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D Crews

Publications and source records attributed to D Crews.

At least 19 recordsLinked to original sources

Sexual dimorphisms in the soma size of neurons in the brain of whiptail lizards (Cnemidophorus species).

Soma area was significantly larger in the anterior hypothalamus-preoptic area of male than female Cnemidophorus inornatus, and it was significantly larger in females than males in the ventromedial hypothalamus. These results parallel those on the volume of the brain areas in these animals, and therefore probably explain at least part of the dimorphisms seen in this sexually reproducing species. Soma size in parthenogenetic C. uniparens also parallels volume.

Animals

Steroid hormone-induced male sex determination in an amniotic vertebrate.

In many reptiles, sex is determined by the temperature at which the eggs are incubated (i.e., temperature-dependent sex determination, or TSD). Past studies have shown that exogenous steroid hormones can override the effects of temperature and induce female sex determination. However, past attempts to induce male sex determination have consistently failed. In the present study, sex determination was studied in a turtle with TSD. By utilizing an incubation temperature regimen that resulted in approximately a 1:1 sex ratio in the control group, sex determination was shown to be sensitive to both exogenous androgen and estrogen treatments: androgen induced the production of male hatchlings, whereas estrogen induced the production of female hatchlings. This is the first report of an amniotic vertebrate in which an exogenous steroid hormone induces male sex determination.

Animals

Interactions between progesterone and androgens in the stimulation of sex behaviors in male little striped whiptail lizards, Cnemidophorus inornatus.

Progesterone is believed to have a suppressive or inhibitory role in regulating androgen-dependent sex behaviors in male mammals and birds. Previous studies in this laboratory have revealed that in the little striped whiptail lizard (Cnemidophorus inornatus), progesterone (P) can stimulate sex behavior in a proportion of the males. The present study sought to determine (i) interactions between androgens and P in activating sex behaviors, and (ii) the overlap in behavioral sensitivities to androgens and P in male C. inornatus. With an increasing length of castration the behavioral sensitivity of males to exogenous P was reduced. However, priming of castrated males with subthreshold doses of exogenous dihydrotestosterone (DHT) greatly facilitated subsequent behavioral responses to exogenous P. Progesterone treatments of castrated males were more effective at reinstating sex behaviors in males that exhibited high-intensity sex behaviors prior to castration compared to males that exhibited low-intensity sex behaviors. Finally, exogenous DHT is more effective at reinstating sex behaviors in P-sensitive males than in P-insensitive males. These data are discussed in light of possible mechanisms underlying the unusual behavioral effects of P in the Cnemidophorus model system.

Androgens

Individual variation in intensity of sexual behaviors in captive male Cnemidophorus inornatus.

The present studies investigated the source of individual variation in intensity of sexual behaviors in captive male whiptail lizards, Cnemidophorus inornatus. No correlation was found between an individual's circulating concentration of dihydrotestosterone, testosterone, or corticosterone at the time of capture or in the laboratory and their level of sexual behaviors observed in the laboratory. A large percentage of males that initially exhibited low intensity courtship remained low intensity courters, although some became more reliable courters following 6 months of acclimation to the laboratory. Similarly, following castration and androgen replacement, most low intensity courters continued to exhibit weak and infrequent sexual behaviors. The data suggest that individual variation in sexual behaviors exhibited by captive male C. inornatus is not due to (i) low circulating concentrations of androgens, (ii) elevated circulating concentrations of corticosterone, or (iii) different profiles of testicular steroidogenesis. Rather, the source of differences may lie in (i) an inability to respond to androgens, (ii) an inability to exhibit sexual behavior, or (iii) non-hormonal stress related to captivity.

Animals

Hydroxysteroid dehydrogenase activity associated with sexual differentiation in embryos of the turtle Trachemys scripta.

Many turtles exhibit temperature-dependent sex determination. We have examined the hypothesis that incubation temperature causes a differential expression of steroidogenic enzymes in embryonic turtles. The activities of three steroidogenic enzymes were studied histochemically in turtle (Trachemys scripta) embryos at different developmental stages: sexually undifferentiated (Stage 15), differentiating (Stage 17), and differentiated (Stage 26; i.e., hatchling). Steroidogenic enzymes were detected in several tissues prior to, during, and after gonadal differentiation in embryos incubated at both male-producing and female-producing temperatures. In all embryos, ene-5-3 beta-hydroxysteroid dehydrogenase (HSDH) was detected only in adrenal tissue. 3 alpha-HSDH was localized in adrenal tissue, as well as in the mesonephros and liver. 17 beta-HSDH was evident in mesonephric, hepatic, and gut tissues. In hatchlings, ene-5-3 beta-HSDH and 3 alpha-HSDH were evident in the adrenal gland, whereas 3 alpha-HSDH and 17 beta-HSDH were present in the mesonephros and liver. While there was some variation in the activities of these enzymes during development, no temperature-specific pattern was apparent. At no stage were the enzymes observed in genital ridge/gonad. Our results show that T. scripta embryos possess enzymes necessary for steroid hormone synthesis. The segregated distributions of the enzymes suggest that a multi-organ regulatory system may mediate embryonic steroidogenesis. Our results do not indicate the genital ridge/gonad the principle site of steroid synthesis, although it may possess other enzymes that influence steroidogenesis.

