Research and development in 2000: directions and priorities for the world's poultry science community.

The challenges and targets facing the world's poultry science community in the immediate future are reviewed in the context of meeting the dietary needs for animal protein of the world population. The prior need to provide for the increasing demand for cereals, oil seeds, and grain legumes for human consumption is assessed at having a reasonable chance of success. If this need is met, the requirement for extra feed resources for increased poultry production targets is also assessed as having a reasonable chance of success. A major component of this equation is the prediction of improved efficiency of poultry production of a similar order to that of the last 50 yr arising from 1) extension of the 20th century revolution in poultry technology to over 50% of the world population compared with the present 20 to 25%; 2) recent advances in genetics, nutrition, health, housing, and husbandry still awaiting application in industry; 3) future applications from current and future research in molecular biotechnology, nutrition, health, and reproduction; and 4) the development of efficient, small-scale, extensive poultry production systems especially in countries where over 25% of the world population will still not be able to afford the products of a modern, intensive poultry industry, even in 50 yr. These challenges, targets, and predictions simply cannot be met unless the world's poultry science community increases its own efficiency, its professional initiatives to deal with the real challenges, and its social initiatives to influence socio-economic decisions on national and world stages.

Genetic analysis of triploidy in a selected line of chickens.

We investigated the pattern of inheritance of maternal meiotic errors responsible for a high frequency of triploid progeny in a selected line of chickens. For the genetic analysis, F1 and backcross populations were produced from crosses between normal diploid individuals of the triploidy line and a control line. Triploid embryos were produced by 35% and 67% of reciprocal F1 females and by 24% and 67% of reciprocal backcross females. These results exclude autosomal recessive and sex-linked recessive or sex-linked dominant inheritance. A single autosomal dominant gene is also not likely to be responsible. However, the results are consistent with the determination of triploidy by a single autosomal gene with no dominance, and an even better fit is obtained by two loci, an autosomal gene with no dominance and a sex-linked gene. The results cannot exclude a multifactorial mode of inheritance, but the rapid response to selection for triploidy and consistent expression of the meiotic errors in different genotypes suggest that meiotic mutations at one or two loci are the most plausible genetic basis for the trait.

Electron microscopy of the seminiferous epithelium in the triploid (ZZZ and ZZW) fowl, Gallus domesticus.

The ultrastructure of cells of the seminiferous epithelium is described in both triploid ZZZ and ZZW fowls, especially cytological aberrations influencing the development of morphologically abnormal spermatids and spermatozoa. Mitotic divisions in triploid fowls are similar to those in diploid fowls, but meiosis in both the ZZZ and ZZW triploid fowl is abnormal and many defective gametes are produced. The majority of spermatids have structural abnormalities and this is attributed to the aneuploidy expected from meiotic chromosome pairing in a triploid. Some spermatids were observed to possess an irregularly shaped head, 2 centriolar complexes, 2 acrosomes and 2 tails and this type differentiated into large-headed spermatoza. Such spermatids and spermatozoa are suggested to be near diploid due to their large nuclear size and 2 sets of organelles. Other spermatids, observed with only 1 centriolar complex, 1 acrosome and 1 tail, are suggested to be near haploid. Differentiation of these spermatids is similar to the process observed in diploid fowls except that there is a high percentage of abnormally shaped heads. The fact that morphologically normal spermatozoa were rare is consistent with the expectation that very few spermatozoa would be exactly haploid. In both ZZZ and ZZW triploid fowls, triple pairing of chromosomes is observed in synaptonemal complexes (SC) during meiosis and recombination nodules are distributed at random in the central regions of the triple pairing SC. In ZZW triploid fowls gamete formation is more severely affected than in ZZZ triploid fowls and it is suggested that many of the degenerative changes occurring in ZZW fowls could be attributed to those spermatids that possess a W sex chromosome, since normal male fowls are the homogametic sex (ZZ) and normal spermatids and spermatozoa contain only a Z sex chromosome.

Development of the gonads in the triploid (ZZW and ZZZ) fowl, Gallus domesticus, and comparison with normal diploid males (ZZ) and females (ZW).

