Correlated responses to long-term selection for clutch length in dwarf brown-egg layers carrying or not carrying the naked neck gene

Unite Mixte de Recherches Genetique et Divaersite Animales Institut National de la Recherche Agronomique/ Institut National Agronomique Paris-Grignon, 78352 Jouy-en-Josas Cedex, France.

Two dwarf brown-egg layer lines, differing in their genotype for the naked neck gene (NA), line L2 (NA*NA/*NA) and line L1 (NA*N/*N), have been selected for 16 generations for increased average clutch length. A control line from the same base population, dwarf and segregating for the NA gene, was maintained by random mating. Genetic parameters were estimated by a multivariate derivative-free restricted maximum likelihood procedure, and the NA gene effect was estimated within the unselected control line. The studied traits included clutch traits, egg production traits, abnormal eggs, egg weight at 36 wk, and BW at 42 wk. The average clutch length, egg number, and maximum clutch length were normalized using the Box-Cox transformation. In response to 16 generations of direct selection for increased average clutch length, other egg production traits, such as laying rate and total egg number, have been indirectly improved in a dwarf layer genetic background. The estimated heritabilities were 0.406 to 0.424 for transformed average clutch length (TCL), 0.373 to 0.411 for transformed egg number (TEN), 0.529 to 0.559 for age at first egg (AFE), 0.275 to 0.282 for laying rate (LR), 0.455 for dutch number (CN), and 0.319 for the number of double-yolked eggs (DYEN). The TCL had high genetic correlations with TEN (0.777), LR (0.863), maximum clutch length (0.902), and CN (-0.845). Selection for increased average clutch length was an effective method for increasing egg production. Line L2 showed a higher egg weight than L1, which indicates that the combined effect of NA and DW genes was favorable to maintain egg weight when egg number could be improved. Line L1 showed a higher number of DYEN, suggesting that the regulation of follicular maturation was changed in this line.