Selection for high and low threshold body weight at first egg in broiler strain females. 4. Photoperiodic drive in the selection lines and in commercial layers and broiler breeders

Genetic parameter estimation and molecular foundation of chicken egg-laying trait

The age of first egg (AFE) in chicken can affect early and even life-time egg production performance to some extent, and therefore is an important economic trait that affects production efficiency. To better understand the genetic patterns of AFE and other production traits including body weight at first egg (BWA), first egg weight (FEW), and total egg number from AFE to 58 wk of age (total-EN), we recorded the production performance of 2 widely used layer breeds, white leghorn (WL) and Rhode Island Red (RIR) and estimated genetic parameters based on pedigree and production data. The results showed that the heritability of AFE in both breeds ranged from 0.4 to 0.6, and AFE showed strong positive genetic and phenotypic correlations to BWA as well as FEW, while showing strong negative genetic and phenotypic correlations with total-EN. Furtherly, by genome-wide association analysis study (GWAS), we identified 12 and 26 significant SNPs to be related to AFE in the 2-layer breeds, respectively. A total of 18 genes were identified that could affect AFE based on the significant SNP annotations obtained, but there were no gene overlapped in the 2 breeds indicating the genetic foundation of AFE could differ from breed to breed. Our results provided a deeper understanding of genetic patterns and molecular basement of AFE in different breeds and could help in the selection of egg production traits.

Interspecific variation in photo-induced GnRH plasticity among nomadic cardueline finches

Changes in response to photoperiod are of fundamental importance to seasonal cycles in avian reproductive development. In this study we present data on photo-induced changes in gonadotropin-releasing hormone (GnRH) immunoreactivity associated with the development of photorefractoriness in males of 3 closely related species of cardueline finch: common redpoll (Carduelis flammea), pine siskin (C. pinus) and white-winged crossbill (Loxia leucoptera). All are nomadic, mid- to high-latitude breeders with varying levels of reproductive flexibility: redpolls are typically seasonal, pine siskins more flexibly seasonal and white-winged crossbills temporally opportunistic. Males were exposed to either long (20L:4D) or short days (5L:19D), beginning in January, and GnRH was assessed via immunocytochemistry 4.5 months later. Plasma LH, testis size, and molt score were also measured. Significant reductions in the number of irGnRH cells detected, cross-sectional cell area and optical density of these cells occurred in both redpolls and siskins on long, as compared with short, days. These decreases in irGnRH, coupled with spontaneous testicular regression and onset of molt were consistent with the development of absolute photorefractoriness on constant long days. In contrast, the tendency for numbers of GnRH immunopositive cells and mean measures of GnRH immunoreactivity (cell area, optical density and axonal fiber density) to decrease were much more moderate in white-winged crossbills held on long versus short days. Although none of the four measures of GnRH immunoreactivity differed significantly between treatments in crossbills, significant shifts in the frequency distribution of irGnRH neurons to include greater numbers of less immunoreactive cells suggested that antigenic GnRH content was reduced in many cells. These photo-induced shifts in optical densities of GnRH cells in long-day, as compared with short-day crossbills followed, to a lesser extent, the pattern in pine siskins and common redpolls. For all three species, patterns of decrease in GnRH immunoreactivity, gonadal regression and molt suggest that some form of refractoriness to photostimulation develops on constant long days. However, more moderate tendencies for GnRH immunoreactivity to decrease in white-winged crossbills, coupled with limited gonadal regression and molt on long days suggest that absolute photorefractoriness might develop more slowly in this taxon than in others, or alternatively, a condition arises that is more similar to relative photorefractoriness. These data indicate that relatively small differences in the response of the neuroendocrine system to stimulatory cues such as day length may lead to remarkably different capabilities for reproductive flexibility even within a closely related family of birds.