Animals

Sites of estrogen uptake in embryonic Trachemys scripta, a turtle with temperature-dependent sex determination.

Female sex determination can be induced in embryonic red-eared slider turtles (Trachemys scripta) by exogenous estrogen, as well as by incubation at warm temperature. In the present study, estrogen target areas were identified in embryos before (stage 15), during (stage 18), and after (stage 22) the critical period for sex determination. Both hyperfilm and emulsion autoradiography were used to localize tritium accumulation after the injection of radiolabeled 17 beta-estradiol. Site-specific tritium-labelling was found at all stages, notably in the mesonephros at stage 15, in the mesonephros and oviduct at stage 18, and in the mesonephros, oviduct, and the interrenal gland at stage 22. Few if any cells in the gonad were tritium-labeled at any stage. The large number of estrogen-concentrating cells in the mesonephros and interrenal and the lack of binding to gonadal tissues indicates that estrogen action on gonadal differentiation during the period of sex determination may be indirect.

Animals

Sexual behavior and 2-deoxyglucose uptake in male red-sided garter snakes (Thamnophis sirtalis parietalis).

The [14C]2-deoxyglucose (2-DG) technique was used to study patterns of neural activity associated with the species-typical courtship behavior of male red-sided garter snakes (Thamnophis sirtalis parietalis). Males in this species court females intensely during the first month following spring emergence from their prolonged winter hibernation. Autoradiographic methods were used to measure the accumulation of radioactive label in various regions through the brains of male garter snakes that courted females, males that failed to court females, and males not exposed to females. Male garter snakes that actively courted females showed a pronounced increase in 2-DG accumulation, and therefore presumably neural activity, in the region of the anterior hypothalamus/preoptic area, relative to males that did not actively court females. Males exposed to females (regardless of whether they courted or not) showed widespread, non-specific increases in 2-DG uptake relative to males not exposed to females. The results indicate the utility of the 2-DG technique for studying complex, species-typical behaviors in vertebrates.

Animals

Specificity of steroid hormone-induced sex determination in a turtle.

The specificity of steroid hormone-induced sex determination was investigated in the red-eared slider, Trachemys scripta, a turtle with temperature-dependent sex determination. All eggs were incubated at either a female-producing temperature (31 degrees C) or a male-producing temperature (26 degrees C) and received control or experimental treatments at stage 17-18 of embryonic development. A variety of treatments induced female sex determination at the male-producing temperature. Oestradiol-17 beta, diethylstilboestrol (DES) (an oestrogen agonist) and norethindrone (NET) (a progestin with reputed oestrogenic as well as anti-oestrogenic properties) were the most effective in inducing female sex determination. Other reputed oestrogen antagonists/partial agonists (i.e. tamoxifen, nafoxidine and clomiphene citrate) were also capable of inducing female sex determination, but to a lesser extent. A high dosage of testosterone resulted in the production of some females (7 of 15 hatchlings) whereas dihydrotestosterone had no detectable effect on sex determination. This latter finding suggests that testosterone could be acting via aromatization to oestradiol-17 beta. A few females resulted from eggs that had been treated with aromatase inhibitor, 1,4,6-androstatrien-3,17-dione (ATD) (3 of 97), the antiandrogen hydroxyflutamide (1 of 55) and progesterone (3 of 36), suggesting the possibilities of non-specific effects of these compounds when used in large dosages. Alternatively, metabolites of these compounds may be oestrogenic. Collectively, the results at the male-producing temperature are consistent with the hypothesis that steroid-induced female sex determination is mediated via an oestrogen-specific receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals

Behavioural endocrinology and reproduction: an evolutionary perspective.

