The forms, uses and significance of genetic variation in endocrine systems

Hormones and the Evolution of Complex Traits: Insights from Artificial Selection on Behavior

Although behavior may often be a fairly direct target of natural or sexual selection, it cannot evolve without changes in subordinate traits that cause or permit its expression. In principle, changes in endocrine function could be a common mechanism underlying behavioral evolution because they are well positioned to mediate integrated responses to behavioral selection. More specifically, hormones can influence both motivational (e.g., brain) and performance (e.g., muscles) components of behavior simultaneously and in a coordinated fashion. If the endocrine system is often "used" as a general mechanism to effect responses to selection, then correlated responses in other aspects of behavior, life history, and organismal performance (e.g., locomotor abilities) should commonly occur because any cell with appropriate receptors could be affected. Ways in which behavior coadapts with other aspects of the phenotype can be studied directly through artificial selection and experimental evolution. Several studies have targeted rodent behavior for selective breeding and reported changes in other aspects of behavior, life history, and lower-level effectors of these organismal traits, including endocrine function. One example involves selection for high levels of voluntary wheel running, one aspect of physical activity, in four replicate High Runner (HR) lines of mice. Circulating levels of several hormones (including insulin, testosterone, thyroxine, triiodothyronine) have been characterized, three of which-corticosterone, leptin, and adiponectin-differ between HR and control lines, depending on sex, age, and generation. Potential changes in circulating levels of other behaviorally and metabolically relevant hormones, as well as in other components of the endocrine system (e.g., receptors), have yet to be examined. Overall, results to date identify promising avenues for further studies on the endocrine basis of activity levels.

Thyroid hormones and daily gain in cattle. Relationships between plasma total thyroxine, free thyroxine, triiodothyronine and average daily gain during the first year of life in Belgian blue and Friesian cattle

Correlation between circulating thyroid hormones and live-weight gain during the first year of life were observed in four trials on cattle of the two main Belgian breeds: Belgian Blue and Friesian. Thyroid hormones assayed and considered as potential predictors of growth rate were: thyroxine (T4), triiodothyronine (T3) and free thyroxine (FT4). Triiodothyronine uptake (T,U), thyroid stimulating hormone (TSH) and free thyroxine index (FTI) were also estimated.

No significant correlation was found between live-weight gain and T4 or T3 in 13 Belgian Blue heifers (trial 1). In 49 bulls submitted to a performance test, T4 and FTI increased from 3 to 12 months whilst T3U decreased (trial 2). Significant correlations between live-weight gain and T4 and between live-weight gain and FTI were obtained from samples taken between 66 and 95 days. Live-weight gain was correlated with T3U when animals were between 96 and 125 days, and 126 to 155 days of age. It is possible that the interaction between total T4 and T3U is important in controlling growth. In 13 young calves, beef merit, expressed as live-weight gain in 1 year, was highly correlated with T4 concentration at 8 to 10 days of age (trial 3). T3 and FT4 did not seem to be directly associated with the growth processes. In trial 4, live-weight gains of 42 young animals housed on a commercial farm were negatively correlated with T4 and with FTI. The conflicting results encountered may perhaps be explained by the lack of information on the use of anabolic preparations in field conditions. T4 concentration and FTI may be considered as potential parameters for the estimation of beef merit but the results are insufficient to draw definite conclusions.

The EDGE hypothesis: epigenetically directed genetic errors in repeat-containing proteins (RCPs) involved in evolution, neuroendocrine signaling, and cancer