Gonadal development in fowls aged from 1 day to more than 4.5 years was studied in 63 ZZW and 16 ZZZ triploid crossbreds and compared with normal diploid males (ZZ) and females (ZW). In the ZZW fowl, the right gonad developed into a testis (although this occurred earlier in the ZZ genotype), and a structurally-abnormal excurrent duct system containing some malformed spermatids and spermatozoa was associated with the gonad of young adults. The left gonad was an ovotestes at hatching and no excurrent ducts were associated with it. The ovarian component was much less developed than that in the ZW genotype-it started to degenerate by 1 week of age, and most of the oocytes had disappeared by about 3 weeks of age. The seminiferous tubules developed in the medullary region, but only abnormal spermatozoa were produced. Leukocytes infiltrated both gonads at about 9 months of age and the seminiferous epithelium had degenerated in most fowls over 1 year old. In ZZZ fowl, gonadal and excurrent duct development was normal, but occurred earlier than in the ZZ genotype. However, meiosis and spermiogenesis were abnormal and malformed spermatozoa were produced. The heads of spermatozoa from the ducts deferens were about 1.4-times longer in the ZZZ genotype than in the ZZ genotype, indicating that the former may be producing some diploid spermatozoa.

Heat shock protein gene HSP108 and a replication histone gene cluster are linked in the chicken.

In work aimed at extending the chicken genome linkage map, a heat shock protein gene, HSP108, was shown by restriction fragment length polymorphism (RFLP) analysis to be linked to one of two replication histone gene clusters located on chromosome 1. Linkage was estimated using the LOD score method, on segregation data from seven pedigreed three-generation families (245 individuals). The maximum likelihood estimate of the recombination fraction (theta) was 0.215.

Decline in heterozygosity under full-sib and double first-cousin inbreeding in Drosophila melanogaster.

The effects of inbreeding on heterozygosities and reproductive fitness were determined by carrying out full-sib and double first-cousin inbreeding in Drosophila melanogaster populations for up to 18 generations. Parents were scored each generation for five or six polymorphic enzyme loci, and progeny numbers per pair were recorded. Inbreeding depression, in the form of significant reductions in progeny numbers and significant extinction of lines, was observed. Heterozygosity decreased at a significantly slower rate than predicted, being about 80% of expected. The full-sib and double first-cousin treatments showed similar disagreement with expectations over comparable ranges of inbreeding. Natural selection was shown to favor heterozygotes in the inbred lines. Associative overdominance was the most probable explanation for the slower than expected decline in heterozygosity.

Association of the major histocompatibility complex with avian leukosis virus infection in chickens.

1. Association of the B blood group, the major histocompatibility complex (MHC) in chickens, with avian leukosis virus (ALV) infection shown by shedding of group-specific (gs) antigen was studied in an Australorp line selected for short oviposition interval to improve egg production. Three haplotypes (B8a, B9a and B21) were segregating in this line at frequencies of 66.7, 15.6 and 17.8%, respectively, averaged over three generations. 2. The relative risk (odds ratio) of a hen becoming a gs-antigen shedder was calculated for progenies of the dams shedding gs-antigen and those of non-shedding dams separately and pooled over three generations. In the progenies of shedding dams, the relative risk was not significantly different from 1.0 for the three haplotypes. In contrast, in the progenies of non-shedding dams, the relative risk was 0.67, 0.48 and 2.53 for B8a, B9a and B21, respectively, with the last two ratios being significantly different from 1.0. 3. The average effect of haplotype substitution on probability of shedding was estimated from a linear logistic model. The estimates (relative to zero for B8a) for B9a and B21, respectively, were -0.26 and 0.03 among the progenies of shedding dams, and -0.16 and 0.87 among the progenies of non-shedding dams. The last estimate only was highly significant. 4. These results suggest that the three haplotypes were similar in susceptibility to congenital infection through hatching eggs, but differed in susceptibility to post-hatching infection from other infected birds.

Chromosome analysis of early embryonic mortality in layer and broiler chickens.