It is a fact that those interested in immediate causation tend to be unaware of the great advances in evolutionary biology. Similarly, most scientists interested in ecological and evolutionary questions ignore advances in neurobiology and molecular biology. Quite simply, 'reductionists see little to be gained from holistic studies, and whole organism biologists do not recognize the value of molecular analysis' (Prosser 1986). This philosophical gap and lack of communication between molecular and physiological biologists with organismal and evolutionary biologists makes it difficult to be a generalist. Yet if we are to understand past, present, and perhaps even future behaviour, we must study how the different levels of biological organization are integrated. If done with foresight, it can lead to new discoveries not only in evolution and ecology, but also in physiology and even molecular biology. One of the first things we are impressed by is the great variety of animals, particularly their behaviours and their physiologies. With so many differences, are there any generalities? With the establishment of evolutionary theory, evidence that there is 'unity in diversity' has come with discoveries of common anatomical features, the cell cycle, conservation of intermediary metabolism, and the genetic code, to name but a few. While in vertebrates there appears to be a conservation of the neural circuits underlying sex behaviour, it is still too early to state the extent to which this concept can be extended to the hormonal mechanisms underlying behaviour. This chapter has documented how some widely-held assumptions are generalities only in a very restricted sense. I have tried to show how much of our conceptual understanding of the behavioural endocrinology stems from extensive studies on relatively few species. According to (Beach 1979), there are '... two cardinal rules that should govern not only the construction of animal models for human behaviour, but for all interspecific comparisons regardless of the behaviour and the species involved. The first rule is that meaningful comparisons are based not upon the formal characteristics of behaviour, but upon its causal mechanisms and functional outcomes.... The second rule is that the validity of interspecific generalization cannot exceed the reliability of intraspecific analysis. Significant comparison of a particular type of behaviour in two different species is impossible unless and until the behaviour has been adequately analyzed in each species by itself.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals

The relationship between reproductive state and "sexually" dimorphic brain areas in sexually reproducing and parthenogenetic whiptail lizards.

The anterior hypothalamus-preoptic area and ventromedial hypothalamus are sexually dimorphic in the reproductively active whiptail lizard Cnemidophorus inornatus. The anterior hypothalamus-preoptic area, which is involved in the control of male-typical copulatory behaviors, is larger in males, whereas the ventromedial hypothalamus, which is involved in the control of female-typical receptivity, is larger in females. In the parthenogenetic whiptail lizard C. uniparens, which is a direct descendant of C. inornatus and exhibits both male-like and female-like pseudosexual behaviors, both brain areas are comparable in size to those of female C. inornatus. This study was conducted to determine whether these brain areas change in size in either species or sex during a time of year when these animals are reproductively inactive, or after removal of the gonads. In male C. inornatus both brain areas changed during reproductive inactivity (either seasonally or surgically induced) and became equivalent to the size characteristic of reproductively active female C. inornatus. When corrected for brain size, the anterior hypothalamus-preoptic area was significantly smaller in intact hibernating and castrated males than in intact males from the summer breeding season. Conversely, the ventromedial hypothalamus was significantly larger in intact hibernating and castrated males than in intact males from the summer breeding season. The two brain areas were not significantly different among the groups of female C. inornatus or parthenogenetic C. uniparens. These results suggest that 1) the brain of whiptail lizards may differentiate seasonally and 2) the female state may be a neutral one to which the male brain reverts during reproductive inactivity.

Aging

Trans-seasonal action of androgen in the control of spring courtship behavior in male red-sided garter snakes.

Gonadal steroids have traditionally been found to have short-term effects on the mating behavior of adult vertebrates, such that increased steroid hormone levels influence behavior over the course of a few days. Long-term effects also have been observed in which embryonic exposure to steroid hormone influences the sexual behavior of adults. The generality of the paradigms that have resulted from this work must be viewed with caution as all of the species studied exhibit a particular reproductive pattern. Here it is shown that in the adult red-sided garter snake, the seasonal androgen peak in the summer has a delayed action (8 months) on the male's readiness to court the next spring following emergence from hibernation. This suggests that the interval between the exposure to and the effects of steroid hormones in adult vertebrates may range from short latencies to long latencies and that the present dichotomy of organizing (permanent) versus activating (transient) effects of steroid hormones may not apply in certain instances.

Animals

Synergism between temperature and estradiol: a common pathway in turtle sex determination?

In many reptiles, the temperature at which the eggs are incubated determines the sex of the hatchlings. Administration of estradiol will counteract the masculinizing effects of a male-producing temperature, resulting in female hatchlings. To address whether temperature and estrogen are biologically equivalent, two experiments were conducted with the red-eared slider turtle, Trachemys scripta. In the first experiment, varying dosages of estrogen were administered at Stage 17 (the middle of the temperature-sensitive window) to eggs maintained at two temperatures, 26 degrees C (which normally produces all males) and 28.2 degrees C (which produces mostly males but lies at the threshold of the transition from male- to female-producing temperatures). Results indicate that estrogen and temperature exert a synergistic effect on sex determination. In the second experiment, estrogen was administered at different stages of embryonic development. The results indicate an estrogen-sensitive period ranging from Stage 14 through Stage 21, a period similar to the temperature-sensitive period for this species. The results of these experiments are consistent with the hypothesis that temperature and estradiol act in a common pathway in temperature-dependent sex determination.

Animals

Chronology and morphology of temperature-dependent sex determination.

