Complex traits analysis of chicken growth using targeted genetical genomics

Dissecting the genetic control of complex trait variation remains very challenging, despite many advances in technology. The aim of this study was to use a major growth quantitative trait locus (QTL) in chickens mapped to chromosome 4 as a model for a targeted approach to dissect the QTL. We applied a variant of the genetical genomics approach to investigate genome-wide gene expression differences between two contrasting genotypes of a marked QTL. This targeted approach allows the direct quantification of the link between the genotypes and the genetic responses, thus narrowing the QTL-phenotype gap using fewer samples (i.e. microarrays) compared with the genome-wide genetical genomics studies. Four differentially expressed genes were localized under the region of the QTL. One of these genes is a potential positional candidate gene (AADAT) that affects lysine and tryptophan metabolism and has alternative splicing variants between the two genotypes. In addition, the lysine and glycolysis metabolism pathways were significantly enriched for differentially expressed genes across the genome. The targeted approach provided a complementary route to fine mapping of QTL by characterizing the local and the global downstream effects of the QTL and thus generating further hypotheses about the action of that QTL.

The chicken polydactyly (Po) locus causes allelic imbalance and ectopic expression of Shh during limb development

Point mutations in the intronic ZRS region of Lmbr1, a limb specific cis-regulatory element of Sonic hedgehog (Shh), are associated with polydactyly in humans, cats, and mice. We and others have recently mapped the dominant preaxial polydactyly (Po) locus in Silkie chickens to a single nucleotide polymorphism (SNP) in the ZRS region. Using polymorphisms in the chicken Shh sequence, we confirm that the ZRS region directly regulates Shh expression in the developing limb causing ectopic Shh expression in the anterior leg, prolonged Shh expression in the posterior limb, and allelic imbalance between wt and Slk Shh alleles in heterozygote limbs. Using Silkie legs, we have explored the consequences of increased Shh expression in the posterior leg on the patterning of the toes, and the induction of preaxial polydactyly.

Overlap of quantitative trait loci for early growth rate, and for body weight and age at onset of sexual maturity in chickens

Critical age, weight and body composition have been suggested as necessary correlates of sexual maturity. A genome scan to identify quantitative trait loci (QTL) for age and body weight at first egg (AFE and WFE) was conducted on 912 birds from an F2broiler–layer cross using 106 microsatellite markers. Without a covariate, QTL for body WFE were detected on chromosomes 2, 4, 8, 27 and Z and a single QTL for AFE was detected on chromosome 2. With AFE as a covariate, additional QTL for body WFE were found on chromosomes 1 and 13, with abdominal fat pad as covariate a QTL for body WFE was found on chromosome 1. With body WFE as covariate, additional QTL for AFE were found on chromosomes 1, 3, 4, 13 and 27. The QTL generally acted additively and there was no evidence for epistasis. Consistent with the original line differences, broiler alleles had positive effects on body WFE and negative effects on AFE, whereas the phenotypic correlation between the two traits was positive. The mapped QTL for body WFE cumulatively accounted for almost half the body weight difference between the chicken lines at puberty. Overlapping QTL for body WFE and body weight to 9 weeks of age indicate that most QTL affecting growth rate also affect body WFE. The co-localisation of QTL for body weight, growth and sexual maturity suggests that body weight and growth rate are closely related to the attainment of sexual maturity and that the genetic determination of growth rate has correlated effects on puberty.

Gallin; an antimicrobial peptide member of a new avian defensin family, the ovodefensins, has been subject to recent gene duplication

Background

Egg white must provide nutrients and protection to the developing avian embryo. One way in which this is achieved is an arsenal of antimicrobial proteins and peptides which are essentially extensions of the innate immune system. Gallin is a recently identified member of a family of peptides that are found in egg white. The function of this peptide family has not been identified and they are potentially antimicrobial.