1. Twenty-three lines of chickens, obtained from grandparent stocks of 4 Australian breeding companies, were analysed to determine the incidence of early embryonic mortality attributable to chromosome abnormalities. The lines included 10 layer strains, consisting of 6 White Leghorn, 2 New Hampshire and 2 Australorp lines, and 13 broiler lines. 2. A total of 10,730 eggs was examined after 3 d incubation; of these 9746 (90.8%) were fertile. Abortive embryonic development was observed in 1379 (14.1%) of the fertile eggs. This consisted of 952 (69.0%) dead and dying embryos, including 646 malformed and 427 (31.0%) membranes without embryos. 3. Early embryonic mortality was found to vary from 9.8 to 26.8% (average 16.4%) in broiler lines and from 8.0 to 27.9% (average 11.9%) in layer lines. 4. Among 898 abortive embryos analysed, 112 had abnormal chromosomes consisting of 27 haploids, 38 haploid-euploids, 24 triploids, 16 diploid-polyploids, 4 aneuploids, 2 tetraploids and 1 translocation. 5. In broilers and layers respectively, chromosome abnormalities were responsible for 4.4 to 28.1% (average 11.8%) and 7.4 to 25.0% (average 13.4%) of the early embryonic mortality. 6. The overall frequency of chromosome abnormalities in all fertile eggs varied from 0.7 to 3.7% for the broiler lines and 0.7 to 3.4% for the layer lines.

Heterozygosity at protein loci in inbred and outbred lines of chickens.

Levels of heterozygosity at six polymorphic protein marker loci were determined by electrophoresis in 24 lines of poultry, encompassing 17 White Leghorn inbred lines (WLI) (with inbreeding coefficients, F, ranging from .946 to .988), five Australorp inbred lines (AusI) (with F values ranging from .924 to .961), and two randombred lines (one White Leghorn and one Australorp). Fixation was observed at one locus in WLI lines, and at two loci in the AusI lines. Segregation at the other loci was observed in the inbred lines of the two breeds. Observed heterozygosity in the inbred lines markedly exceeded the expectations under inbreeding theory. In White Leghorns, reproductive fitnesses for heterozygotes were superior to homozygotes in the inbred lines, but not in the control. Consequently, natural selection operating through associative overdominance appears to be responsible for the higher than expected heterozygosities in the inbred lines.

Avian leukosis virus and selection on oviposition interval in Australorp lines.

1. Shedding of group-specific antigen of avian leukosis virus (ALV) into egg albumen was determined in three Australorp lines: AS line that had been selected primarily for short oviposition interval, ASS line that had been derived from the AS line and developed as a commercial dam line for egg laying, and AC line that had been kept as a randombred control. 2. The proportion of shedders was 13.1 to 16.7% in the AS line in 1984-88, 16.3% in the ASS line in 1984 (before culling of the shedders), and 6.1% and 6.6% in the AC line in 1984 and 1988, respectively. 3. In the AS line, shedders were 1.8% lower in rate of lay to 300 d of age, 1.3 g lower in average egg weight at 34 weeks of age, 5.6% lower in hatchability of fertile eggs and 0.24 h shorter in oviposition interval than non-shedders. In the ASS line (1984 only), the differences between shedders and non-shedders were in the same direction, but in magnitude greater for rate of lay and smaller for oviposition interval. 4. The shedders were favoured by the artificial selection because of their shorter oviposition interval and this appeared to be responsible for the higher levels of ALV shedding in the selection lines.

Cytological evidence of maternal meiotic errors in a line of chickens with a high incidence of triploidy.

Direct evidence of the nature of maternal meiotic errors in a selected line of chickens with a high incidence of triploidy was obtained by using cytologically marked paternal gametes derived from a closely related avian species. Matings were made by artificial insemination of female chickens of the selection line and a control line with semen from ring-necked male pheasants. A total of five triploid, one pentaploid, and 21 diploid hybrid embryos were karyotyped. Each triploid hybrid embryo contained one set of paternal pheasant chromosomes and two sets of maternal chicken chromosomes, providing irrefutable cytological evidence that the triploids were derived from diploid ova produced by females of the selection line. The pentaploid hybrid contained one set of paternal pheasant chromosomes and four sets of maternal chicken chromosomes, indicating that it had been derived from a tetraploid ovum. Females of the selection line are thought to have a genetically mediated susceptibility to nondisjunction which is responsible for the high incidence of meiotic errors. Evidence is provided that the non-disjunction occurs at both meiosis I and meiosis II.

