Isogenic Individuals Research Articles

An expression microarray approach for the identification of metastable epialleles in the mouse genome

Genetic loci displaying environmentally responsive epigenetic marks, termed metastable epialleles, offer a solution to the paradox presented by genetically identical yet phenotypically distinct individuals. The murine viable yellow agouti (Avy) metastable epiallele exhibits stochastic DNA methylation and histone modifications associated with coat color variation in isogenic individuals. The distribution of Avy variable expressivity shifts following maternal nutritional and environmental exposures. To characterize additional murine metastable epialleles, we utilized genome-wide expression arrays (N = 10 male individuals, 3 tissues per individual) and identified candidates displaying large variability in gene expression among individuals (Vi = inter-individual variance), concomitant with a low variability in gene expression across tissues from the three germ layers (Vt = inter-tissue variance), two features characteristic of the Avy metastable epiallele. The CpG island in the promoter of Dnajb1 and two contraoriented ERV class II repeats in Glcci1 were validated to display underlying stochasticity in methylation patterns common to metastable epialleles. Furthermore, liver DNA methylation in mice exposed in utero to 50 mg bisphenol A (BPA)/kg diet (N = 91) or a control diet (N = 79) confirmed environmental lability at validated candidate genes. Significant effects of exposure on mean CpG methylation were observed at the Glcci1 Repeat 1 locus (p < 0.0001). Significant effects of BPA also were observed at the first and fifth CpG sites studied in Glcci1 Repeat 2 (p < 0.0001 and p = 0.004, respectively). BPA did not affect methylation in the promoter of Dnajb1 (p = 0.59). The characterization of metastable epialleles in humans is crucial for the development of novel screening and therapeutic targets for human disease prevention.

Evolution of Robustness to Noise and Mutation in Gene Expression Dynamics

Phenotype of biological systems needs to be robust against mutation in order to sustain themselves between generations. On the other hand, phenotype of an individual also needs to be robust against fluctuations of both internal and external origins that are encountered during growth and development. Is there a relationship between these two types of robustness, one during a single generation and the other during evolution? Could stochasticity in gene expression have any relevance to the evolution of these types of robustness? Robustness can be defined by the sharpness of the distribution of phenotype; the variance of phenotype distribution due to genetic variation gives a measure of ‘genetic robustness’, while that of isogenic individuals gives a measure of ‘developmental robustness’. Through simulations of a simple stochastic gene expression network that undergoes mutation and selection, we show that in order for the network to acquire both types of robustness, the phenotypic variance induced by mutations must be smaller than that observed in an isogenic population. As the latter originates from noise in gene expression, this signifies that the genetic robustness evolves only when the noise strength in gene expression is larger than some threshold. In such a case, the two variances decrease throughout the evolutionary time course, indicating increase in robustness. The results reveal how noise that cells encounter during growth and development shapes networks' robustness to stochasticity in gene expression, which in turn shapes networks' robustness to mutation. The necessary condition for evolution of robustness, as well as the relationship between genetic and developmental robustness, is derived quantitatively through the variance of phenotypic fluctuations, which are directly measurable experimentally.

Brain weights correlate with behavioral parameters in individual inbred mice housed in a common and enriched environment

Fifty 8-week-old Balb/c mice were individually identified and housed together in a large and enriched environment for 5 months. Maze and open field exploration, response to an aversive noise, swimming, and induced grooming tests were applied to each mouse in an initial search for possible relationships between brain morphology and spontaneous behavior in isogenic individuals living in a complex social and physical environment. The tasks generated 39 quantitative behavioral indices which include locomotion, rearing, still, and grooming bout frequencies, latencies, total, and mean bout durations. At the end of the tests, the 7-month-old mice were sacrificed and the fresh weights of their whole brain, cerebellum, brain stem, diencephalon, telencephalon, and prosencephalon were rapidly obtained. Behavioral data have wide variations and do not adjust to normal population distributions. Means of the same parameter differ between tests. A Spearman correlation matrix of all data yielded many significant correlations between indices of the same task which can be interpreted in terms of time budget and sequence probability. Significant correlations between indices of different tests suggest diverse emotionalities, exploratory strategies, and motor skills. The correlations between body and brain weights and among separate brain regions were not significant. There were several low but significant correlations between brain weights and behavioral indices. Such correlations, the resulting factors, and significant behavioral differences between mice with large and small brains suggest that mice displaying low motor activity in novel environments have larger brains and forebrain/hindbrain ratios than mice with high activity, and that animals with high scores of some specific behaviors have larger brain areas physiologically related to such behaviors.(ABSTRACT TRUNCATED AT 250 WORDS)

Relationship between the temperature given at the time of fertilized egg formation and bisexual reproduction pattern in the deriving strain of the rotifer Brachionus plicatilis.

Temperature given at the time of fertilized (resting) egg formation in the rotifer Brachionus plicatilis was discussed, from a point of view that it may cause great difference in bisexual reproductivity among the deriving strains. Fertilized eggs were produced at five temperatures (17, 20, 25, 27 and 30°C) in sea water. Neonates from the eggs were employed as stem mothers of respective experimental strains. Every strain consisted of isogenic individuals through keeping parthenogenesis, which was ensured by the individual culture in 0.1ml sea-water medium. Media were renewed at regular intervals of 12, 24 or 36 hours at 30, 25 or 20°C respectively in order to prevent the accumulation of metabolites which is an exciting cause of bisexual reproduction.The temperature given in the process of fertilized egg formation determines the bisexual reproduction pattern of deriving strains. Among the strains derived from the fertilized eggs which were produced at high temperature over 25°C, the appearance rate of mictic females has no relation to temperature. Among those, however, derived from the eggs originating in low temperature below 25°C, it varies inversely with temperature below 25°C, but not affected above 25°C.

Discordant abnormalities in monozygotic twins

Congenital abnormalities are more often seen in monozygotic twins, especially with a monochorionic placenta, than in dizygotic twins. Although abnormalities are more frequently concordant in monozygotic than in dizygotic twins, discordance for the abnormalities among the former is strikingly frequent. Among 24 consecutive pairs of monochorionic (i.e., monozygotic) twins, five sets were discordant for various phenotypic traits. During the same period, among 79 pairs of dichorial twins, an abnormality was noted in only one pair, and this was discordant. The twin method has not been extensively applied to the study of the relative contributions of heredity and environment in the development of congenital disease. Its use in such studies is described and the various factors which may account for intrapair differences in isogenic individuals are discussed.