Results

We have confirmed that there are at least 3 forms of the gallin gene in the chicken genome in 3 separate lines of chicken, all the forms are expressed in the tubular cells of the magnum region of the oviduct, consistent with its presence in egg white. mRNA expression levels are in the order 10,000 times greater in the magnum than the shell gland. The conservation between the multiple forms of gallin in the chicken genome compared with the conservation between gallin and other avian gallin like peptides, suggests that the gene duplication has occurred relatively recently in the chicken lineage. The gallin peptide family contains a six cysteine motif (C-X₅-C-X₃-C-X₁₁-C-X₃-C-C) found in all defensins, and is most closely related to avian beta-defensins, although the cysteine spacing differs. Further support for the classification comes from the presence of a glycine at position 10 in the 41 amino acid peptide. Recombinant gallin inhibited the growth of Escherischia coli (E. coli) at a concentration of 0.25 μM confirming it as part of the antimicrobial innate immune system in avian species.

Conclusions

The relatively recent evolution of multiple forms of a member of a new defensin related group of peptides that we have termed ovodefensins, may be an adaptation to increase expression or the first steps in divergent evolution of the gene in chickens. The potent antimicrobial activity of the peptide against E. coli increases our understanding of the antimicrobial strategies of the avian innate immune system particularly those of the egg white and the evolution of the defensin family. The potential of this peptide and others in the family can now be investigated in a number of novel antimicrobial roles.

Chronic stress alters glucocorticoid receptor and mineralocorticoid receptor mRNA expression in the European starling (Sturnus vulgaris) brain

Although the glucocorticoid response to acute short-term stress is an adaptive physiological mechanism that aids in the response to and survival of noxious stimuli, chronic stress is associated with a negative impact on health. In wild-caught European starlings (Sturnus vulgaris), chronic stress alters the responsiveness of hypothalamic-pituitary-adrenal (HPA) axis as measured by the acute corticosterone response. In the present study, we investigated potential underlying neuroendocrine mechanisms by comparing glucocorticoid receptor and mineralocorticoid receptor mRNA expression in the brains of chronically and nonchronically-stressed starlings. Hypothalamic paraventricular nucleus, but not hippocampal, glucocorticoid receptor mRNA expression in chronically-stressed birds was significantly lower compared to controls, suggesting changes in the efficacy of corticosterone negative feedback. In addition, chronically-stressed birds showed a significant decrease in hippocampal MR mRNA expression. Together, these results suggest that chronic stress changes the brain physiology of wild birds and provides important information for the understanding of the underlying mechanisms that result in dysregulation of the HPA axis in wild animals by chronic stress.

New hypotheses on the function of the avian shell gland derived from microarray analysis comparing tissue from juvenile and sexually mature hens

Activation of the shell gland region of the avian oviduct is mediated by ovarian steroids. To understand more extensively how shell glands are maintained and function, we have compared gene expression in the shell glands from juvenile and laying hens using a chicken cDNA microarray. Average expression profiles of juvenile and sexually mature shell glands were compared resulting in the identification of 266 differentially regulated genes. Reverse transcription quantitative polymerase chain reaction confirmed expression differences. The differentially expressed genes included several with known involvement in shell gland function, including ion transport and shell matrix proteins. There were also many unpredicted differentially expressed genes, and for some we propose hypotheses for their functions. These include those encoding (a) osteoprotegerin, a decoy death receptor for receptor activator of nuclear factor NFkB ligand (RANKL) and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), that in the shell gland, may prevent apoptosis and/or may have an endocrine effect by preventing RANKL's action on bone osteoclasts that mobilize stored calcium; (b) prostatic acid phosphatase (ACPP) and prostate stem cell antigen (PSCA) that could play a role in sperm physiology within the shell gland; (c) urea transporter (SLC14A2) that could provide a novel anti-microbial defence; (d) bactericidal/permeability-increasing protein-like 2 (BPIL2), and other potential anti-microbials that have not previously been documented in the chicken. These new hypotheses, if borne out experimentally, will lead to a greater understanding of shell gland function including the processes involved in eggshell formation and anti-microbial activity.

