The role of insulin-like growth factor-1 (IGF-1) in growth and reproduction in female brown house snakes (Lamprophis fuliginosus)

Complex ontogeny of sexual size dimorphism in a female-larger gecko: Implications of determinate growth for lizard body size and life-history evolution

Ectothermic vertebrates such as reptiles were assumed to be indeterminate growers, which means that there is no terminal point in time or size for growth in their lifetime. In recent years, evidence for the determinate nature of growth in lizards has accumulated, necessitating a re-examination of models of their ontogeny and evolution of sexual size dimorphism (SSD). In the female-larger gecko Paroedura vazimba, we monitored post-embryonic growth over a period of 15 months. After hatching, females grew faster than males but also reached their final body size, that is, closed growth of their vertebrae, earlier than males. The closure of bone growth in females correlates with the onset of reproductive maturation. We compared this pattern with the previously minutely studied, male-larger species Paroedura picta, where we documented determinate growth as well. We propose a model to explain the evolutionary switches in the direction of SSD in lizards based on bipotential effects of ovarian hormones on growth. In this model, male growth is assumed to require no male-specific growth modifier, such as sex-limited hormonal regulators, while growth is feminized by ovarian hormones in females. Low levels of ovarian hormones can promote bone growth, but high levels associated with maturation of the reproductive organs promote senescence of bone growth plates and thus cessation of bone growth. We suggest that models on growth, life-history and evolution of body size in many lizards should acknowledge their determinate nature of growth.

The maternal energetic environment affects both egg and offspring phenotypes in green anole lizards (Anolis carolinensis)

Animals exist in dynamic environments that may affect both their own fitness and that of their offspring. Maternal effects might allow mothers to prepare their offspring for the environment in which they will be born via several mechanisms, not all of which are well understood. Resource scarcity and forced resource allocation are two scenarios that could affect maternal investment by altering the amount and type of resources available for investment in offspring, albeit in potentially different ways. We tested the hypothesis that maternal dietary restriction and sprint training have different consequences for the offspring phenotype in an oviparous lizard (Anolis carolinensis). To do this, we collected and reared eggs from adult diet-manipulated females (low-diet [LD] or high-diet [HD]) and sprint-trained females (sprint trained [ST] or untrained [UT]) and measured both egg characteristics and hatchling morphology. ST and LD mothers laid both the fewest and heaviest eggs, and ST, UT, and LD eggs also had significantly longer incubation periods than the HD group. Hatchlings from the diet experiment (LD and HD offspring) were the heaviest overall. Furthermore, both body mass of the mother at oviposition and change in maternal body mass over the course of the experiment had significant and sometimes different effects on egg and offspring phenotypes, highlighting the importance of maternal energetic state to the allocation of maternal resources.

Development of male-larger sexual size dimorphism in a lizard: IGF1 peak long after sexual maturity overlaps with pronounced growth in males

Squamate reptiles have been considered to be indeterminate growers for a long time. However, recent studies demonstrate that bone prolongation is stopped in many lizards by the closure of bone growth plates. This shift in the paradigm of lizard growth has important consequences for questions concerning the proximate causes of sexual size dimorphism. The traditional model of highly plastic and indeterminate growth would correspond more to a long-term action of a sex-specific growth regulator. On the other hand, determinate growth would be more consistent with a regulator acting in a sex-specific manner on the activity of bone growth plates operating during the phase when a dimorphism in size develops. We followed the growth of males and females of the male-larger Madagascar ground gecko (Paroedura picta) and monitored the activity of bone growth plates, gonad size, levels of steroids, expression of their receptors (AR, ESR1), and expression of genes from the insulin-like growth factor network (IGF1, IGF2, IGF1R, and IGF2R) in livers. Specifically, we measured gene expression before the onset of dimorphic growth, at the time when males have more active bone growth plates and sexual size dimorphism was clearly visible, and after a period of pronounced growth in both sexes. We found a significant spike in the expression of IGF1 in males around the time when dimorphism develops. This overexpression in males comes long after an increase in circulating testosterone levels and sexual maturation in males, and it might be suppressed by ovarian hormones in females. The results suggest that sexual size dimorphism in male-larger lizards can be caused by a positive effect of high levels of IGF1 on bone growth. The peak in IGF1 resembles the situation during the pubertal growth spurt in humans, but in lizards, it seems to be sex-specific and disconnected from sexual maturation.