Heterosis of growth and reproductive traits in fowl

Long-term divergent selection for high (HH) and low (LL) BW at 56 d of age in White Plymouth Rock chickens resulted in deviations from a typical chronological timeline for reproductive development. Line HH chickens of the 42nd generation of selection (S(42)) were heavier at 28, 56, 168, and 266 d of age, as well as at sexual maturity than those from line LL of the same generation, and commenced egg production at a younger age (188 d vs. 217 d). Once the LL chickens reached sexual maturity, 99.5% of their ovulations resulted in normal oviposits, in contrast to 94.6% for HH females. Heterosis, as measured by differences of F(1) progeny from the parental lines, can have a positive or negative sign. Although negative heterosis does not imply inferiority, results may be confusing because, depending on the trait, positive or negative can biologically be the phenotype that is preferred. Heterosis was calculated using coefficients of variation and means for reciprocal crosses from S(41) parents of the selected lines. Growth and reproductive traits were measured in this experiment. In almost all cases, heterosis calculated using coefficients of variation had a narrower range of magnitudes.

Effect of photoperiod and quantitative feed restriction in a broiler strain on onset of lay in females and onset of semen production in males: a genetic hypothesis

The effects of feed restriction and photoperiod on body weight (WTSXM) and age (ASXM) at onset of mature semen production in males and onset of lay in females of a broiler female line were examined. Feeding treatments were as follows: ad libitum (AL) and quantitative feed restriction (QR). Photoperiod treatments were as follows: open shed (OS), in which the chicks were reared under naturally increasing daylight with supplemental light; dark room (DR), in which chicks were reared under short days, gradually increasing from 6 h of light (L) to 11.5L; and dark room to open shed (DR-OS), in which chicks reared under short days in the dark room were transferred to the open shed at 149 d. Treatment effects were similar in direction in males and females, suggesting similar control of entry into reproduction of the two sexes, although within a given treatment, males matured earlier than females. Effects of feeding and photoperiod treatments were additive. Relative to QR, the AL feeding treatment increased WTSXM by 832 g for males and 1,089 g for females; ASXM was decreased by 15.8 d for males and 9.3 d for females. Relative to OS, the DR photoperiod increased WTSXM by 591 g for males and 513 g for females; ASXM was increased by 17.5 d for males and 26.8 d for females. The DR-OS photoperiod increased WTSXM by 86 and 169 g for males and females, respectively; ASXM was increased by 6.1 d and 4.9 d, respectively. Under DR, the delay in onset of sexual maturity caused by QR was not due to failure to reach threshold body weight or age and, hence, appears to have resulted from the feed restriction itself. A similar delay for QR was found under OS but might have been due to failure to reach threshold body weight. It is speculated that the delay in onset of sexual maturity caused by feed restriction may be an ecological adaptation or, alternatively, a result of nutritional imbalance.

Two-way selection for high and low responsiveness to photostimulation in broiler strain males

Previous studies indicate that responsiveness to photoperiod (PR) has deteriorated in White Rock heavy breed females and males, and that selection for increased PR in females has a positive effect on egg production. In order to determine whether PR can be affected by selection in males, a two-way selection experiment for high and low PR, as expressed through early and late onset of mature semen production under suboptimal photoperiod, was implemented in White Rock heavy breed males. In each selection line, the selected males in each generation were mated to unselected females of the same line. After four generations of selection, males and females of the two selection lines did not differ significantly in 6-wk body weight. Males of the high-PR line, however, achieved mature semen production 18.9 d earlier (P < 0.01) and at a body weight 587 g lighter (P < 0.01) than the males of the low-PR line. Females of the high-PR line came into lay 25.9 d earlier (P < 0.01) and at a body weight 375 g lighter (P < 0.01) than females of the low-PR line; they also produced 13.9 more eggs over a 14-wk test period (P < 0.01). These results show that it is possible to affect PR by selection in males, with parallel effects on female PR and reproductive performance. This result carries the potential of improving reproductive performance of male and female broiler chickens through mass selection for early sexual maturity among males.

