the adult leopard gecko are permanently organized by both embryonic temperature and gonadal sex during development. (Endocrinology 140: 4501‐ 4508, 1999)

Developmental effects on intersexual and intrasexual variation in growth and reproduction in a lizard with temperature-dependent sex determination

The mechanisms that control growth and reproduction have received considerable attention by molecular and cellular endocrinologists, yet there has been relatively little effort to link these two aspects of physiology. On the other hand, evolutionary biologists have long commented on the relationship between growth and reproduction in many species, yet have generally neglected the mechanisms underlying such complex traits. An approach that integrates the multiple proximate levels promises to provide significant insight into the evolution of neuroendocrine control mechanisms. In this chapter, we take this approach in reviewing environmental influences on growth and reproduction in the leopard gecko, Eublepharis macularius. In this species, incubation temperature during embryonic development not only determines gonadal sex, but also underlies within-sex differences in growth, adult morphology, aggressiveness, reproductive physiology and behaviour, and brain organization. Thus, the leopard gecko is an excellent model to elucidate the developmental interactions among the environment and the endocrine and nervous systems that control growth and reproduction.

Organization and Activation of Sexual and Agonistic Behavior in theLeopard Gecko, Eublepharis macularius

Gonadal sex is determined by the temperature experienced during incubation in the leopard gecko (Eublepharis macularius). Furthermore, both factors, incubation temperature and gonadal sex, influence adult sexual and agonistic behavior in this species. Yet it is unclear whether such differences in behavior are irreversibly organized during development or are mediated by differences in hormone levels in adulthood. To address this question, we gonadectomized adult females and males generated from a female-biased (30°C) and a male-biased (32.5°C) incubation temperature and treated them with equivalent levels of various sex steroids. We found that 17β-estradiol (E2) activated sexual receptivity in females but not males, suggesting an organized sex difference in behavioral sensitivity to E2. There were also organized and activated sex differences in attractivity to stimulus males. Although females were more attractive than males when treated with E2, both sexes were equally unattractive when treated with dihydrotestosterone (DHT) or testosterone (T). Likewise, sex differences in aggressive and submissive behavior were organized and activated. Attacks on stimulus males were activated by T in males but not in females. In contrast, hormones did not influence flight behavior in males but did affect female submissiveness. Overall, males also evoked more attacks by stimulus males than did females. Nevertheless, females and males treated with androgens evoked more attacks than animals of the same sex that were treated with cholesterol or E2. Incubation temperature had some weak effects on certain behaviors and no effect on others. This suggests that temperature effects in gonadally intact geckos may be due primarily to differences in circulating levels of hormones in adulthood. We conclude that gonadal sex has both organizational and activational effects on various behaviors in the leopard gecko.

The oxysterol 27-hydroxycholesterol regulates α-synuclein and tyrosine hydroxylase expression levels in human neuroblastoma cells through modulation of liver X receptors and estrogen receptors–relevance to Parkinson’s disease

J. Neurochem. (2011) 119, 1119–1136.

Distribution of androgen and estrogen receptor mRNA in the brain and reproductive tissues of the leopard gecko, Eublepharis macularius

Incubation temperature during embryonic development determines gonadal sex in the leopard gecko, Eublepharis macularius. In addition, both incubation temperature and gonadal sex influence behavioral responses to androgen and estrogen treatments in adulthood. Although these findings suggest that temperature and sex steroids act upon a common neural substrate to influence behavior, it is unclear where temperature and hormone effects are integrated. To begin to address this question, we identified areas of the leopard gecko brain that express androgen receptor (AR) and estrogen receptor (ER) mRNA. We gonadectomized adult female and male geckos from an incubation temperature that produces a female‐biased sex ratio and another temperature that produces a male‐biased sex ratio. Females and males from both temperatures were then treated with equivalent levels of various sex steroids. Region‐specific patterns of AR mRNA expression and ER mRNA expression were observed upon hybridization of radiolabeled (35S) cRNA probes to thin sections of reproductive tissues (male hemipenes and female oviduct) and brain. Labeling for AR mRNA was very intense in the epithelium, but not within the body, of the male hemipenes. In contrast, expression of ER mRNA was prominent in most of the oviduct but not in the luminal epithelium. Within the brain, labeling for AR mRNA was conspicuous in the anterior olfactory nucleus, the lateral septum, the medial preoptic area, the periventricular preoptic area, the external nucleus of the amygdala, the anterior hypothalamus, the ventromedial hypothalamus, the premammillary nucleus, and the caudal portion of the periventricular nucleus of the hypothalamus. Expression of ER mRNA was sparse in the septum and was prominent in the ventromedial hypothalamus, the caudal portion of the periventricular nucleus of the hypothalamus, and a group of cells near the torus semicircularis. Many of these brain regions have been implicated in the regulation of hormone‐dependent, sex‐typical reproductive and agonistic behaviors in other vertebrates. Consequently, these nuclei are likely to control such behaviors in the leopard gecko and also are candidate neural substrates for mediating temperature effects on behavior. J. Comp. Neurol. 437:385–397, 2001.