Reproductive fitness and artificial selection in animal breeding: culling on fitness prevents a decline in reproductive fitness in lines of Drosophila melanogaster selected for increased inebriation time.

The maintenance of reproductive fitness in lines subjected to artificial selection is one of the major problems in animal breeding. The decline in reproductive performance has neither been predictable from heritabilities and genetic correlations, nor have conventional selection indices been adequate to avoid the problem. Gowe (1983) has suggested that the heritabilities of reproductive traits are non-linear, with heritabilities being higher on the lower fitness side. Consequently, he has predicted that culling on reproductive fitness in artificial selection lines will be effective in preventing the usual declines in fitness. An experimental evaluation of Gowe's prediction has been carried out by comparing fitnesses of replicated lines of three treatments: selection for increased inebriation time without culling on fitness (HO), selection for inebriation time with culling of 20% (4/20) of selected females on reproductive fitness (HS), and unselected controls (C). Response to selection for inebriation time in the two selection treatments was similar. After 25 generations, the competitive index, a measure of reproductive fitness, was significantly lower in the HO treatment than the HS treatment, while the HS treatment did not differ from the control lines or the base population. These results demonstrate for the first time that culling on reproductive fitness in selection lines can be used to prevent the usual decline in reproductive performance.

Live haploid-diploid and other unusual mosaic chickens (Gallus domesticus).

Sixteen haploid-diploid chickens, four diploid-triploids, and one haploid-diploid-triploid chicken have been found in a layer line of crossbred origin. Live haploid-euploid vertebrates have not been reported previously. Flow cytometric analysis revealed 1-80% haploid red blood cells (RBCs) in the haploid-euploid chickens and 5-30% triploid RBCs in the diploid-triploids. Mosaic cell lines were also found in leukocytes and many tissues. The haploid-diploid chickens were all phenotypically normal and fertile, but the phenotype and fertility of the diploid-triploid chickens depended on their sex-chromosome complements and the ratio of diploid to triploid cells. The haploid cell lines are thought to originate from supernumerary spermatozoa, and the diploid-triploids from binucleated oocytes with meiotic errors. It is likely that genetic factors are partly responsible for the occurrence of these mosaic chickens.

Analyses of oviposition times and intervals in a wide range of layer flocks under normal and continuous lighting regimes.

Distributions of oviposition times and intra-clutch oviposition intervals in continuous light (CL) and normal light (NL) environments were studied in 39 populations of 27 different genotypes. The populations showed a range of mean intervals from 23.1 to 27.6 h in CL, arising from the history of selection directly on interval in some populations. The data were analysed to infer empirical relations of descriptive statistics of the distribution to each other and to rate of lay to 301 d of age (PRL). In NL, as the mean intra-clutch interval decreased, more intervals tended to accumulate against the barrier of 24 h imposed by the light-dark cycle and the variability of intervals declined rapidly. Similar, but less striking, relations were found in CL above a mean interval of about 24 h. Under CL, the mean intra-clutch interval (CIM) decreased beneath 24 h and the variability of intervals tended to show a steep increase. The rate of lay to 301 d of age increased linearly at a rate of 6.3% for each hour by which CIM decreased to about 24 h, but no further below that level. Oviposition time in NL was advanced with a decrease in CIM, apparently in two linear phases of differing slopes, perhaps reflecting different physiological bases of the change. The degree of entrainment in NL, measured by the proportion of eggs laid in the model 8 h, increased with reduction of CIM. The distribution of oviposition times in CL showed a great deal of variation among the populations and departed significantly (P less than 0.05) from the uniform rectangular distribution, in all but three populations. The proportion of eggs laid in the modal 8 h of the day, suggested as an indicator of sensitivity to uncontrolled timing cues in CL, was positively correlated with CIM. Free-running periods were estimated for individual pullets from sequences of oviposition times in CL. The mean period for a population declined linearly with CIM, but only when CIM had decreased below about 25.75 h, suggesting that the period of endogenous circadian rhythm might have been altered in some populations with low CIM's. The proportion of eggs laid in the modal 8 h of the free-running period, perhaps an indicator for regularity of the circadian rhythm, was negatively correlated with CIM.