Functional remodeling of adrenal steroidogenic tissue by food deprivation in the lizard, Sceloporus undulatus

The present study examined how food availability interacts with age to modulate lizard adrenal steroidogenic function at the cellular level. Adult male and juvenile male and female Eastern Fence Lizards (Sceloporus undulatus) underwent a period of food deprivation with or without a shorter re-feeding period. Lizards maintained on a full feeding regimen served as the controls. Across the feeding regimens, plasma corticosterone of adult lizards was unchanged whereas that of food-deprived juvenile lizards was increased nearly 7 times and this increase was normalized by a short re-feeding period. Freshly dispersed adrenocortical cells derived from these lizards were incubated with ACTH and the production of selected steroids was measured by highly specific radioimmunoassay. Net maximal steroid rates of juvenile cells were 161% greater than those of adult cells. Adult and juvenile progesterone rates were consistently suppressed by food deprivation (by nearly 48%) and were normalized by a re-feeding period, whereas divergent effects were seen with corticosterone and aldosterone rates. Food deprivation suppressed corticosterone rates of adult cells by 43% but not those of juvenile cells. In a reciprocal manner, food deprivation had no significant effect on aldosterone rates of adult cells, but it suppressed those of juvenile cells by 52%. A short re-feeding period normalized most rates in both adult and juvenile cells and further augmented the adult aldosterone rate by 54%. The effect of the feeding regimens on ACTH sensitivity varied with life stage and with steroid. The overall sensitivity of adult cells to ACTH was nearly three times that of juvenile cells. Collectively, the data presented here and data from previous work indicate that food restriction/deprivation in Sceloporus lizard species causes a functional remodeling of the adrenocortical tissue. Furthermore, life stage adds more complexity to this remodeling.

Expression of insulin-like growth factors depends on both mass and resource availability in female green anoles (Anolis carolinensis)

The insulin and insulin-like signaling (IIS) network is an important mediator of cellular growth and metabolism in animals, and is sensitive to environmental conditions such as temperature and resource availability. The two main hormones of the IIS network, insulin-like growth factor 1 (IGF1) and insulin-like growth factor 2 (IGF2), are present in all vertebrates, yet little is known regarding the responsiveness of IGF2 in particular to external stimuli in non-mammalian animals. We manipulated diet (low or high quantity of food: low and high diet group, respectively) in adult green anole (Anolis carolinensis) females to test the effect of energetic state on hepatic gene expression of IGF1 and IGF2. The absolute expression of IGF2 in female green anoles was 100 times higher than that of IGF1 regardless of diet treatment, and IGF1 and IGF2 expression interacted with post-treatment body mass and treatment, as did the expression of the purported housekeeping genes glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and eukaryotic elongation factor 2 (EEF2). The low diet group showed a negative relationship between body mass and gene expression for all genes, whereas the relationships between body mass and gene expression in the high diet group were either absent (in the case of IGF1) or positive (for all other genes). After accounting for total change in mass, the low diet group expressed IGF2, GAPDH and EEF2 at higher levels compared with individuals in the high diet group of a similar change in mass. These results illustrate that expression of IGF1 and IGF2, and of the housekeeping genes is affected by energe-tic status in reptiles.

Effects of experimental increase in insulin-like growth factor 1 on feather growth rate, moult intensity and feather quality in a passerine bird

Moulting is a crucial, yet often overlooked life-history stage in many animals, when they renew their integumental structures. This life-history stage is an energetically demanding somatic growth event that has particular importance in birds because feathers play a crucial role in flight, insulation and communication. Somatic growth processes are regulated by the evolutionarily conserved peptide hormone insulin-like growth factor 1 (IGF-1). However, the role of IGF-1 in feather growth remains unknown. In this study, we captured 41 juvenile free-living bearded reedlings (Panurus biarmicus) that had started their first complete moult and brought them into captivity. Then, we manipulated their circulating IGF-1 levels using poly-(lactic-co-glycolid acid) microparticles (microspheres) that provide a sustained release of IGF-1. The treatment increased IGF-1 levels but did not affect the feather growth rate. However, 2 weeks after the treatment, birds in the increased IGF-1 group were moulting more feathers simultaneously than the controls and were at a more advanced stage of moult. Birds with experimentally increased IGF-1 levels had better quality feathers (measured by a lower number of fault bars) than the controls. These results suggest that an increase in IGF-1 does not speed up feather growth, but may alter moult intensity by initiating the renewal of several feathers simultaneously. This may shorten the overall moulting time but may imply costs in terms of IGF-1-induced oxidative stress.

Nature or Nurture: Can Prey-Based Diets Influence Species-Specific Physiological Performance Traits of Epidermal Lipid Content and Cutaneous Water Loss?

