No description available.

No description available.

Due to the importance of insulin-like growth factor-I (IGF-I) in regulating many physiological and metabolic processes, the IGF-I gene was chosen as a candidate gene to study trait associated polymorphisms in chickens. A PstI restriction fragment length polymorphism (RFLP) was detected at the 5' region of the gene and mapped to about 7 Kb upstream of the published promoter sequence. Analysis for association of the marker with traits of economic importance in an unselected, random-bred population of 359 White Leghorns revealed a significant association with egg weight (P ≤ 0.05) and specific gravity (P ≤ 0.05). There was also a trend for association with juvenile body weight (P = 0.08) but not adult body weight. For egg weight the PstI (-/-) genotype was associated with lower egg weight as compared to the heterozygote or the PstI (+/+) genotype. The PstI marker also was found to be significantly associated with differences in trait correlations. A regulatory loop that co-ordinated feed consumption, body weight, egg weight and rate of egg laying was detected, and this regulatory loop differed among the IGF-I genotypic classes. In the PstI (+/-) genotype, the degree of correlation between some of the traits was time dependent, while in the PstI (+/+) genotype it remained constant through the different periods of measurement. Since IGF-I is known to play an important role in immune functions, the association of the IGF-I genotypes with immune traits was also investigated. A significant association was found for delayed type hypersensitivity, interferon production and T-cell count (P ≤ 0.05). Individuals belonging to the PstI (+/-) genotypic class exhibited higher immune response, reflected by the delayed type hypersensitivity reaction and antibody the interactive effects of marker genotypes in the GH, GH-receptor and IGF-I genes on traits and trait correlations indicated that the three are part of an epistatic pathway, wherein the phenotypic consequences of

Chicken genomic DNA containing 5 kb of the 5$ sp prime$ end of the growth hormone (GH) receptor gene and 12 kb of the region up-stream was cloned and a restriction map was constructed. Using subcloned fragments as probes, a HindIII polymorphism was detected in both egg layer and in meat-type chickens. This polymorphic site was mapped at 7 kb up-stream of the coding region of the GH-receptor gene and a PCR assay for the polymorphism was developed to facilitate genotyping of large numbers of chickens. / Alleles of the GH-receptor gene and the GH gene were analyzed for association with traits in chicken strains of different genetic origins. In egg layers, association was significant for juvenile body weight, egg weight, feed consumption and feed efficiency for egg mass (P $<$ 0.05). In meat-type chickens, the GH-receptor allele associated with high juvenile body weight in egg layers was co-selected with leanness. A comparison of the genotype classes revealed that for several traits there was significant interaction between the GH and GH-receptor genotype. The results indicated that there are variants of the genes of the GH-axis which affect traits in White Leghorns and that the effect of a genetic variation in one gene may depend on the variation in another gene.

No description available.

Five experiments were conducted to study genetic influences on growth and early egg production in chickens. Parental lines included White Plymouth Rocks divergently selected 25 generations for high (H) and for low (L) 56-day body weight and a closed population of Jersey Black Giants (JJ). F₁ generation crosses (sire listed first) included HL, LH, JL, and JH, and an F₂ generation from HL x HL matings. Comparisons also include dwarf and nondwarf chickens from the H and L lines. Symbols for dwarfs and nondwarfs within the high line were HD and HN and within the low line were LD and LN. Growth patterns of dwarfs more closely approximated that of the H than that of the L line, regardless of whether measured in a chronological or physiological context. Although body components were usually smaller for dwarfs than their within-line nondwarf counterparts, on a relative body weight basis, dwarfs were more uniform than nondwarfs. Dwarfs were also more uniform than normals for age at first egg. The dw allele reduced yolk formation and ovulation rate, resulting in less erratic ovulation and lowering the incidence of defective eggs. Differences between reciprocal crosses were minor as were recombination effects for the traits measured. There was little, if any, evidence of heterosis for body weight and skeletal and lean traits prior to 57 days of age, at sexual maturity, or 60 days after the onset of lay. When present, heterosis was general for most traits measured with the exception of those associated with adiposity which were population dependent. Modest heterosis was observed for the age of inflection of the growth curve, age at sexual maturity, egg production traits, and body weight change during a 60-day laying period. Hybrid vigor for egg production traits resulted in more normal eggs via regularity of yolk development and synchronization of ovulation. The onset of sexual maturity was accompanied by the achievement of a population-dependent body composition, which was generally independent of age and body weight. Under ad libitum feeding, lean carcass traits appeared to be more critical to the onset of sexual maturity than adiposity, as evidenced by greater differences between mature and immature pullets for the former than for the latter traits. The exception to this pattern was the line selected for low juvenile body weight where adiposity traits appeared to be primary. / Ph. D.