The chicken type III GnRH receptor homologue is predominantly expressed in the pituitary, and exhibits similar ligand selectivity to the type I receptor

Two GnRH isoforms (cGnRH-I and GnRH-II) and two GnRH receptor subtypes (cGnRH-R-I and cGnRH-R-III) occur in chickens. Differential roles for these molecules in regulating gonadotrophin secretion or other functions are unclear. To investigate this we cloned cGnRH-R-III from a broiler chicken and compared its structure, expression and pharmacological properties with cGnRH-R-I. The broiler cGnRH-R-III cDNA was 100% identical to the sequence reported in the red jungle fowl and white leghorn breed. Pituitary cGnRH-R-III mRNA was approximately 1400-fold more abundant than cGnRH-R-I mRNA. Northern analysis indicated a single cGnRH-R-III transcript. A pronounced sex and age difference existed, with higher pituitary transcript levels in sexually mature females versus juvenile females. In contrast, higher expression levels occurred in juvenile males versus sexually mature males. Functional studies in COS-7 cells indicated that cGnRH-R-III has a higher binding affinity for GnRH-II than cGnRH-I (K(d): 0.57 vs 19.8 nM) with more potent stimulation of inositol phosphate production (ED(50): 0.8 vs 4.38 nM). Similar results were found for cGnRH-R-I, (K(d): 0.51 vs 10.8 nM) and (ED(50): 0.7 vs 2.8 nM). The initial rate of internalisation was faster for cGnRH-R-III than cGnRH-R-I (26 vs 15.8%/min). Effects of GnRH antagonists were compared at the two receptors. Antagonist #27 distinguished between cGnRH-R-I and cGnRH-R-III (IC(50): 2.3 vs 351 nM). These results suggest that cGnRH-R-III is probably the major mediator of pituitary gonadotroph function, that antagonist #27 may allow delineation of receptor subtype function in vitro and in vivo and that tissue-specific recruitment of cGnRH-R isoforms has occurred during evolution.

Polymorphisms in eggshell organic matrix genes are associated with eggshell quality measurements in pedigree Rhode Island Red hens

Novel and traditional eggshell quality measurements were made from up to 2000 commercial pedigree hens for a candidate gene association analysis with organic eggshell matrix genes: ovocleidin-116, osteopontin (SPP1), ovocalyxin-32 (RARRES1), ovotransferrin (LTF), ovalbumin and ovocalyxin-36, as well as key genes in the maintenance and function of the shell gland [estrogen receptor (ESR1) and carbonic anhydrase II (CAII)]. Associations were found for (i) ovalbumin with breaking strength and shell thickness; (ii) ovocleidin-116 with elastic modulus, shell thickness and egg shape; (iii) RARRES1 with mammillary layer thickness; (iv) ESR1 with dynamic stiffness; (v) SPP1 with fracture toughness and (vi) CAII with egg shape. The marker effects are as large as 17% of trait standard deviations and could be used to improve eggshell quality.

Photoperiodic response curves for plasma LH concentrations and age at first egg in female broiler breeders

The objective of this work was to define precisely the response curve for photoinduced luteinizing hormone (LH) release in feed-restricted meat-type (broiler) breeder females and to compare it with the photoperiodic response curve for advance in age at first egg (AFE). Birds with a mean body weight of 2.0kg at 20 weeks of age were transferred from an 8 to a 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 14 or 18-h photoperiod; change in plasma LH was measured 4d after photostimulation and subsequent individual AFE recorded. The first significant increase in LH secretion was seen in birds transferred to an 11.5-h photoperiod, but no further significant increases in LH were observed in birds transferred to longer photoperiods. A photoperiodic response curve based on a meta-analysis of changes in photoinduced LH secretion observed in this study and data from an earlier experiment using dwarf broiler breeders indicated a critical daylength of about 9.5h and a saturation daylength of approximately 13h. Similarly, the first significant advance in AFE occurred in birds transferred to an 11-h photoperiod, but with no further significant increases seen in birds transferred to photoperiods >11h. A response curve for photoinduced advances in AFE was produced by meta-analysis using data from the present study and from an earlier investigation involving fewer, more widely spaced photoperiods. It is concluded, in female broiler breeders, that the photoperiodic response curves for photoinduced LH release and AFE are similar, with the point at which the responses begin to rise steeply (classical critical daylength) occurring at 9.5h and the asymptote (classical saturation daylength) at 13h. Functionally, however, the minimum photoperiod to achieve a significant change in either LH secretion or advance in AFE is between 11 and 11.5h.