Epidermal lipids serve as the primary barrier to cutaneous water loss (CWL) and play a significant role in water conservation and homeostasis. Previous studies have shown the correlation between increased aridity of habitats and the amount of epidermal lipids among species. Generally, increased amounts of epidermal lipids lower skin permeability. Species-specific differences in CWL and prey preferences between two sympatric snake species, the Northern Cottonmouth (Agkistrodon piscivorus) and the Eastern Copperhead (Agkistrodon contortrix), motivated us to question if prey-base can result in these observed species-specific differences in CWL. We experimentally controlled the diets for a captive colony of Northern Cottonmouths (A. piscivorus) by feeding either fish (Notemigonus crysoleucas) or mice (Mus musculus) to investigate if diet can affect the quantity and quality of epidermal lipids and the rates of CWL. Snakes fed mice gained consistently more mass, but diet treatments did not affect growth rate. We found no significant differences in quantitative lipid content or rates of CWL between diet treatments. An analysis for qualitative lipid content using infrared spectrophotometry also showed no diet effect, thus suggesting that lipid content and CWL are strong species-specific physiological performance traits not influenced by recent dietary history. While there is some evidence that epidermal permeability may be variable under certain environmental conditions (e.g., humidity), our findings show that diet has no effect and that a shift in prey preference may not influence or enhance physiological performance for decreasing CWL.

Gene expression of the IGF hormones and IGF binding proteins across time and tissues in a model reptile

The insulin and insulin-like signaling (IIS) network regulates cellular processes including pre- and postnatal growth, cellular development, wound healing, reproduction, and longevity. Despite their importance in the physiology of vertebrates, the study of the specific functions of the top regulators of the IIS network, insulin-like growth factors (IGFs) and IGF binding proteins (IGFBPs), has been mostly limited to a few model organisms. To expand our understanding of this network, we performed quantitative gene expression of IGF hormones in liver and qualitative expression of IGFBPs across tissues and developmental stages in a model reptile, the brown anole lizard (Anolis sagrei). We found that lizards express IGF2 across all life stages (preoviposition embryos to adulthood) and at a higher level than IGF1, which is opposite to patterns seen in laboratory rodents but similar to those seen in humans and other vertebrate models. IGFBP expression was ubiquitous across tissues (brain, gonad, heart, liver, skeletal muscle, tail, and regenerating tail) in adults, apart from IGFBP5, which was variable. These findings provide an essential foundation for further developing the anole lizard as a physiological and biomedical reptile model, as well as expanding our understanding of the function of the IIS network across species.

Rapid morphological divergence following a human-mediated introduction: the role of drift and directional selection

Theory predicts that when populations are established by few individuals, random founder effects can facilitate rapid phenotypic divergence even in the absence of selective processes. However, empirical evidence from historically documented colonisations suggest that, in most cases, drift alone is not sufficient to explain the rate of morphological divergence. Here, using the human-mediated introduction of the silvereye (Zosterops lateralis) to French Polynesia, which represents a potentially extreme example of population founding, we reassess the potential for morphological shifts to arise via drift alone. Despite only 80 years of separation from their New Zealand ancestors, French Polynesian silvereyes displayed significant changes in body and bill size and shape, most of which could be accounted for by drift, without the need to invoke selection. However, signatures of selection at genes previously identified as candidates for bill size and body shape differences in a range of bird species, also suggests a role for selective processes in driving morphological shifts within this population. Twenty-four SNPs in our RAD-Seq dataset were also found to be strongly associated with phenotypic variation. Hence, even under population founding extremes, when it is difficult to reject drift as the sole mechanism based on rate tests of phenotypic shifts, the additional role of divergent natural selection in novel environments can be revealed at the level of the genome.

Utilization of an endocrine growth index, insulin-like growth factor binding protein (IGFBP)-1b, for postsmolt coho salmon in the Strait of Georgia, British Columbia, Canada

