Joseph P. Hegmann

Estimating genetic correlations from inbred strains.

Genetic correlations measure the extent of pleiotropic effects of polygenes on pairs of characters or the closeness of linkage between sets of loci influencing the traits and held in allelic (gametic) disequilibrium. Their importance for research lies primarily in predicting correlated responses of one trait to selection based on values for another, and secondarily in analyzing the complex organization of biological systems. Genetic correlations appear to limit the rate and set the direction of multivariate evolution. In view of this, efficient methods for estimating genetic correlations may be essential for understanding the role of behavior in adaptation and for predicting behavioral change in evolution. In this paper we present methods for the estimation of genetic correlations from inbred strain comparisons. Estimates from inbred strains are relatively easy to obtain and appear to be valid when compared to those derived from more demanding parent-offspring comparisons and to correlated responses to selection.

Phenotypic and Genetic Covariance Structure in Milkweed Bug Life History Traits

Variation in life history characteristics is critical for any species because it influences the timing and extent of population growth. When the variability is caused by gene differences among individuals, changes in population growth differ among genotypes. Gene frequencies will change at loci with effects on life history traits, and selection and microevolution of life history traits should occur. However, if many life history characteristics vary among individuals, there may be pairwise covariance caused, in part, by genetic linkage disequilibrium or by pleiotropy. Change caused by selection acting on additive genetic variance for any single character will be accompanied by change in other genetically correlated characters. Darwin (1859; see Darwin 1958) commented on this situation in reference to domestication and artificial selection, which he said “will almost certainly modify unintentionally other parts of the structure, owing to the mysterious laws of correlation” (p. 35). The importance of genetic correlations in the context of life history variables and natural selection is the primary motivation for the work reported here.

Genetic differences influencing behavioral temperature regulation in small mammals. I. Nesting byMus musculus

Nesting behavior was found to differ for animals of five different inbred strains ofMus musculus reared in the same environment, indicating heritable differences in level of nesting byMus. For two separate crosses, hybrid animals built larger nests than did animals of the inbred parental strains. In addition, from data of one of the crosses and derived generations, a very low heritability of nesting but substantial dominance variance were found. This pattern of results is expected of characters which have been the target of natural selection. MaleMus were found to build larger nests than females of all groups tested. These findings suggest that nesting byMus musculus represents required thermoregulatory behavior.

Neuromuscular transmission in the gastropod mollusc Helisoma trivolvis: Identification of motoneurons

SummaryIntracellular records from three muscles in the gastropod mollusc Helisoma trivolvis are described and correlated with extracellular electromyograms and tension recordings. These data are then used to formulate criteria for the identification of motoneurons in the central nervous system.1.Individual muscle fibers in the three muscles studied were small in diameter (less than 10 μ), arranged in a parallel fashion, and heavily invested by connective tissue (Fig. 1).2.Initial identification of neurons in the central nervous system as excitatory motoneurons to particular muscles was based on a number of criteria: a) each action potential in that neuron was followed by an extracellular electromyogram response in the muscle of relatively constant latency; b) in the case of the posterior jugalis, the action potentials of the motoneuron accounted for all extracellularly recorded activity in the muscle during normal function (Fig. 3) ; and c) procion yellow dye injections and antidromic stimulation showed that the axons of such neurons leave the CNS in nerve trunks shown by electrical stimulation to contain the excitatory axons to specific muscles.3.A single basic pattern of muscle innervation, which was comparable to the vertebrate motor unit, was observed. The posterior jugalis appears to be controlled by a single motoneuron whereas the columellar muscle appears to be under the control of a number of motoneurons. Intracellular recordings gave no indication of multiple innervation of individual muscle fibers. High gain intracellular records revealed miniature excitatory junctional potentials but inhibitory junctional potentials were never observed in any of the muscles studied.

Genetic differences influencing behavioral temperature regulation in small mammals. II. Genotype-environment interactions.

The importance of genotype by temperature interactions contributing to individual differences in nesting behavior has been demonstrated using two inbred strains ofMus musculus. Exposure to low ambient temperature increased amounts of cotton used by both the high-nesting (BALB/cJ) and low-nesting (C57BL/6J) strains. The larger total nesting scores of BALB/cJ mice compared to those of C57BL/6J mice resulted from differential increases, depending on temperature, in the amount of cotton used across days, so that the strain differences were greater in both rate of increase and total cotton used for animals tested at 5°C than for those tested at 26°C. The correlation between per gram food consumption and weight of nests was large and negative for animals tested at 5°C, and low for animals tested at 26°C, indicating a metabolic advantage in the cold for animals which built large nests. It is suggested that demonstration of a genotype-environment interaction contributing to differences among natural populations for a specific phenotype provides evidence that the character has been modified by selection acting through the environmental variable being studied.

