EFFECTS OF THE SEX-LINKED DWARF GENE ON PLASMA SOMATOTROPIC AND THYROID-HORMONE LEVELS AND ON ENERGY-METABOLISM OF LEGHORN AND BROWN EGG-TYPE LAYING HENS AND THEIR RECIPROCAL CROSSES

The effects of the dwarfing gene on somatotrophic (GH, IGF-I) and thyr eotrophic (T-4, T-3) hormones and on energy metabolism in laying hens were studied. Leghorn laying hens (L) and brown-egg laying hens (BE) w ere used to investigate a possible influence of a genetic background o n the expression of the dwarf gene. Birds came from the mating of hete rozygous sires (Dwdw) with dwarf females (dw-) from pure lines (L x L, and BE x BE) and their reciprocal crosses. In this way hens were of a normal growth or carrying the dwarf gene and were full sisters or hal f sibs. Estimation of complete energy balance suggests that the dwarfi ng gene has no significant advantage in improving the net energetic ef ficiency of production in comparison to normal hens. Also, maintenace requirements did not differ significantly among genotypes. Plasma conc entrations of the assayed hormones followed the pattern generally foun d in juvenile dwarf chickens. GH levels were low in normal-sized hens of all genotypes. The presence of the dwarf gene increased GH levels i n leghorn hens and both reciprocal crosses. However,in dwarf brown-egg laying hens, GH levels were low and not different from normal hens. H eterosis was obtained in dwarfs only for GH and to a lesser extent for IGF-I. Superiority over purebreds was quite high for GH in both recip rocal dwarf crosses, yielding significant positive heterosis. Hens of all genotypes bearing the dwarf gene had significantly reduced plasma IGF-I and T-3 concentrations compared to their normal-sized counterpar ts whereas the opposite was true for plasma T-4 levels. Results sugges t that the dwarf gene fan be differentially expressed in relation to t he genetic back ground.