Monitoring the growth of salmon during their early marine phase provides insights into prey availability, and growth rates may be linked to risks of size-dependent mortality. However, the measurement of growth rate is challenging for free-living salmon in the ocean. Insulin-like growth factor (IGF)-I is a growth-promoting hormone that is emerging as a useful index of growth in salmon. In addition, laboratory-based studies using coho salmon have shown that one of circulating IGF-binding proteins (IGFBPs), IGFBP-1b, is induced by fasting and thus could be used as an inverse index of growth and/or catabolic state in salmon. However, few studies have measured plasma levels of IGFBP-1b in salmon in the wild. We measured plasma IGFBP-1b levels for postsmolt coho salmon collected in the Strait of Georgia and surrounding waters, British Columbia, Canada, and compared regional differences in IGFBP-1b to ecological information such as seawater temperature and stomach fullness. Plasma IGFBP-1b levels were the highest in fish from Eastern Johnstone Strait and relatively high in Queen Charlotte Strait and Western Johnstone Strait, which was in good agreement with the poor ocean conditions for salmon hypothesized to occur in that region. The molar ratio of plasma IGF-I to IGFBP-1b, a theoretical parameter of IGF-I availability to the receptor, discriminated differences among regions better than IGF-I or IGFBP-1b alone. Our data suggest that plasma IGFBP-1b reflects catabolic status in postsmolt coho salmon, as highlighted in fish in Eastern Johnston Strait, and is a useful tool to monitor negative aspects of salmon growth in the ocean.

A clinal polymorphism in the insulin signaling transcription factor foxo contributes to life-history adaptation in Drosophila

A fundamental aim of adaptation genomics is to identify polymorphisms that underpin variation in fitness traits. In Drosophila melanogaster, latitudinal life-history clines exist on multiple continents and make an excellent system for dissecting the genetics of adaptation. We have previously identified numerous clinal single-nucleotide polymorphism in insulin/insulin-like growth factor signaling (IIS), a pathway known from mutant studies to affect life history. However, the effects of natural variants in this pathway remain poorly understood. Here we investigate how two clinal alternative alleles at foxo, a transcriptional effector of IIS, affect fitness components (viability, size, starvation resistance, fat content). We assessed this polymorphism from the North American cline by reconstituting outbred populations, fixed for either the low- or high-latitude allele, from inbred DGRP lines. Because diet and temperature modulate IIS, we phenotyped alleles across two temperatures (18°C, 25°C) and two diets differing in sugar source and content. Consistent with clinal expectations, the high-latitude allele conferred larger body size and reduced wing loading. Alleles also differed in starvation resistance and expression of insulin-like receptor, a transcriptional target of FOXO. Allelic reaction norms were mostly parallel, with few GxE interactions. Together, our results suggest that variation in IIS makes a major contribution to clinal life-history adaptation.

Exploring the mechanistic link between corticosterone and insulin-like growth factor-1 in a wild passerine bird

Background

Physiological regulators of life history trade-offs need to be responsive to sudden changes of resource availability. When homeostasis is challenged by unpredictable stressors, vertebrates respond through a set of physiological reactions, which can promote organismal survival. Glucocorticoids have been traditionally recognized as one of the main regulators of the physiological stress response, but the role of an evolutionarily more conserved pathway, the hypothalamic-pituitary-somatotropic (HPS) axis producing insulin-like growth factor-1 (IGF-1) has received much less attention. Although IGF-1 is known to affect several life history traits, little is known about its role in the physiological stress response and it has never been studied directly in adult wild animals.

Methods

In this study, we combined field observations with a controlled experiment to investigate how circulating levels of IGF-1 change in response to stress and whether this change is due to concomitant change in glucocorticoids in a free-living songbird, the bearded reedling Panurus biarmicus. We used a standard capture-restraint protocol in field observation, in which we took first and second (stress induced: 15 minutes later) samples. In a follow-up experiment, we used a minimally invasive oral corticosterone manipulation.

Results

We showed that corticosterone levels significantly increased while IGF-1 levels significantly decreased during capture and handling stress. However, change in corticosterone levels were not related to change in IGF-1 levels. We found that experimentally elevated corticosterone levels did not affect IGF-1 levels.

Discussion

Our results are the first to highlight that circulating IGF-1 levels are responsive to stress independently from glucocorticoids and suggest that the HPS axis is an autonomous physiological pathway that may play an important role as regulator of life-history decisions.

Growth factor gene IGF1 is associated with bill size in the black-bellied seedcracker Pyrenestes ostrinus

Pyrenestes finches are unique among birds in showing a non-sex-determined polymorphism in bill size and are considered a textbook example of disruptive selection. Morphs breed randomly with respect to bill size, and differ in diet and feeding performance relative to seed hardness. Previous breeding experiments are consistent with the polymorphism being controlled by a single genetic factor. Here, we use genome-wide pooled sequencing to explore the underlying genetic basis of bill morphology and identify a single candidate region. Targeted resequencing reveals extensive linkage disequilibrium across a 300 Kb region containing the insulin-like growth factor 1 (IGF1) gene, with a single 5-million-year-old haplotype associating with phenotypic dominance of the large-billed morph. We find no genetic similarities controlling bill size in the well-studied Darwin’s finches (Geospiza). Our results show how a single genetic factor may control bill size and provide a foundation for future studies to examine this phenomenon within and among avian species.