Increased food intake stimulates GnRH-I, glycoprotein hormone alpha-subunit and follistatin mRNAs, and ovarian follicular numbers in laying broiler breeder hens

The aim of this study, in 36 week-old laying broiler breeder hens, was to establish the effects on reproductive neuroendocrine gene expression of reinstating ad libitum food intake after moderate food restriction from 2 weeks of age. Seven days of ad libitum feeding increased the number of large pre-ovulatory ovarian follicles and gonadotropin releasing hormone-I (GnRH-I), glycoprotein hormone alpha-subunit and follistatin mRNAs. Plasma luteinizing hormone (LH) was also increased while plasma follicle-stimulating hormone (FSH) was reduced. There were no associated changes in gonadotropin inhibitory hormone (GnIH), LHbeta or FSHbeta mRNAs. The mechanism underlying the increased expression of alpha-subunit and follistatin mRNAs was investigated in vitro by incubating pituitary fragments with pulses of GnRH-I. This treatment increased alpha-subunit and follistatin mRNAs but did not affect gonadotropin beta-subunit mRNAs. It is concluded that lifting food restriction in laying hens increases GnRH-I gene transcription or mRNA stability which may be a consequence, or cause of increased GnRH-I release. This, in turn, increases glycoprotein hormone alpha-subunit and follistatin mRNAs, resulting in increased plasma LH and decreased plasma FSH, respectively.

A QTL for osteoporosis detected in an F2 population derived from White Leghorn chicken lines divergently selected for bone index

Osteoporosis, resulting from progressive loss of structural bone during the period of egg-laying in hens, is associated with an increased susceptibility to bone breakage. To study the genetic basis of bone strength, an F(2) cross was produced from lines of hens that had been divergently selected for bone index from a commercial pedigreed White Leghorn population. Quantitative trait loci (QTL) affecting the bone index and component traits of the index (tibiotarsal and humeral strength and keel radiographic density) were mapped using phenotypic data from 372 F(2) individuals in 32 F(1) families. Genotypes for 136 microsatellite markers in 27 linkage groups covering approximately 80% of the genome were analysed for association with phenotypes using within-family regression analyses. There was one significant QTL on chromosome 1 for bone index and the component traits of tibiotarsal and humeral breaking strength. Additive effects for tibiotarsal breaking strength represented 34% of the trait standard deviation and 7.6% of the phenotypic variance of the trait. These QTL for bone quality in poultry are directly relevant to commercial populations.

Molecular cloning and tissue distribution of a short form chicken leptin receptor mRNA

In mammals, alternative splicing of the leptin receptor (LEPR) produces several C-terminal truncated isoforms that are believed to play a role in the transport, cellular internalisation and degradation of the hormone leptin. The chicken leptin receptor (chLEPR) is similar to its mammalian counterparts in terms of its intron/exon structure and conserved motifs. However, it is unknown whether the chLEPR also undergoes alternative splicing. To test this, structural analysis of intron 19 of the chLEPR, equivalent to the intron in which alternative splicing occurs in mammals, was combined with 3'-rapid amplification of cDNA ends (3'-RACE) to search for chLEPR splice variants. A 44-amino acid alternative exon 20 was identified that is spliced to generate a short isoform of the chLEPR (chLEPR-SF). Comparative sequence analysis of intron 19 identified two regions that are highly conserved between the chicken and mammals, indicating their possible importance as intronic elements in the regulation of alternative splicing of the LEPR in vertebrates. Tissue expression of the chLEPR-SF was lower and more restricted than that of the chLEPR long isoform. Collectively these data demonstrate that the chLEPR is alternatively spliced to produce at least one short isoform, as is the case in mammals.