Trans-generational epigenetic phenomena, such as contamination with endocrine-disrupting chemicals (EDCs) that decrease fertility and the global methylation status of DNA in the offspring, are of great concern because they may affect health, particularly the health of children. However, of even greater concern is the possibility that trans-generational changes in the methylation status of the DNA might lead to permanent changes in the DNA sequence itself. By contaminating the environment with EDCs, mankind might be permanently affecting the health of future generations. In this section, we present evidence from our laboratory and others that trans-generational epigenetic changes in DNA might lead to mutations directed to genes encoding amino acid repeat-containing proteins (RCPs) that are important for adaptive evolution or cancer progression. Such epigenetic changes can be induced "naturally" by hormones or "unnaturally" by EDCs or environmental stress. To illustrate the phenomenon, we present new bioinformatic evidence that the only RCP ontological categories conserved from Drosophila to humans are "regulation of splicing," "regulation of transcription," and "regulation of synaptogenesis," which are classes of genes likely to be important for evolutionary processes. Based on that and other evidence, we propose a model for evolution that we call the EDGE (Epigenetically Directed Genetic Errors) hypothesis for the mechanism by which mutations are targeted at epigenetically modified "contingency genes" encoding RCPs. In the model, "epigenetic assimilation" of metastable epialleles of RCPs over many generations can lead to mutations directed to those genes, thereby permanently stabilizing the adaptive phenotype.

Epigenetics and its implications for behavioral neuroendocrinology

Individuals vary in their sociosexual behaviors and reactivity. How the organism interacts with the environment to produce this variation has been a focus in psychology since its inception as a scientific discipline. There is now no question that cumulative experiences throughout life history interact with genetic predispositions to shape the individual's behavior. Recent evidence suggests that events in past generations may also influence how an individual responds to events in their own life history. Epigenetics is the study of how the environment can affect the genome of the individual during its development as well as the development of its descendants, all without changing the DNA sequence. Several distinctions must be made if this research is to become a staple in behavioral neuroendocrinology. The first distinction concerns perspective, and the need to distinguish and appreciate, the differences between Molecular versus Molar epigenetics. Each has its own lineage of investigation, yet both appear to be unaware of one another. Second, it is important to distinguish the difference between Context-Dependent versus Germline-Dependent epigenetic modifications. In essence the difference is one of the mechanism of heritability or transmission within, as apposed to across, generations. This review illustrates these distinctions while describing several rodent models that have shown particular promise for unraveling the contribution of genetics and the environment on sociosexual behavior. The first focuses on genetically-modified mice and makes the point that the early litter environment alters subsequent brain activity and behavior. This work emphasizes the need to understand behavioral development when doing research with such animals. The second focuses on a new rat model in which the epigenome is permanently imprinted, an effect that crosses generations to impact the descendants without further exposure to the precipitating agent. This work raises the question of how events in generations past can have consequences at both the mechanistic, behavioral, and ultimately evolutionary levels.

Fecal glucocorticoids: a noninvasive method of measuring adrenal activity in wild and captive rodents

To determine the utility of fecal corticosteroid concentration as a measure of chronic stress under laboratory and field conditions, we biochemically and physiologically validated a radioimmunoassay for corticosteroids in three rodent species, house mice (Mus musculus), deer mice (Peromyscus maniculatus), and red-back voles (Clethrionomys gapperi). The biochemical validations demonstrated that the assay accurately and precisely measured corticosteroid concentration in the feces. The physiological validation indicated that the assay was sensitive enough to detect the stress associated with (a) brief handling and bleeding of animals, (b) chronic caloric restriction, (c) exposure to a novel environment, and (d) exposure to a novel cold environment. Our results suggest that fecal measurements reflect stress levels experienced by these animals approximately 6-12 h before defecation. Therefore, given a judicious trapping and trap-monitoring protocol, this assay has considerable utility for measuring the stress levels at which animals actually exist in the field.

The uses of intraspecific variation in aging research

The artificial creation of genetically long-lived populations of several invertebrate species has illustrated how researchers may take advantage of genetic variation within a species to investigate the nature and mechanisms of aging. The advantage of studying intraspecific variation is that populations will be generally similar except for the relevant demographic differences. Also, there are reasons to suspect that genetic mechanisms of aging may differ from mechanisms associated with life extension via environmental manipulations such as caloric restriction. However creating a long-lived mammalian aging model will be expensive and time consuming. An alternative approach is to seek to identify naturally occurring slowly aging populations to contrast mechanistically with a reference population. Ecologists have already noted that demographic alterations of the appropriate type are frequently associated with populations from differing latitudes, differing altitudes, or from islands. Therefore, it is likely that genetically longer- (and shorter)-lived mammal populations of the same species already exist in nature, and could potentially be exploited to inquire into the genetics and mechanisms of aging and longevity. Of particular interest is the indication that some island populations of house mice may exhibit extended longevity compared with laboratory strains.