Pyrenestes finches have a bill size polymorphism thought to be maintained by disruptive selection. Here, the authors identify a single candidate region, including insulin-like growth factor 1, differentiating small and large bill size morphs and a wider region differentiating the mega-billed morph.

Hormonally Mediated Increases in Sex-Biased Gene Expression Accompany the Breakdown of Between-Sex Genetic Correlations in a Sexually Dimorphic Lizard

The evolution of sexual dimorphism is predicted to occur through reductions in between-sex genetic correlations (r_mf) for shared traits, but the physiological and genetic mechanisms that facilitate these reductions remain largely speculative. Here, we use a paternal half-sibling breeding design in captive brown anole lizards (Anolis sagrei) to show that the development of sexual size dimorphism is mirrored by the ontogenetic breakdown of r_mf for body size and growth rate. Using transcriptome data from the liver (which integrates growth and metabolism), we show that sex-biased gene expression also increases dramatically between ontogenetic stages bracketing this breakdown of r_mf. Ontogenetic increases in sex-biased expression are particularly evident for genes involved in growth, metabolism, and cell proliferation, suggesting that they contribute to both the development of sexual dimorphism and the breakdown of r_mf. Mechanistically, we show that treatment of females with testosterone stimulates the expression of male-biased genes while inhibiting the expression of female-biased genes, thereby inducing male-like phenotypes at both organismal and transcriptomic levels. Collectively, our results suggest that sex-specific modifiers such as testosterone can orchestrate sex-biased gene expression to facilitate the phenotypic development of sexual dimorphism while simultaneously reducing genetic correlations that would otherwise constrain the independent evolution of the sexes.

Short-term methionine supplementation during the early post-partum period in primiparous rabbits improves prolificacy associated with an increase in serum concentrations of IGF-I

The aim of this study was to evaluate the effects of methionine supplementation on energy metabolism and reproductive performance during the early post-partum period in primiparous does. Forty nulliparous New Zealand White does were used. Females were randomized in two groups at calving: the control group (n = 20) was fed with the basal diet, and the methionine group (n = 20) was fed the basal diet plus 1 g/animal/day of methionine from the day of calving to 4 days post-partum. Results showed that methionine supplementation increased (p = 0.032) the concentration of insulin-like growth factor-1 with respect to control group 4 days post-partum. It similarly increased the prolificacy (p = 0.03), the number of kits born alive per litter (p = 0.06) and the body gain weight of the litter during supplementation (p = 0.035). These results were observed despite the does in the methionine group having a deeper negative energy balance than the does in the control group. Finally, methionine supplementation did not affect receptivity (p = 0.23), fertility (p = 0.49), the number of kits born dead per litter (p = 0.86) insulin and metabolites as glucose, non-esterified fatty acids and triglycerides. In conclusion, our results show that methionine supplementation during the first 4 days of the post-partum period in rabbits increases total litter size and the corporal weight of kits and is associated with an increase in blood concentration of IGF-1.

Insulin-like signaling (IIS) responses to temperature, genetic background, and growth variation in garter snakes with divergent life histories

The insulin/insulin-like signaling pathway (IIS) has been shown to mediate life history trade-offs in mammalian model organisms, but the function of this pathway in wild and non-mammalian organisms is understudied. Populations of western terrestrial garter snakes (Thamnophis elegans) around Eagle Lake, California, have evolved variation in growth and maturation rates, mortality senescence rates, and annual reproductive output that partition into two ecotypes: "fast-living" and "slow-living". Thus, genes associated with the IIS network are good candidates for investigating the mechanisms underlying ecological divergence in this system. We reared neonates from each ecotype for 1.5years under two thermal treatments. We then used qPCR to compare mRNA expression levels in three tissue types (brain, liver, skeletal muscle) for four genes (igf1, igf2, igf1r, igf2r), and we used radioimmunoassay to measure plasma IGF-1 and IGF-2 protein levels. Our results show that, in contrast to most mammalian model systems, igf2 mRNA and protein levels exceed those of igf1 and suggest an important role for igf2 in postnatal growth in reptiles. Thermal rearing treatment and recent growth had greater impacts on IGF levels than genetic background (i.e., ecotype), and the two ecotypes responded similarly. This suggests that observed ecotypic differences in field measures of IGFs may more strongly reflect plastic responses in different environments than evolutionary divergence. Future analyses of additional components of the IIS pathway and sequence divergence between the ecotypes will further illuminate how environmental and genetic factors influence the endocrine system and its role in mediating life history trade-offs.