Comb size and estrogen levels toward the onset of lay in broiler and layer strain females under ad libitum and restricted feeding

The time course of comb development and estrogen levels were compared in broiler (BX) and layer (LX) females that consumed feed ad libitum (A) or were subjected to quantitative feed restriction (R). The chicks were reared under short photoperiod [(6 h light (L):18 h dark (D)] until 22 wk of age. At this time, photoperiod was increased in one step to 8L:16 D, and then gradually increased until 14L:10D at 34 wk. There was a significant interaction between genetic type and feeding treatment, such that entry into lay of the LX-R and BX-R females was delayed by 1 and 4 wk, respectively, relative to the LX-A and BX-A birds. Mean comb size of LX-A birds began to increase while still under 6L:18D photoperiod; that of the other treatment groups did not begin to increase until photoperiod was shifted to 8L:16D. Comb size of individual LX-A, LX-R, and BX-A birds began to increase about 8 wk prior to individual onset of lay; that of BX-R birds about 11 wk prior to onset of lay. In all groups, estrogen levels remained low until 3 to 4 wk prior to onset of lay, when they showed a sharp increase. Following onset of lay, estrogen levels of all groups remained high. Critical day length of LX-A birds appears to be lower than that of BX-A birds. In addition, feed restriction per se appears to decrease photoperiod responsiveness of BX birds.

Selection for high and low threshold body weight at first egg in broiler strain females. 7. Effect of photoperiod on body weight and age at onset of mature semen production, in males of the selection lines, and in commercial broiler and layer males

Effect of photoperiod on age and BW at attainment of mature semen production was examined in males of lines selected for high (HL) and low (LL) threshold BW at onset of lay, and in males of a commercial Cornish male line (MC), a commercial broiler breeder female line cross (BX), and a commercial layer cross (LX). Photoperiod treatments were: I, increasing natural spring light; and D, steadily decreasing artificial light provided in a light-controlled room. Under I, age at onset of mature semen production was 168.5, 133.6, 138.1, 124.0, and 118.0 d for HL, LL, MC, BX, and LX males, respectively. Under D, age at onset of mature semen production increased by 18.3, 8.0, 55.9, 60.9, and -2.0 d, respectively. The differential effect of Treatment D on the various genetic stocks is attributed to differences between stocks in photoperiodic drive. Therefore, the very great increase in age at onset of mature semen production in BX and MC males points to a severe reduction in photoperiodic drive in males of these stocks, and to future problems in male fertility in broiler breeder flocks entering lay in the summer and fall. Lines HL, LL, BX, and LX rank the same with respect to photoperiodic drive, whether measured in males as in this experiment, or in females, as in a previous experiment. This result suggests that genetic factors affecting photoperiodic drive come to similar expression in males and females. Thus, selection for increased photoperiodic drive in broiler line males may be able to increase photoperiodic drive in broiler line females, and in this way improve female reproductive performance.

Selection for high and low threshold body weight at first egg in broiler strain females. 6. Threshold weight for onset of photosensitivity in females of the selection lines

A two-way selection program, carried out for high and low threshold BW at onset of lay (TWL) resulted in a marked difference in TWL in the selected lines. Later studies showed that the selection procedure had decreased photoperiodic drive in the high line (HL) as compared to the low line (LL), in this way increasing the number of days required from onset of photosensitivity to onset of lay in the HL as compared to the LL. The present study was carried out to determine whether the difference in photoperiodic drive was sufficient to account for the entire difference in TWL between the HL and LL, or whether it is also necessary to postulate a difference in threshold weight for onset of photosensitivity (TWSM) as well. Birds of the two lines were raised under feed restriction to 22 wk reaching weights (1,881.8 g) for LL, 2,304.8 g for HL), well below anticipated TWSM. They were then provided gradually increasing amounts of feed, resulting in daily gains of 3 to 5 g for the LL and 8 to 9 g for the HL. Under this regimen, the HL entered lay an average of 64.4 d later and at a body weight 752.8 g greater than the LL. On the basis of previous studies of these lines, of the 64.4 d delay in onset of lay in the HL as compared to LL, 21 d can be attributed to the difference in photoperiodic drive. The excess of 43 d must thus represent a difference in TWSM. During this period the HL were gaining about 8.5 g/d, and at 22 wk HL body weight was already 153 g greater than LL body weight. In all then, TWSM of HL is estimated at 518 g greater than that of the LL. Thus, the selection procedure employed for TWL apparently affected TWSM as well as photoperiodic drive.