Probability of an egg cracking during packing can be predicted using a simple non-destructive acoustic test

1. The aim of this investigation was to test the predictive power of the dynamic stiffness measurement to identify eggs which are most likely to crack under field conditions. 2. A representative sample of eggs (n = 1660) was collected from the front of the cages in a commercial battery unit. Egg weight, % damping and dynamic stiffness (Kdyn) were recorded using an acoustic crack detection device. Intact eggs were marked and replaced in the front of the cages. These eggs were subsequently passed through online collection, grading and packing machinery, along with a volume of unmarked eggs. At the end of packing the acoustic test was repeated on the marked eggs, and these were subsequently categorised as being either intact (0) or cracked (1). 3. A logistic regression of the probability of cracking vs Kdyn revealed that as the Kdyn measurement decreases below 15,000 N/m there is a rapid increase in the probability that an egg will crack during routine handling. 4. Additional variables (visit, egg weight,% damping and position in the house (battery [1 to 7], side [1, 2] and tier [1 to 8]) were also fitted to the model but only egg weight, visit and tier effects significantly improved the model fit. 5. This study confirms that the dynamic stiffness measurement can predict the probability of an egg cracking in the field and with high precision. As this measurement also has a high heritability, it could be incorporated into breeding programmes, where it would offer an excellent method to improve eggshell quality and reduce the incidence of cracked eggs.

Identification of a non-mammalian leptin-like gene: characterization and expression in the tiger salamander (Ambystoma tigrinum)

Leptin is well established as a multifunctional cytokine in mammals. However, little is known about the evolution of the leptin gene in other vertebrates. A recently published set of ESTs from the tiger salamander (Ambystoma tigrinum) contains a sequence sharing 56% nucleotide sequence identity with the human leptin cDNA. To confirm that the EST is naturally expressed in the salamander, a 409bp cDNA was amplified by RT-PCR of salamander testis and stomach mRNAs. The coding sequence of the cDNA is predicted to encode 169 amino acids, and the mature peptide to consist of 146 residues, as in mammals. Although the overall amino acid identity with mammalian leptins is only 29%, the salamander and mammalian peptides share common structural features. An intron was identified between coding exons providing evidence that the sequence is present in the salamander genome. Phylogenetic analysis showed a rate of molecular divergence consistent with the accepted view of vertebrate evolution. The pattern of tissue expression of the leptin-like cDNA differed between metamorphosed adult individuals of different sizes suggesting possible developmental regulation. Expression was most prominent in the skin and testis, but was also detected in tissues in which leptin mRNA is present in mammals, including the fat body, stomach, and muscle. The characterization of a salamander leptin-like gene provides a basis for understanding how the structure and functions of leptin have altered during the evolution of tetrapod vertebrates.

Changes in reproductive neuroendocrine mRNAs with decreasing ovarian function in ageing hens