Evolution and Function of the Insulin and Insulin-like Signaling Network in Ectothermic Reptiles: Some Answers and More Questions

The insulin and insulin-like signaling (IIS) molecular network regulates cellular growth and division, and influences organismal metabolism, growth and development, reproduction, and lifespan. As a group, reptiles have incredible diversity in the complex life history traits that have been associated with the IIS network, yet the research on the IIS network in ectothermic reptiles is sparse. Here, we review the IIS network and synthesize what is known about the function and evolution of the IIS network in ectothermic reptiles. The primary hormones of this network-the insulin-like growth factors 1 and 2 (IGFs) likely function in reproduction in ectothermic reptiles, but the precise mechanisms are unclear, and likely range from influencing mating and ovulation to maternal investment in embryonic development. In general, plasma levels of IGF1 increase with food intake in ectothermic reptiles, but the magnitude of the response to food varies across species or populations and the ages of animals. Long-term temperature treatments as well as thermal stress can alter expression of genes within the IIS network. Although relatively little work has been done on IGF2 in ectothermic reptiles, IGF2 is consistently expressed at higher levels than IGF1 in juvenile ectothermic reptiles. Furthermore, in contrast to mammals that have genetic imprinting that silences the maternal IGF2 allele, in reptiles IGF2 is bi-allelically expressed (based on findings in chickens, a snake, and a lizard). Evolutionary analyses indicate some members of the IIS network are rapidly evolving across reptile species, including IGF1, insulin (INS), and their receptors. In particular, IGF1 displays extensive nucleotide variation across lizards and snakes, which suggests that its functional role may vary across this group. In addition, genetic variation across families and populations in the response of the IIS network to environmental conditions illustrates that components of this network may be evolving in natural populations. The diversity in reproductive physiology, metabolic plasticity, and lifespan among reptiles makes the study of the IIS network in this group a potentially rich avenue for insight into the evolution and function of this network. The field would benefit from future studies that discern the respective functions of IGF1 and IGF2 and how these functions vary across taxa, perfecting additional assays for measuring IIS components, and determining the role of IIS in different tissues.

Rapid molecular evolution across amniotes of the IIS/TOR network

The insulin/insulin-like signaling and target of rapamycin (IIS/TOR) network regulates lifespan and reproduction, as well as metabolic diseases, cancer, and aging. Despite its vital role in health, comparative analyses of IIS/TOR have been limited to invertebrates and mammals. We conducted an extensive evolutionary analysis of the IIS/TOR network across 66 amniotes with 18 newly generated transcriptomes from nonavian reptiles and additional available genomes/transcriptomes. We uncovered rapid and extensive molecular evolution between reptiles (including birds) and mammals: (i) the IIS/TOR network, including the critical nodes insulin receptor substrate (IRS) and phosphatidylinositol 3-kinase (PI3K), exhibit divergent evolutionary rates between reptiles and mammals; (ii) compared with a proxy for the rest of the genome, genes of the IIS/TOR extracellular network exhibit exceptionally fast evolutionary rates; and (iii) signatures of positive selection and coevolution of the extracellular network suggest reptile- and mammal-specific interactions between members of the network. In reptiles, positively selected sites cluster on the binding surfaces of insulin-like growth factor 1 (IGF1), IGF1 receptor (IGF1R), and insulin receptor (INSR); whereas in mammals, positively selected sites clustered on the IGF2 binding surface, suggesting that these hormone-receptor binding affinities are targets of positive selection. Further, contrary to reports that IGF2R binds IGF2 only in marsupial and placental mammals, we found positively selected sites clustered on the hormone binding surface of reptile IGF2R that suggest that IGF2R binds to IGF hormones in diverse taxa and may have evolved in reptiles. These data suggest that key IIS/TOR paralogs have sub- or neofunctionalized between mammals and reptiles and that this network may underlie fundamental life history and physiological differences between these amniote sister clades.