Population Crosses and the Genetic Structure of Milkweed Bug Life Histories

The fitness of a given phenotype is a direct consequence of the schedule of behavior, fecundity, and mortality that constitutes a life history. For this reason life histories are major adaptations of unique importance to general Darwinism (Bell 1980). As with other complex adaptations life histories consist not of single characters, but of sets of phenotypic traits that covary and function together (Frazetta 1975). Such sets often are referred to as “strategies” or “tactics” and much theoretical and empirical effort has been devoted to attempting to understand the evolution of the complex known as a “life history strategy” (Bell 1980, Stearns 1976, 1977). Births and deaths are most closely related to fitness and have drawn most of the attention, but behavior is also an important component of life histories, especially as it confers flexibility on where and when to breed (Istock 1978 and Chapter 1, this volume, Nichols et al. 1976, Taylor and Taylor 1977, 1978). Two important elements of insect behavior are migration and diapause (Dingle 1981, Solbreck 1978), and we consider them in our discussion here, along with the more traditional life table statistics that they influence.

Circadian complexes: Circadian rhythms under common gene control

SummaryCircadian rhythms for food and water consumption were measured in five inbred strains of mice under a photoperiod of 16 h light and 8 h dark (16:8 LD), and under constant light (LL).Significant strain differences were observed which indicate that a common gene difference, or set of differences inMus musculus influences both the phase angle (ψ) associating the rhythms with the light-dark cycle, and the periods (τLL) of circadian rhythms for food and water consumption. The biological clock mechanism influenced by this genetic variance is common to both food and water circadian rhythms, and differs among the five inbred strains. A positive genetic correlation was observed between the phase angle (ψ) and the period (τLL) of each rhythm. This observation can be understood in terms of a functional relationship between phase and period proposed by Pittendrigh and Daan (1976b) for the entrainment of a circadian oscillator by a light-dark cycle in nocturnal rodents.These results suggest that circadian rhythms for food and water consumption in mice are regulated by a common physiological mechanism, and would respond to natural selection as a single “circadian complex” under common gene control.

Physiological function and behavioral genetics. I. Genetic variance for peripheral conduction velocity in mice

Individual differences for conduction velocity in the ventral and dorsal caudal nerves of 856 mice were assessed employing a rapid,in vivo procedure. Analysis of conduction velocity differences for mice of six inbred strains (A/J, BALB/cJ, CBA/J, C3H/HeJ, C57BL/6J, and DBA/1J) suggests that individual differences for conduction velocity are influenced by genetic differences. Animals of the BALB/cJ and CBA/J strains displayed significantly higher speeds of conduction than those of the C3H/HeJ and A/J strains. Conduction velocities for DBA/1J and C57BL/6J animals did not differ significantly from those of the other strains. Results of three studies allowing assessment of effects of strain, female and male parent, sex, and interactions among these factors support the findings of the strain comparison with regard to genetic variance for peripheral nerve conduction velocity. In addition, these studies indicate heterosis for fast speed of conduction. More detailed genetic analyses are in progress.

Gene differences modify Aschoff's rule in mice

Nocturnal animals typically display an increase in the free running period of circadian rhythms in response to light in direct proportion to intensity. Here we show that gene differences among inbred strains of mice (Mus musculus) modify the effect of constant bright light on the free running period of a circadian rhythm for wheel running activity. Individuals with the shortest free running periods in dim red light showed the greatest increases in period following exposure to bright light. These effects may be due to gene-imposed differences in the response of a circadian pacemaker to light, or they may be due to gene-imposed differences in visual system function that alter perception of light intensity.

Genetic association between progesterone-induced and maternal nesting in mice.

Increases in nesting during pregnancy may be mediated by progesterone in mice. If the behaviors, maternal nesting (MN) and nesting induced by exogenous progesterone (PN), are controlled by the same physiological mechanism, it would be expected that they share a common genetic basis. The present experiment was designed to quantify the extent of genetic association between PN and MN. At Wesleyan University, baseline nesting was measured on females of 4 inbred strains. Subsequently, half of the mice in each strain received progesterone implants. There were significant increases in nesting due to progesterone treatment. After 21 days, implants were removed and nesting levels returned to baseline. The mice were mated and nesting measured throughout pregnancy. The strain rank order was the same for levels of PN and MN. The genetic correlation between PN and MN estimated from analysis of covariance within and between strains was not significantly different from 1.0. These results were replicated at the University of Iowa. The high genetic correlation implies a common physiological mechanism underlying PN and MN.