Egg production declines with advancing age in the domestic chicken and this is particularly pronounced in breeding stocks of meat type hens (broiler breeders). The objective of this study was to establish whether declining egg production with reproductive ageing in broiler breeders is correlated with plasma LH and FSH, and with mRNAs encoding hypothalamic gonadotrophin-releasing hormone-I (GnRH-I), gonadotrophin inhibitory hormone (GnIH), and gonadotrophin subunits. Comparisons were made between hens at the peak of egg laying (young: 30 weeks) and at the end of a laying year (old: 60 weeks). Old hens were subdivided into laying and out-of-lay groups. Plasma LH and FSH were lower in old than in young laying hens. Compared with old laying hens, old out-of-lay hens had significantly increased plasma FSH but not plasma LH. There were no differences in total hypothalamic GnRH-I and GnIH mRNAs between young and old hens. In old laying hens, the decrease in plasma LH was correlated with decreased gonadotrophin alpha-subunit but not LHbeta mRNAs. The decrease in plasma FSH was not associated with a change in FSHbeta mRNA. In old out-of-lay hens, the increase in plasma FSH was correlated with increased FSHbeta mRNA, while unchanged plasma LH was associated with increased LHbeta mRNA. A regression analysis of all plasma gonadotrophin and gonadotrophin subunit mRNA data collected from the study demonstrated that plasma LH is correlated with alpha-subunit but not LHbeta mRNAs, while plasma FSH is correlated with FSHbeta but not alpha-subunit mRNAs. It is concluded that the decrease in the rate of lay in ageing broiler breeders is not correlated with decreased GnRH-I mRNA nor with increased GnIH mRNA, but it is related to a decrease in alpha-subunit mRNA which may account for the associated reduction in plasma LH but not FSH.

Heritability and genetic correlation of measurements derived from acoustic resonance frequency analysis; a novel method of determining eggshell quality in domestic hens

Cracked eggshells result in economic loss and provide a route for pathogenic organisms to enter the egg. Genetic factors that contribute to shell strength are likely to decrease the risk that an egg will crack when subject to insult. A novel measurement, the dynamic stiffness of the eggshell (Kdyn) was examined to determine if it might be used in the genetic selection of hens with improved eggshell characteristics. The measurement is determined from acoustic resonance frequency analysis. The estimates of heritability for the novel measurement of Kdyn were moderately high and ranged from 0.33 to 0.53 depending on the model used for the estimation. The estimates of genetic correlation of Kdyn with eggshell breaking strength (0.49) and static stiffness (0.57) were positive and relatively large as expected. There was a small negative genetic correlation between Kdyn and egg production from 26 to 50 weeks of age (-0.19) and a moderate one from 58 to 74 weeks of age (-0.36). The moderate heritability and relative independence of Kdyn indicates that this measurement could be used successfully in a breeding programme to improve shell quality and to reduce the incidence of cracks.

Gonadotrophin inhibitory hormone depresses gonadotrophin alpha and follicle-stimulating hormone beta subunit expression in the pituitary of the domestic chicken

Studies performed in vitro suggest that a novel 12 amino acid RF amide peptide, isolated from the quail hypothalamus, is a gonadotrophin inhibitory hormone (GnIH). The aim of the present study was to investigate this hypothesis in the domestic chicken. Injections of GnIH into nest-deprived incubating hens failed to depress the concentration of plasma luteinizing hormone (LH). Addition of GnIH to short-term (120 min) cultures of diced pituitary glands from adult cockerels depressed follicle-stimulating hormone (FSH) and LH release and depressed common alpha and FSHbeta gonadotrophin subunit mRNAs, with no effect on LHbeta subunit mRNA. Hypothalamic GnIH mRNA was higher in incubating (out-of-lay) than in laying hens, but there was no significant difference in the amount of hypothalamic GnIH mRNA in out-of-lay and laying broiler breeder hens at the end of a laying year. It is concluded that avian GnIH may play a role in controlling gonadotrophin synthesis and associated constitutive release in the domestic chicken.