Hormonal induction of steroid sulphatase in the mouse

By comparing steroid sulphatase levels per se, and also ratios to alpha-galactosidase, in 6 sets of mice - normal females, entire and castrated males both with and without exogenous testosterone administration - we obtained support for the contention that induction of this enzyme is in part controlled by male hormones.

Direct and correlated responses to selection for plasma thyroxine levels in mice

Selection was carried out in mice for concentration of thyroxine hormone (T4) in plasma of males at 11 weeks of age over seven generations. Selection was practised for high level in two replicate lines and for low level in two replicate lines, and there was an unselected control. There was a response in both directions, and the divergence of 12·4 ng/ml observed in generation seven was equivalent to about 20% of the base population mean or nearly one phenotypic standard deviation. The realised heritability was 9%.

Plasma thyroxine level had a repeatability of 0·54 when two measurements were made 24 h apart. The responses made at 11 weeks in males were also evident in both males and females at 5 weeks. Plasma tri-iodo thyronine (T3) concentrations showed a correlated response almost as large, relative to the mean level, as that in T4.

Positive correlated responses were observed in total weights of the litter at 12 days, and in individual weights at 3, 6 and 9 weeks, the responses in the early weights being greater relative to their mean. The results suggest that the correlated weight changes were due to genetic responses in maternal characteristics, probably milk production, rather than individual growth.

A review of research on genetic variation in physiological characteristics related to performance in dairy cattle

Genetic influence on physiological characteristics ranges from single gene effects on amino acid substitutions in alternative forms of proteins to quantitative genetic effects on the amounts of enzymes and hormones. The number of loci involved in the control of quantitative variation in physiologically important substances is not known. A number of marker genes that affect blood antigens, serum and milk proteins, and enzymes have been identified in dairy cattle. However, relatively little is known about genetic effects on quantitative physiological traits in dairy cattle. Much more is known about the genetic control of hormones in laboratory animals. About 25% of the variation in milk production of dairy cows results from genetic differences. We need more studies of genetic influences on the various physiological and biochemical processes involved in the secretion of milk to reveal the mechanisms by which genetics influences the quantity and quality of milk produced by individual cows.

Growth hormone function in strains of mice selected for large and small size

The gene (dw) causing hypopituitary dwarfism was transferred by repeated backcrosses into strains of mice differing genetically in growth-rate through previous selection. The dwarf mice lack growth hormone, and the purpose was to find out if the differences in growth-rate between the strains were in any part due to differences in their growth hormone status – amount of hormone or tissue sensitivity. In the absence of growth hormone, i.e. in the dwarfs, the strains still grew at different rates, proving that growth hormone status was not the only cause of their differences. The effect of substituting the dw gene was, however, greater in the large strain than in the unselected control, and less in the small strain than in the control. The growth differences between the strains were therefore in part due to growth hormone. Tissue sensitivity in the three strains was compared by their responses to graded doses of exogenous growth hormone. The large and control strains did not differ, but the small strain had a lower sensitivity. The results suggest that the increased growth-rate of the large strain is partly due to an increased amount, or activity, of its circulating growth hormone, while the reduced growth-rate of the small strain is in part due to a reduced sensitivity of its target organs to growth hormone.

Genetic variation in plasma thyroxine levels and minimal metabolic rates of the mouse, Mus musculus

A study of inbred strains of the mouse using competitive protein-binding techniques revealed significant strain variation in the total plasma thyroxine levels and in the plasma-free thyroxine index. Measurements of minimal metabolic rates also showed strain variation, but the positive correlation which might be expected between the plasma-free thyroxine index and the minimal metabolic rate did not obtain. Possible explanations are discussed.