Genetics of litter size in three maternal lines of rabbits: Repeatability versus multiple-trait models M. Piles,* 1 M. L. Garcı ´a,† O. Rafel,* J. Ramon,* and M. Baselga‡ *Unitat de Cunicultura, IRTA, Torre Marimo ´n s/n., 08140 Caldes de Montbuı ´, Barcelona, Spain; †Divisio ´n de Produccio ´n Animal, Departamento de Tecnologı ´a Agroalimentaria, Universidad Miguel Herna ´ ndez, Ctra. Beniel Km. 3.2, Orihuela 03312, Alicante, Spain; and ‡Departamento de Ciencia Animal, Universidad Polite ´cnica de Valencia, Camino de Vera, 14, 46071 Valencia, Spain ABSTRACT: Variance components were estimated in 3 lines of rabbits selected for litter size at weaning (A, Prat, and V) to test one of the assumptions of the models used for selection: that litter size data at differ- ent parities are repeated measurements of the same trait. Multiple-trait analyses were performed for the total number of kits born (TB), the number of kits born alive (BA), and the number of kits weaned (NW) per litter. Estimates were obtained by REML in multivari- ate analyses, including all of the information of the selection criteria, under a repeatability model or a mul- tiple-trait model, considering litter size at the first, sec- ond, and subsequent parities as different traits. Models included the fixed effects of the physiological status of the female and the year-season of mating day, buck and doe random permanent environmental effects, and doe additive genetic effects. Results indicated that pro- lificacy was determined mainly by doe components and that the service sire had a very small effect. Heritabilit- ies for the first and second parities were greater than the estimates obtained under the repeatability model (0.04 to 0.14 for the repeatability model). In the A and V lines, similar values of heritability were found at the Key words: genetic parameter, litter size, parity, paternal component, rabbit ©2006 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2006. 84:2309–2315 doi:10.2527/jas.2005-622 INTRODUCTION Litter size at birth or weaning has been the objective of selection in several experiments involving rabbit pop- ulations (Baselga et al., 1992; de Rochambeau et al., 1994, 1998; Go ´mez et al., 1996). However, response to selection, when estimated, has been low (de Rocham- beau et al., 1994; Garcı ´a and Baselga, 2002a,b). One of 1 Corresponding author: miriam.piles@irta.es Received October 28, 2005. Accepted April 10, 2006. 2309 first and second parities, but in the Prat line heritability at the second parity was always greater than at the first and greater parities (values of 0.21, 0.17, and 0.15 for TB, BA, and NW, respectively, in second parities of the Prat line). Genetic correlations between the same traits at different parities were approximately 0.8 for all traits in line A, but much lower in the other 2 lines. On average, the values were 0.64 for TB, 0.48 for BA, and 0.39 for NW between the first and second parities, and 0.65 for TB, 0.56 for BA, and 0.45 for NW between the first and third and greater parities. Genetic correla- tions between the second and greater parities showed the greatest values (approximately 0.8) in lines A and Prat for all traits, but they were lower in line V (0.63 for BA and 0.37 for NW). The heterogeneity of heritabilities and genetic correlations between parities lower than 0.9 suggests that litter size at different parities could be considered as different traits when genetic evalua- tions are performed. However, when the accuracies of predicted breeding values under a multiple-trait and a repeatability model were calculated, assuming the first to be the true model, the values obtained were nearly the same for all traits in all lines. the arguments proposed to explain this result is that one of the assumptions of the models used for selec- tion—that litter size at different parities are repeated measurements of the same trait—is not correct. In rab- bits, Baselga et al. (1992) found evidence of differences in heritabilities and genetic correlations less than 1 for litter size at different parities in 2 populations selected for litter size at weaning. Nevertheless, the estimation of variance components was approached using a statis- tical method that did not allow all of the available infor- mation to be optimally used; therefore, the results ob- tained could have been biased. In swine, several au- thors have recommended considering prolificacy at Published December 8, 2014