Plasma concentrations of luteinising hormone during the ovulatory cycle in hens selected for reduced oviposition interval and maintained in continuous light or a 24 h light:dark cycle.

Plasma luteinising hormone (LH) concentrations were measured during the ovulatory cycle in lines of Australorps and White Leghorns selected for reduced oviposition interval and maintained under continuous light and noise. Selection significantly increased plasma LH concentrations in mid-sequence ovulatory cycles of Australorps but not in the White Leg-horns. Selection in the Australorps apparently increased the rate of ovarian follicular maturation, resulting in more frequent LH peaks. The effect of selection on plasma LH concentrations is a function of the lighting condition to which the hens are exposed.

Effects of selection for reduced oviposition interval on plasma concentrations of luteinising hormone during the ovulatory cycle in hens on a 24 h lighting cycle.

The effects of selection under continuous light for reduced oviposition interval within a sequence of eggs on plasma luteinising hormone (LH) concentrations during the ovulatory cycle were investigated in lines of Australorps and White Leghorns maintained on a 24-h light:dark cycle. Selection did not significantly alter LH concentrations during mid-sequence cycles in either breed. In the control lines, concentrations were higher in the Australorps than in the White Leghorns before and at peak in mid-sequence cycles. The increase in egg production resulting from selection was associated with an advance in the mean time of lay of mid-sequence eggs in both breeds and, in the Australorps, with more pre-ovulatory LH peaks being initiated at the beginning of the open period of the ovulatory cycle. Selection reduced the time an egg spends in the oviduct and, in the Australorps, significantly reduced the interval between a pre-ovulatory LH peak and the oviposition of the egg currently in the shell gland. We suggest that selection under continuous light for reduced oviposition interval increased the rate of ovarian follicular maturation by a mechanism which did not involve an increase in pre-ovulatory concentration of plasma LH in a 24-h light-dark cycle.

Cross-breeding performance of white Leghorn and Australorp lines selected under continuous light for short interval between eggs: full-year egg production and efficiency of food utilisation.

White Leghorn (LS) and Australorp (AS) lines selected for short oviposition interval and their control lines (LC and AC), together with three strains of Australian commercial layers, were evaluated for crossbreeding performance. The estimates of selection effect on crossbreeding performance in lines LS and AS respectively were 3.5% and 15.2% for survivors' egg number, -3.3% and -2.0% for average egg weight, and 4.3% and 7.4% for efficiency of food utilisation. Compared to the commercial strains, the LS X AS cross was 13 to 15% higher for egg number, 7 to 10% lower for egg weight and 2 to 7% better for efficiency of food utilisation. The LC X AC cross laid more but smaller eggs than the commercial strains. This raised the question of how effective conventional selection in the commercial nucleus flocks had been in improving egg number in the previous 10 years, a period when the control lines had been bred without deliberate selection.

Increase in oviposition interval due to the sex-linked dwarf gene in white Leghorns and Australorps.

Oviposition patterns in dwarf and normal pullets were compared after the dw gene had been introduced into a White Leghorn and an Australorp strain, which had been selected for reduced oviposition interval. The dw gene increased oviposition interval by about 2 h and reduced clutch length by two-thirds in both strains. It also decreased the occurrence of intervals shorter than 21 h and of membranous and soft-shelled eggs in the White Leghorn strain, but had little effect on these traits in the Australorp. About two-thirds of the 17% difference in rate of lay between dwarf and normal pullets could be attributed to the difference in oviposition interval. It is suggested that the dw gene may be useful in overcoming certain physiological barriers when selecting for shorter oviposition interval.