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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.

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.

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.

The possibility that egg production in broiler breeder hens may be increased by selection for reduced oviposition interval under continuous light was investigated by comparing the pattern of pre-ovulatory releases of plasma luteinising hormone (LH) and the associated ovipositions in the same broiler hens maintained under normal cycles (15.25 h light/d) or continuous light. The lighting conditions had no effect on plasma concentrations of LH before and at the pre-ovulatory LH peak in first, mid-sequence or terminal ovulatory cycles. Plasma LH concentrations were similar during first, mid-sequence and terminal ovulatory cycles. Mid-sequence oviposition intervals and the interval between a mid-sequence LH peak and its associated oviposition were longer under continuous light than under normal lighting. Pre-ovulatory releases of LH occurred during a restricted period of day in both lighting conditions. Under continuous lighting they were probably entrained by the daily pattern of restricted feeding. Any selection programme for reduced oviposition interval under continuous lighting in broiler breeder hens should take into account the entraining effects of the daily pattern of feeding.

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.

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.

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.

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.