Selection differentials and selection coefficients

Proof is offered that s ig for all forms of selection where fitness is a nondecreasing function of a normally distributed phenotype (called fitness potential); s is selection coefficient, i is standard selection differential, and g is standard differential effect of a locus genotype.

Evidence is presented that the cost of selection does not limit genic polymorphism; that truncation selection is not necessary for high cost-efficiency; and that opposing directions of selection in a heterogeneous environment do not reduce cost-efficiency critically.

     

Strategies of adaptation: interpopulation selection differentials

Spatial and temporal variation of selection intensities has been analyzed using published data on Crow's indices in 278 human populations, representing various anthropological types, religions, cultures and a wide spectrum of environmental conditions. Statistical analysis has shown that populations with different types of economy have different adaptive strategies. In tribal populations both components of selection-differential fertility and differential mortality-have equal contributions, vary according to ecological factors and show no decline with time. Urban populations of the industrially developed countries show dramatic selection relaxation; the second component becomes negligible due to a sharp decrease in childhood mortality; fertility and its variance are reducing due to cultural factors (family planning).     

Estimation of phenotypic selection differentials for predicting genetic responses to ratio-based selection

Ratios of the phenotypic values of two traits may be used as selection criteria in animal and plant breeding to improve the ratio traits themselves or to effect changes in their two component (numerator and denominator) traits. Prediction of genetic responses to ratio-based selection would facilitate quantitative analysis and evaluation of selection based on ratios. Methods for predicting such responses are derived and presented here. They employ expressions for the truncation value of a ratio and for the phenotypic selection differentials of the numerator and denominator traits. The derivation of these expressions is based upon the assumption that the phenotypic values of each of these traits are normally distributed. Worked examples relating to livestock and crop improvement are included to demonstrate how responses to selection for ratios may be predicted.     

Neuronal glycosylation differentials in normal, injured and chondroitinase-treated environments

Glycosylation is found ubiquitously throughout the central nervous system (CNS). Chondroitin sulphate proteoglycans (CSPGs) are a group of molecules heavily substituted with glycosaminoglycans (GAGs) and are found in the extracellular matrix (ECM) and cell surfaces. Upon CNS injury, a glial scar is formed, which is inhibitory for axon regeneration. Several CSPGs are up-regulated within the glial scar, including NG2, and these CSPGs are key inhibitory molecules of axonal regeneration. Treatment with chondroitinase ABC (ChABC) can neutralise the inhibitory nature of NG2. A gene expression dataset was mined in silico to verify differentially regulated glycosylation-related genes in neurons after spinal cord injury and identify potential targets for further investigation. To establish the glycosylation differential of neurons that grow in a healthy, inhibitory and ChABC-treated environment, we established an indirect co-culture system where PC12 neurons were grown with primary astrocytes, Neu7 astrocytes (which overexpress NG2) and Neu7 astrocytes treated with ChABC. After 1, 4 and 8 days culture, lectin cytochemistry of the neurons was performed using five fluorescently-labelled lectins (ECA MAA, PNA, SNA-I and WFA). Usually α-(2,6)-linked sialylation scarcely occurs in the CNS but this motif was observed on the neurons in the injured environment only at day 8. Treatment with ChABC was successful in returning neuronal glycosylation to normal conditions at all timepoints for MAA, PNA and SNA-I staining, and by day 8 in the case of WFA. This study demonstrated neuronal cell surface glycosylation changes in an inhibitory environment and indicated a return to normal glycosylation after treatment with ChABC, which may be promising for identifying potential therapies for neuronal regeneration strategies.     

Reduction of selection differentials in finite populations with a nested full-half sib family structure

The effect of family structure is of increasing importance in modern breeding schemes, because increased intraclass correlations between relatives due to improved breeding value estimation methods use all family information, and increased family sizes are possible with improved reproduction rates. In addition, reduction of the generation intervals in modern breeding schemes leads to increased intraclass (family) correlations, because young animals have little information on individual or on progeny performance. This paper derives an approximation for the selection differential in a population divided into families. The result is then extended to an approximation for the selection differentials in populations that are divided into full sib families within paternal half sib families. The approximation is compared with Monte Carlo results, from which it is concluded that the approximation is satisfactory (i.e., rarely more than 5% in error). In some practical situations the approximation is shown to be not more than 2% in error. With high intraclass correlations and few animals selected, the reduction of the selection differentials is maximal. When breeding values are based on family information and the family structure is not accounted for, overestimation of the selection differentials can be up to 61%.     

Heterogeneous Variances in Repeated Measures

A general approach to repeated measures analysis described in MCGILCHRIST and CULLIS (1991) is extended to general linear models in which there is a homogeneous correlation structure such as a stationary process but the variances are heterogeneous over time. Models for the variance heterogeneity are suggested and results are applied to data examining the effect of enzootic pneumonia on young pigs.