Development and evolution of hidden regulators: selective breeding for an infantile phenotype.

Mother-infant separation in the rat has been used as an analytical tool to reveal biosocial processes underlying infant physiology and behavior. The same strategy has guided a project in which selective breeding for an infantile behavior has provided insights into how biological systems become recruited and integrated as expressions of temperamental affective responses. Two lines of rats (High and Low USV lines) were selectively bred based on rates of USV emission to maternal separation and isolation at postnatal day (P) 10. After many generations of breeding, the High and Low lines show widespread and distinctly different profiles of physiology and behavior in the first 3 weeks of life. Insights gained from longitudinal studies suggest that selection may work by reorganizing developmental processes, not just a given trait, over the postnatal period. As animal models, the lines have the potential to provide valuable tools for understanding developmental mechanisms underlying genetic and developmental risk for depression/anxiety syndromes in children and adults.