Abstract
In order to investigate the possibility of using the dwarf gene for egg production, two dwarf brown-egg laying lines were selected for 16 generations on average clutch length; one line (L1) was normally feathered and the other (L2) was homozygous for the naked neck gene NA. A control line from the same base population, dwarf and segregating for the NA gene, was maintained during the selection experiment under random mating. The average clutch length was normalized using a Box-Cox transformation. Genetic variability and selection response were estimated either with the mixed model methodology, or with the classical methods for calculating genetic gain, as the deviation from the control line, and the realized heritability, as the ratio of the selection response on cumulative selection differentials. Heritability of average clutch length was estimated to be 0.42 ± 0.02, with a multiple trait animal model, whereas the estimates of the realized heritability were lower, being 0.28 and 0.22 in lines L1 and L2, respectively. REML estimates of heritability were found to decline with generations of selection, suggesting a departure from the infinitesimal model, either because a limited number of genes was involved, or their frequencies were changed. The yearly genetic gains in average clutch length, after normalization, were estimated to be 0.37 ± 0.02 and 0.33 ± 0.04 with the classical methods, 0.46 ± 0.02 and 0.43 ± 0.01 with animal model methodology, for lines L1 and L2 respectively, which represented about 30% of the genetic standard deviation on the transformed scale. Selection response appeared to be faster in line L2, homozygous for the NA gene, but the final cumulated selection response for clutch length was not different between the L1 and L2 lines at generation 16.
Highlights
The sex-linked dwarf gene, DW, has been described for many years [19] and is known to improve food efficiency and egg production in dam lines used for broiler production, as reviewed by Mérat [24]
The L1 selected line was normally feathered, homozygous for the non-naked neck allele (NA*N), the L2 line was homozygous for the naked neck allele (NA*NA), and control line C was segregating for the three possible genotypes at the NA locus
After pooling the data of G6, G8 and G10 to G16 of the control line, the analysis showed a positive effect of the NA gene on the mean and on the coefficient of variation of the clutch length with 3.09, 3.28, 3.34 and 35.0%, 41.2%, 42.8%, for NA*N/NA*N, NA*NA/NA*N and NA*NA/NA*NA genotypes, respectively
Summary
The sex-linked dwarf gene, DW, has been described for many years [19] and is known to improve food efficiency and egg production in dam lines used for broiler production, as reviewed by Mérat [24]. In egg-laying strains, the DW gene has been shown to decrease egg production [6], and more clutch length [1]. Selection for clutch length can be proposed as a specific approach for improving egg production of dwarf layers. Clutch length has been shown to be moderately to highly heritable, with a high genetic correlation with egg number [4,22,39]. A selection experiment was initiated in 1985, with the aim to improve clutch length in two lines of dwarf brown-egg layers, differing by their genotype for the NA gene. In addition to the investigation of the genetic variability of clutch length in dwarf layers, this experiment made it possible to examine the effect of the combination of two major genes, DW and NA, on selection response
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