A Model of Imprinting Evolved from Neurophysiological Concepts

The imprinting process is described by a model evolved from physiological data. For the case of sexual imprinting in zebra finches a second sensitive phase, occurring at sexual maturity, is considered. The first sensitive phase is not restricted exclusively to the imprinting of sexual objects. Properties of the model allow for the explanation of apparent reversibility of imprinting. The same physiological mechanisms involved in the organization of simple neuronal connections are assumed to operate in the highly complex networks associated with imprinting phenomena.     

Equivalence of multibreed animal models and hierarchical Bayes analysis for maternally influenced traits

Background

It has been argued that multibreed animal models should include a heterogeneous covariance structure. However, the estimation of the (co)variance components is not an easy task, because these parameters can not be factored out from the inverse of the additive genetic covariance matrix. An alternative model, based on the decomposition of the genetic covariance matrix by source of variability, provides a much simpler formulation. In this study, we formalize the equivalence between this alternative model and the one derived from the quantitative genetic theory. Further, we extend the model to include maternal effects and, in order to estimate the (co)variance components, we describe a hierarchical Bayes implementation. Finally, we implement the model to weaning weight data from an Angus × Hereford crossbred experiment.

Methods

Our argument is based on redefining the vectors of breeding values by breed origin such that they do not include individuals with null contributions. Next, we define matrices that retrieve the null-row and the null-column pattern and, by means of appropriate algebraic operations, we demonstrate the equivalence. The extension to include maternal effects and the estimation of the (co)variance components through the hierarchical Bayes analysis are then straightforward. A FORTRAN 90 Gibbs sampler was specifically programmed and executed to estimate the (co)variance components of the Angus × Hereford population.

Results

In general, genetic (co)variance components showed marginal posterior densities with a high degree of symmetry, except for the segregation components. Angus and Hereford breeds contributed with 50.26% and 41.73% of the total direct additive variance, and with 23.59% and 59.65% of the total maternal additive variance. In turn, the contribution of the segregation variance was not significant in either case, which suggests that the allelic frequencies in the two parental breeds were similar.

Conclusion

The multibreed maternal animal model introduced in this study simplifies the problem of estimating (co)variance components in the framework of a hierarchical Bayes analysis. Using this approach, we obtained for the first time estimates of the full set of genetic (co)variance components. It would be interesting to assess the performance of the procedure with field data, especially when interbreed information is limited.     

Water transport and IIF parameters for a connective tissue equivalent

Understanding the biophysical processes that govern freezing injury of a tissue equivalent (TE) is an important step in characterizing and improving the cryopreservation of these systems. TEs were formed by entrapping human dermal fibroblasts (HDFs) in collagen or in fibrin gels. Freezing studies were conducted using a Linkam cryostage fitted to an optical microscope allowing observation of the TEs cooled under controlled rates between 5 and 130 degrees C/min. Typically, freezing of cellular systems results in two biophysical processes that are both dependent on the cooling rate: dehydration and/or intracellular ice formation (IIF). Both these processes can potentially be destructive to cells. In this study, the biophysics of freezing cells in collagen and fibrin TEs have been quantified and compared to freezing cells in suspension. Experimental data were fitted in numerical models to extract parameters that governed water permeability, E(Lp) and L(pg), and intracellular ice nucleation, omega(o) and kappa(o). Results indicate that major differences exist between freezing HDFs in suspension and in a tissue equivalent. During freezing, 55% of the HDFs in suspension formed IIF as compared to 100% of HDFs forming IIF in collagen and fibrin TE at a cooling rate of 130 degrees C/min. Also, both the water permeability and the IIF parameters were determined to be higher for HDFs in TEs as compared to cell suspensions. Between the TEs, HDFs in fibrin TE exhibited higher values for the biophysical parameters as compared to HDFs in collagen TE. The observed biophysics seems to indicate that cell-cell and cell-matrix interactions play a major role in ice propagation in TEs.     

Genetic variation in the midcontinental population of sandhill cranes,Grus canadensis

Three subspecies of sandhill crane (Grus canadensis) are recognized in the Midcontinental population, the lesser (Grus c. canadensis), Canadian (G. c. rowani), and greater (G. c. tabida). Blood samples collected on the population's primary spring staging area in Nebraska, U.S.A., were used to resolve the genetic relationship among these subspecies. Phylogenetic analysis of 27 G. canadensis, by DNA sequencing of a 675 bp region of the mtDNA, supports the subspecies designations of G. c. canadensis and G. c. tabida. G. c. rowani individuals were intermediate with each of the other two subspecies. Genetic divergence ranged from 6.5 to 14.5% between G. c. canadensis and G. c. tabida, 0.5 to 6.6% within G. c. canadensis, and 0.1 to 6.0% within G. c. tabida. Sufficient DNA for analysis was obtained from shed feathers indicating a source of genetic material that does not require the capture or sacrifice of the birds. Other genetic markers and methods, including satellite telemetry, are required for obtaining detailed information on crane distributions as needed to establish effective management units for the MCP.     

Bootstrap estimates of the statistical accuracy of thresholds obtained from psychometric functions

Estimates of the accuracy of a threshold obtained from a psychometric function are often based on asymptotic theory. When the number of trails is small, however, these estimates may be untrustworthy. A computer program is described that uses a move reliable bootstrap approach to obtaining estimates of the standard deviation and confidence limits of a threshold and of the slope and spread of the psychometric function for any criterion level of performance.     

Single cell responses from the optic tectum of the zebra finch (Taeniopygia guttata castanotis Gould)

Responses of neurons of the optic tectum, the prominent, highly laminated mesencephalic station of the tectofugal visual pathway in birds, to computer-generated and other visual stimuli were examined in zebra finches. Our study shows that the contralateral retina projects to the tectum in topographic order. The representation of the visual field is tilted against the horizon by 22°. The representation of the contralateral hemifield extends to the ipsilateral side by 15°. Most neurons have receptive fields with excitatory centres of different shapes and inhibitory surround. A new type of neuronal receptive field is described which has an excitatory centre and a surround which is movement sensitive and preferably excited by very small spots. The first type of neurons is mostly located in upper tectal layers, the latter only in deeper layers. Excitatory centre sizes increase with depth, and there is a tendency of smaller receptive fields in the foveal region. The representation of the frontal visual field does not show specializations which could be expected if it were used for fixation of grain during pecking. Our results are in accordance with previous behavioural experiments. Accepted: 30 April 1999