Nutritional modulation of IGF-1 in relation to growth and body condition in Sceloporus lizards

Nutrition and energy balance are important regulators of growth and the growth hormone/insulin-like growth factor (GH/IGF) axis. However, our understanding of these functions does not extend uniformly to all classes of vertebrates and is mainly limited to controlled laboratory conditions. Lizards can be useful models to improve our understanding of the nutritional regulation of the GH/IGF-1 axis because many species are relatively easy to observe and manipulate both in the laboratory and in the field. In the present study, the effects of variation in food intake on growth, body condition, and hepatic IGF-1 mRNA levels were measured in (1) juveniles of Sceloporus jarrovii maintained on a full or 1/3 ration and (2) hatchlings of Sceloporus undulatus subjected to full or zero ration with or without re-feeding. These parameters plus plasma IGF-1 were measured in a third experiment using adults of S. undulatus subjected to full or zero ration with or without re-feeding. In all experiments, plasma corticosterone was measured as an anticipated indicator of nutritional stress. In S. jarrovii, growth and body condition were reduced but lizards remained in positive energy balance on 1/3 ration, and hepatic IGF-1 mRNA and plasma corticosterone were not affected in comparison to full ration. In S. undulatus, growth, body condition, hepatic IGF-1 mRNA, and plasma IGF-1 were all reduced by zero ration and restored by refeeding. Plasma corticosterone was increased in response to zero ration and restored by full ration in hatchlings but not adults of S. undulatus. These data indicate that lizards conform to the broader vertebrate model in which severe food deprivation and negative energy balance is required to attenuate systemic IGF-1 expression. However, when animals remain in positive energy balance, reduced food intake does not appear to affect systemic IGF-1. Consistent with other studies on lizards, the corticosterone response to reduced food intake is an unreliable indicator of nutritional stress. Further studies on ecologically relevant variation in food intake are required to establish the importance of nutrition as an environmental regulator of the GH/IGF axis. Within the range of positive energy balance, the potential involvement of molecular signals in growth regulation requires further investigation.

Natural variation in steroid hormone profiles of male Timber Rattlesnakes, Crotalus horridus, in northwest Arkansas

We describe the seasonal profile of circulating steroid hormones (testosterone and corticosterone) in relation to the breeding season in free ranging male Timber Rattlesnakes, Crotalus horridus, over the course of three active seasons. In addition, we examine variation in steroid concentrations across years and in relation to body condition. We found that seasonal profiles of plasma testosterone were different compared to other crotalines with similar mating patterns. Concentrations of testosterone were elevated above baseline in the three months leading up to the single late summer breeding season. Testosterone peaked in July at the onset of the breeding season and dropped to baseline during the peak months of breeding (August and September). Testosterone concentrations also varied annually. Although the exact cause of annual variation could not be established, our results indicate that weather patterns may have driven observed differences. Testosterone concentrations were positively related to body condition, indicating that testosterone production is modulated according to energetic status (particularly in the two months prior to the breeding season). Corticosterone did not vary seasonally or with any measured variable, a result similar to other studied crotalines. Our results highlight the importance of long-term descriptive studies of the regulatory mechanisms that underlie behavior and physiology in diverse taxa, as these mechanisms can vary greatly within and among populations and are valuable in elucidating the intrinsic and extrinsic sources of such variation.

A general mechanism for conditional expression of exaggerated sexually-selected traits

Sexually-selected exaggerated traits tend to be unusually reliable signals of individual condition, as their expression tends to be more sensitive to nutritional history and physiological circumstance than that of other phenotypes. As such, these traits are the foundation for many models of sexual selection and animal communication, such as "handicap" and "good genes" models. Exactly how expression of these traits is linked to the bearer's condition has been a central yet unresolved question, in part because the underlying physiological mechanisms regulating their development have remained largely unknown. Recent discoveries across animals as diverse as deer, beetles, and flies now implicate the widely conserved insulin-like signaling pathway, as a common physiological mechanism regulating condition-sensitive structures with extreme growth. This raises the exciting possibility that one highly conserved pathway may underlie the evolution of trait exaggeration in a multitude of sexually-selected signal traits across the animal kingdom.

Rates of molecular evolution vary in vertebrates for insulin-like growth factor-1 (IGF-1), a pleiotropic locus that regulates life history traits