The hypothalamic hormones, thyrotrophin-releasing hormone (TRH) and gonadotrophin-releasing hormone (GnRH), play pivotal roles in the growth and sexual maturation of chickens. In chickens, TRH regulates the release and synthesis of thyrotrophin (TSH) and also acts as a growth hormone-releasing factor. GnRH stimulates the release and synthesis of gonadotrophins (LH and FSH). TRH and GnRH are released and stored in the median eminence, and both hormones are transported into the pituitary gland via the hypophysial portal circulation. TRH and GnRH exert their physiological functions by binding to their specific receptors (TRH receptor and GnRH receptor, respectively) on the surface of cells in the pituitary gland. The activated receptors couple to guanine nucleotide-binding regulatory proteins (G proteins), Gq and/or G11, which in turn triggers the secondary messenger [1,2- diacylglycerol (DAG) and inositoltrisphosphate (IP3)] signalling cascade. The signalling generates the physiological effects of the hormones. The TRH-R and GnRH-R are members of G-protein coupled receptor (GPCR) family. The objective of this thesis was to clone and characterise the chicken TRH and GnRH receptors as useful tools for investigating the regulatory roles of TRH and GnRH receptors in the growth and sexual maturation of chickens. In addition, sequence information of the receptors would potentially assist in elucidating the binding sites and the molecular nature of the processes involved in receptor activation.

M. S.

M.S.

The heritable variation of body weight in two lines of chickens divergently selected for high and low juvenile body weight, as determined from parental line, F₁, F₂ and backcross generations, was principally additive. Feed intake during the first few weeks post hatching was influenced by potence in the direction of the high-weight line, while no comparable pattern was noted for early water intake. Heterosis was observed for feed efficiency which was associated with an equivalent negative heterosis for percentage carcass fat. Feed intake of chicks from the high-weight line and reciprocal crosses were restricted to that of chicks from the low-weight line. This restriction resulted in a greater depression in body weight of the high-weight line than of cross chicks. In contrast, a comparable restriction of water consumption resulted in a similar (10%) depression of body weight among both high-weight line and cross chicks. The effect of feed and water restriction on feed:water ratios was dependent upon mating type. The amount of feed introduced into the crop of chicks from the high-weight line via overfeeding was similar to their ad libitum intake. Chicks from the low-weight line could be overfed at earlier ages and to a greater degree than either high-weight or cross chicks. The degree of overfeeding was associated with the relative size of certain gastrointestinal components. Differences among populations were reflected in concomitant changes in body weight and carcass fat. Comparisons among the parental line and F₁ progeny provided evidence for nonadditive genetic variation for hedonic sensitivity towards quinine sulfate and dextrose, though in opposite directions. Additive genetic variation appeared to influence the preference ratios for both stimuli at super-threshold concentrations. These data suggest the absence of a simple relationship among growth, feed intake control and body composition. It is hypothesized that in addition to their commonality, different genetic systems exist for the expression of high and low body weight as well as their various correlates. The implications of these systems are discussed within the context of feed intake control and future genetic changes in growth. / Ph. D.