Temperature sensitivity and gonadal differentiation were studied in a turtle, Trachemys scripta, with temperature-dependent sex determination. Sex determination was sensitive to both the duration and magnitude of incubation temperature. Temperature exerted an "all or none" effect on the ovarian or testicular nature of most gonads, but affected the length of ovaries in a graded fashion. Collectively, the results indicate that sex determination is controlled by the quantitative effect of temperature during a period beginning prior to (histologically detectable) sexual differentiation and extending to a time when initial sex specific changes are evident in the gonads.

Animals

Estrogen and sex reversal in turtles: a dose-dependent phenomenon.

Exogenous estradiol benzoate (EB) or estradiol-17 beta (E2) caused dose-dependent gonadal feminization of slider turtle (Trachemys scripta) embryos incubated at a male-producing temperature (26 degrees), suggesting that sex reversal requires a threshold dosage of these hormones. Even at dosages resulting in mixtures of males and females, nearly all hatchlings had normal-appearing ovaries or testes. Only 7 of 241 hatchlings had gonads that had not differentiated fully into ovaries or testes. Thus, with rare exception, estrogens exerted an "all or none" effect. The transport of hormone into the embryo varied with mode of administration and E2 was more effective than was EB at the lowest dosage used. These studies suggest either that exogenous estrogen is capable of coordinating cortical/medullary development in the gonad so that intersexes are prevented or that estrogen acts "upstream" of the developmental processes responsible for coordinating gonadal development.

Animals

The effects of intracranial implantation of estrogen on receptivity in sexually and asexually reproducing female whiptail lizards, Cnemidophorus inornatus and Cnemidophorus uniparens.

The ventromedial hypothalamus (VMH) is an important site in the neuroendocrine control of sexual receptivity in mammals. This study was conducted to determine if the VMH was also involved in estrogen induction of receptivity in whiptail lizards. Estradiol benzoate (EB) was implanted into the VMH of ovariectomized Cnemidophorus inornatus, a sexually reproducing species, and C. uniparens, a parthenogenetic species which displays "pseudosexual" behaviors similar to the sexual behaviors typical of both male and female C. inornatus. In both species, EB was significantly more effective in eliciting receptivity when implanted in the VMH than in other locations in the brain. These results support the idea that, as in mammals, the VMH is an important location of estrogen action in the control of receptive behaviors in both sexually and asexually reproducing whiptail lizards.

Animals

The effects of progesterone on sexual behavior in male green anole lizards (Anolis carolinensis).

It is well known that androgen-dependent sexual behaviors in male mammals and birds are inhibited by exogenous progesterone (P). However, recent research on male whiptail lizards (Cnemidophorus inornatus) indicates that P can stimulate sexual and copulatory behavior. We report here both antiandrogenic and synandrogenic actions of P on sexual behavior in males of another reptile, the green anole lizard (Anolis carolinensis). Earlier reports on birds and mammals are reviewed and discussed in relation to a possible physiological role of P in influencing sexual behavior in male vertebrates.

Animals

Concentrations of sex steroid hormones in pregnant and fetal Mongolian gerbils.

Sex steroid concentrations in the plasma of 24-day pregnant Mongolian gerbils (Meriones unguiculatus) and their male and female fetuses were measured using radioimmunoassays. It was found that, on Day 24 of gestation: (a) androgen levels were higher in those male fetuses developing adjacent to no female fetuses than in those male fetuses developing between two female fetuses and (b) androgen levels were higher in those female fetuses developing between two male fetuses than in those female fetuses with no immediate, male neighbours. Further, plasma taken from 24-day pregnant dams that had exhibited vaginal opening at a relatively early age had significantly lower androgen levels and significantly higher estradiol levels than did plasma taken from 24-day pregnant dams that had exhibited relatively late vaginal opening. The data provide direct evidence of hormonal mediation of previously described differences both in the morphology and reproductive biology of male and female adult gerbils as a function both of their fetal intrauterine locations relative to members of the other sex and of the age at vaginal introitus of their respective dams.

Animals

The effects of exogenous neuropeptide Y on feeding and sexual behavior in the red-sided garter snake (Thamnophis sirtalis parietalis).

Actively courting adult male red-sided garter snakes were injected with one of two dosages of neuropeptide Y (NPY) or saline into the third cerebral ventricle. Courtship and feeding responses were assessed prior to surgery, 4-5 h and 24 h post-surgery. Feeding behavior was tested by the presentation of a favored food, an earthworm. NPY significantly reduced courtship behavior scores at both low (0.16 nmol) and high (0.77 nmol) doses 4-5 h after surgery. Snakes also took much longer to reach the criterion courtship score at the high dose. Feeding occurred only in snakes treated with NPY. At the low dose 29% ate at 4-5 h and 24 h after surgery; 43% of the snakes treated with the high dose ate 4-5 h post-surgery. When tested the day following injection, animals were courting at levels comparable to presurgery levels and refused food.

Animals