A study of association between genetic markers in candidate genes and reproductive traits in one generation of a commercial broiler breeder hen population

Markers of alleles for three physiological candidate genes for reproductive traits, growth hormone (GHR), gonadotropin-releasing hormone receptor (GNRHR) and neuropeptide Y (NPY) were assessed for the association with the total egg production, number of double-yolked eggs and age at first egg in a single generation of a broiler breeder (Gallus gallus) pedigree dam line. Single-nucleotide polymorphisms and deletions were detected in the GHR, GNRHR and NPY genes. Genotypes were identified using a PCR-RFLP assay. The frequency of restriction enzyme+/-alleles in the population was for GHR 0.68 (NspI-) and 0.32 (NspI+), for NPY 0.78 (DraI+) and 0.22 (DraI-) and for GNRHR 0.54 (Bpu1102I+) and 0.46 (Bpu1102I-). Trait data from a total of 772 hens in 67 sire families from one generation of the pedigree dam line were recorded. However, the analysis used only the offspring of heterozygous sires to reduce the influence of selection and genetic background (n=33 sire families for GHR; n=14 sire families for NPY; n=36 sire families for GNRHR). A dominance effect of NPY on age at first egg and an additive effect of GNRHR on the number of double-yolked eggs were found (P<0.05).

Early expression of chicken gonadotropin-releasing hormone-1 in the developing chick

Gonadotropin-releasing hormone (GnRH), which is essential for reproductive function, is made by neurones that migrate from the nasal region into the brain during early embryonic development. This migration begins in chick when the olfactory pit is formed. This is approximately the time that GnRH neurones can be detected immunocytochemically. The present study investigated (i). how early in development the GnRH gene is expressed and (ii). the sites of its expression. Accordingly, reverse transcriptase-polymerase chain reaction (PCR) and in situ hybridization were performed on chick embryos before gastrulation up until the stage by which GnRH neurones have begun to migrate into the central nervous system. Primers were made to the 5'- and 3'-UTR region of the message for cGnRH-I, the form of the peptide that is essential for reproductive function in the chicken. PCR product was found in all stages and the sequences of products from all stages were identical. Thus, the GnRH gene is expressed continuously throughout embryonic development. In situ hybridization with a digoxygenin labelled riboprobe revealed staining along the primitive streak immediately before gastrulation. In later stages, cGnRH-I gene expression was seen in association with the anterior neural ridge. The expression was subsequently restricted to a narrow, clearly defined region, which is associated with the presumptive nasal cavity and olfactory placode. Later, GnRH neurones could be seen in their migratory routes by both in situ hybridization and immunocytochemistry. Expression of the GnRH gene has been described in preimplantation stages in mammals and there is evidence that the neuropeptide plays a role in formation and maintenance of the placenta. What role (if any) it may play in early avian development remains unknown. The demonstration of sites of GnRH expression during the early period of neurulation suggests that GnRH neurones arise before olfactory placode formation.

Acceleration of maturation of FSH and LH responses to photostimulation in prepubertal domestic hens by oestrogen

Egg laying begins in domestic hens, reared on short daylengths, at about day 147 of age and is advanced by photostimulation after but not before about day 42 of age. The development of this response at day 42 may be facilitated by oestrogen. This hypothesis was investigated in prepubertal hens, reared on short daylengths, by comparing the effects of oestrogen treatment on pituitary and plasma FSH and LH responses to photostimulation (16 h light:8 h dark) for 1 week at days 34 and 54 of age. Oestradiol benzoate (0.5 mg kg(-1)) was injected i.m. on alternate days for 1 week before and after photostimulation. At day 34, pituitary LH content increased after photostimulation but plasma LH and FSH concentrations did not increase. At day 54, pituitary FSH content and plasma FSH and LH concentrations increased after photostimulation, whereas pituitary LH content did not increase. At days 34 and 54, oestrogen treatment decreased pituitary FSH and LH contents but did not block the stimulatory effect of photostimulation on pituitary FSH. At day 34 but not at day 54, photostimulation combined with oestrogen treatment increased plasma FSH and LH concentrations. Plasma LH but not plasma FSH concentration increased after GnRH-I injection at days 34 and 54. These observations are consistent with the hypothesis that, in prepubertal female chickens, maturation of the neuroendocrine mechanism mediating photoinduced FSH and LH release may be mediated by oestrogen. This effect of oestrogen on photoinduced LH release may be mediated by increased GnRH-I release or enhanced pituitary responsiveness to GnRH-I. It is proposed that neuroendocrine mechanisms controlling photoinduced FSH release may involve oestrogen-responsive interactions between pituitary paracrine factors, including activins and follistatin.

