Population size and selection intensity effects on long-term selection response in mice

Area of Science:

• Animal Breeding and Genetics
• Quantitative Genetics
• Population Genetics

Background:

• Effective population size (Ne) and selection intensity (i) are crucial for long-term genetic gain.
• Understanding the interplay of Ne, i, inbreeding depression, and genetic drift is vital for optimizing animal breeding programs.
• Previous studies have explored selection responses, but long-term effects across varying Ne and i require further investigation.

Purpose of the Study:

• To evaluate the long-term response to within full-sib family selection for postweaning gain under different effective population sizes (Ne) and selection intensities (i).
• To assess the impact of inbreeding depression and genetic drift on selection response in lines with varying Ne.
• To investigate the effectiveness of crossbreeding in mitigating negative effects and enhancing selection response.

Main Methods:

• Lines with different Ne (4, 8, 16 parental pairs) and i (50% - I, 25% - M) were subjected to long-term selection for postweaning gain.
• Realized heritabilities (hR-2) and genetic correlations were estimated across two phases of selection (0-14 and >14 generations).
• Crossbreeding experiments were conducted between selected lines and a control line to evaluate the effects of inbreeding depression and genetic drift.

Main Results:

• Realized heritabilities declined significantly after 14 generations, particularly in lines with smaller Ne.
• Crossbred lines exhibited higher hR-2 compared to the second selection phase in some lines, suggesting improved response.
• Total response to selection was significantly lower in lines with lower Ne and higher in lines with higher Ne, independent of selection intensity.

Conclusions:

• Long-term selection for postweaning gain is challenged by declining heritability, likely due to inbreeding depression and genetic drift, especially in smaller effective population sizes.
• Crossbreeding can partially overcome these limitations by alleviating inbreeding effects.
• Both inbreeding depression and genetic drift were found to be equally important factors influencing the differences in total response among lines with varying Ne.