Insulin-like growth factor-1 (IGF-1) is a member of the vertebrate insulin/insulin-like growth factor/relaxin gene family necessary for growth, reproduction, and survival at both the cellular and organismal level. Its sequence, protein structure, and function have been characterized in mammals, birds, and fish; however, a notable gap in our current knowledge of the function of IGF-1 and its molecular evolution is information in ectothermic reptiles. To address this disparity, we sequenced the coding region of IGF-1 in 11 reptile species-one crocodilian, three turtles, three lizards, and four snakes. Complete sequencing of the full mRNA transcript of a snake revealed the Ea-isoform, the predominant isoform of IGF-1 also reported in other vertebrate groups. A gene tree of the IGF-1 protein-coding region that incorporated sequences from diverse vertebrate groups showed similarity to the species phylogeny, with the exception of the placement of Testudines as sister group to Aves, due to their high nucleotide sequence similarity. In contrast, long-branch lengths indicate more rapid divergence in IGF-1 among lizards and snakes. Additionally, lepidosaurs (i.e., lizards and snakes) had higher rates of non-synonymous:synonymous substitutions (dN/dS) relative to archosaurs (i.e., birds and crocodilians) and turtles. Tests for positive selection on specific codons within branches and evaluation of the changes in the amino acid properties, suggested positive selection in lepidosaurs on the C domain of IGF-1, which is involved in binding affinity to the IGF-1 receptor. Predicted structural changes suggest that major alterations in protein structure and function may have occurred in reptiles. These data propose new insights into the molecular co-evolution of IGF-1 and its receptors, and ultimately the evolution of IGF-1's role in regulating life-history traits across vertebrates.

Serum IGF-I in middle age covaries with reproductive life-history traits in British men and women

OBJECTIVES

The growth hormone/insulin-like growth factor-I (GH/IGF-I) axis may be an important component of individual life-history variation. This study examined whether IGF-I levels covary with reproductive life-history traits in a long-term British cohort study (the National Child Development Study).

METHODS

Using data on up to 5,252 individuals (2,431 men and 2,821 women) born in March 1958, relationships of serum IGF-I at age 45 with pubertal age, time to pregnancy (TTP), age at first reproduction (AFR), and lifetime reproductive success (LRS) were analyzed using general linear models.

RESULTS

IGF-I showed modest non-linear associations with pubertal age, being low in women who first menstruated, and men whose voice broke, in the oldest respective age categories. In women, but not men, IGF-I was negatively related to TTP. IGF-I was positively associated with AFR in both sexes and tended to be negatively related to LRS in women only. Some, but not all, relationships were attenuated after controlling for multiple potential confounders.

CONCLUSION

These results are consistent with the hypothesis that the GH/IGF-I axis partially underlies individual variance in reproductive outcomes, through either direct endocrine effects on reproductive function or behavior, indirect effects on other traits, or a combination of the above. Several limitations of this study include the late age at which blood samples were collected and the difficulty of disentangling social and biological factors contributing to the traits studied. Am. J. Hum. Biol., 2012. © 2012 Wiley Periodicals, Inc.

Mediation of vertebrate life histories via insulin-like growth factor-1

Life-history traits describe parameters associated with growth, size, survival, and reproduction. Life-history variation is a hallmark of biological diversity, yet researchers commonly observe that one of the major axes of life-history variation after controlling for body size involves trade-offs among growth, reproduction, and longevity. This persistent pattern of covariation among these specific traits has engendered a search for shared mechanisms that could constrain or facilitate production of variation in life-history strategies. Endocrine traits are one candidate mechanism that may underlie the integration of life history and other phenotypic traits. However, the vast majority of this research has been on the effects of steroid hormones such as glucocorticoids and androgens on life-history trade-offs. Here we propose an expansion of the focus on glucocorticoids and gonadal hormones and review the potential role of insulin-like growth factor-1 (IGF-1) in shaping the adaptive integration of multiple life-history traits. IGF-1 is a polypeptide metabolic hormone largely produced by the liver. We summarize a vast array of research demonstrating that IGF-1 levels are susceptible to environmental variation and that IGF-1 can have potent stimulatory effects on somatic growth and reproduction but decrease lifespan. We review the few studies in natural populations that have measured plasma IGF-1 concentrations and its associations with life-history traits or other characteristics of the organism or its environment. We focus on two case studies that found support for the hypothesis that IGF-1 mediates adaptive divergence in suites of life-history traits in response to varying ecological conditions or artificial selection. We also examine what we view as potentially fruitful avenues of research on this topic, which until now has been rarely investigated by evolutionary ecologists. We discuss how IGF-1 may facilitate adaptive plasticity in life-history strategies in response to early environmental conditions and also how selection on loci controlling IGF-1 signaling may mediate population divergence and eventual speciation. After consideration of the interactions among androgens, glucocorticoids, and IGF-1 we suggest that IGF-1 be considered a suitable candidate mechanism for mediating life-history traits. Finally, we discuss what we can learn about IGF-1 from studies in free-ranging animals. The voluminous literature in laboratory and domesticated animals documenting relationships among IGF-1, growth, reproduction, and lifespan demonstrates the potential for a number of new research questions to be asked in free-ranging animals. Examining how IGF-1 mediates life-history traits in free-ranging animals could lead to great insight into the mechanisms that influence life-history variation.