Changes in basal hypothalamic chicken gonadotropin-releasing hormone-I and vasoactive intestinal polypeptide associated with a photo-induced cycle in gonadal maturation and prolactin secretion in intact and thyroidectomized starlings (Sturnus vulgaris)

Chicken gonadotropin-releasing hormone-I (GnRH-I) and the avian prolactin-releasing hormone, vasoactive intestinal polypeptide (VIP), were measured in the basal hypothalamus in male starlings during photo-induced gonadal growth and the subsequent development and maintenance of reproductive photorefractoriness. Comparisons were made with thyroidectomized birds, which maintain breeding condition irrespective of changes in photoperiod. In intact birds, basal hypothalamic GnRH-I increased four-fold after photostimulation and then decreased 115-fold over 12 weeks to values characteristic of long-term photorefractoriness. Pituitary and plasma prolactin increased after photostimulation, reaching peak values when the testes were regressing, and returned to low values in long-term photorefractory birds. Basal hypothalamic VIP did not change after photostimulation in intact birds. In photostimulated thyroidectomized birds, values for basal hypothalamic GnRH-I and VIP, and for pituitary and plasma prolactin, remained no different to those of nonphotostimulated intact birds. These observations confirm that reproductive photorefractoriness is related to a decrease in hypothalamic GnRH-I. However, photorefractoriness in terms of prolactin secretion is not similarly related to a decrease in basal hypothalamic VIP. The mechanisms responsible for the decrease in prolactin in long-term photorefractory birds and for the total lack of photoperiodic responses in thyroidectomized birds remain unresolved.

Effect of exogenous oestradiol and lighting regime on age at first egg in domestic pullets

1. Groups of ISA Brown pullets were transferred from 8- to 16-h photoperiods at 34, 44 or 54 d. In each group, 12 birds were injected on alternate days over a 12-d period starting 6 d before the change in photoperiod with beta-oestradiol-3-benzoate (1 mg/kg body weight) or with arachis oil vehicle (controls). Short-day controls were similarly injected from 28 to 40 d. Long-day (16 h) controls were also included in the trial but were not injected. Age at first egg (AFE) was recorded and plasma luteinising hormone (LH) concentrations were measured around the time of oestradiol treatment. 2. Mean AFE for birds photostimulated at 34 d was not significantly different from short-day controls. Birds photostimulated at 44 and 54 d matured at similar ages but 3 weeks earlier than short-day controls (P<0.05). 3. There was a tendency for oestradiol to advance AFE for birds photostimulated at 34 d (P=0.15) but to delay AFE following photostimulation at 44 d (P=0.23). Oestradiol significantly delayed AFE for the birds photostimulated at 54 d (P=0.01). 4. Plasma LH levels during 6 d of oestradiol injection but before transfer from 8- to 16-h photoperiods tended to fall between 28 and 34 d, were relatively constant between 38 and 44 d, but declined significantly between 48 and 54 d. Following photostimulation at 34 d, increases in plasma LH levels for oestradiol-injected birds were significantly greater than for controls. Oestradiol treatment had no significant effect on changes in plasma LH concentrations after photostimulation at 44 or 54 d. 5. This trial confirms previous work showing that pullets are unresponsive to photostimulation before 6 weeks of age. It also demonstrates that raising circulating oestrogen levels by injecting 0.5 mg/kg oestradiol benzoate on alternate days enhances the LH response to photostimulation at 34-d, but only very slightly sensitises a 34-d old bird to an increase in photoperiod which, 10 d later, is capable of advancing AFE in control birds by 24 d. Increased circulating oestrogen might be a factor which allows pullets to advance AFE in response to an increase in daylength.