Changes in the Control of the Hypothalamic-Pituitary Gonadal Axis Across Three Differentially Selected Strains of Laying Hens (Gallus gallus domesticus)

Abstract

Genetic selection for earlier sexual maturation and extended production cycles in laying hens has significantly improved reproductive efficiency. While limited emphasis has been placed on the underlying physiological changes, we hypothesize that modifications in the control of the hypothalamic-pituitary gonadal (HPG) axis have occurred. Thus, three strains of White leghorn derivatives were followed from hatch to 100 weeks of age (woa), including Lohmann LSL-lite (n = 120) as current commercial hens, heritage Shaver White leghorns (n = 100) as 2000s commercial equivalents, and Smoky Joe hens (n = 68) as 1960s commercial equivalents. Body weight (BW) and egg production were monitored, and blood samples were collected throughout to monitor estradiol (E2) concentrations. Tissue samples were collected at 12, 17, 20, 25, 45, 60, 75, and 100 woa to capture changes in mRNA levels of key genes involved in the HPG axis and monitor ovarian follicular pools. All hens, regardless of strain, age or photoperiod laid their first egg within a 64-gram BW window and, as E2 levels increased prior to photostimulation (PS) in Lohmann and Shaver hens, a metabolic trigger likely induced sexual maturation. However, increased levels of Opsin 5 (OPN5) were observed during the maturation period. Although an elevation in gonadotrophin-releasing hormone I (GnRH-I) mRNA levels was associated with early maturation, no changes in gonadotrophin-inhibitory hormone (GnIH) mRNA levels were observed. Nonetheless, a significant shift in pituitary sensitivity to GnRH was associated with maturation. Throughout the trial, Lohmann, Shaver, and Smoky Joe hens laid 515, 417, and 257 eggs, respectively (p < 0.0001). Results show that the extended laying persistency in Lohmann hens was supported by sustained pituitary sensitivity to GnRH-I, recurrent elevations in follicle-stimulating hormone (FSH) mRNA levels, and five cyclical elevations in E2 levels. This was also associated with a consistently higher pool of small white ovarian follicles. In summary, our results demonstrate first that, regardless of photoperiodic cues, meeting a specific narrow body weight threshold is sufficient to initiate sexual maturation in Leghorn chicken derivatives. Furthermore, recurrent increases in E2 and FSH may be the key to sustain extended laying period, allowing modern layers to double their reproductive capacity compared to their 1960s-counterparts.

Bone and eggshell quality throughout an extended laying cycle in three strains of layers spanning 50 years of selection

“…Apart from 60 woa, Lohmann and Shaver hens had a higher percentage of laying birds than the Smoky Joe ( P < 0.05). In the case of Smoky Joe hens, this was associated with spontaneous moulting, as reported in Hanlon et al., (2021) .…”

“…The relative tibia weight of the Smoky Joe did not change throughout, while the relative tibia weight of Lohmann and Shaver hens declined up to 20 and 45 woa, respectively. This is likely a reflection of a combination of the slower growth rate of the Smoky Joe established in Hanlon et al. (2021) and lower cumulative production.…”

“…This translated to egg weight increasing by 0.7 g per 100-g increase in pullet live body weight at the onset of lay ( Bouvarel et al., 2011 ). This may explain the smaller size of the eggs at the beginning of the cycle and partly explain the continuation of small eggs produced by Smoky Joe hens as they remained lighter than the other strains during peak production ( Hanlon et al., 2021 ). With Smoky Joe hens and eggs remaining lighter throughout the entire laying cycle, the present study suggests that body weight may play a larger role in controlling egg weight throughout the production cycle than initially suspected, as demonstrated by Leeson and Summers (2005) .…”

“…Three strains of laying hens were used in this study to examine the impact of genetic selection for improved sexual maturation, production rate, and laying persistency on bone and eggshell characteristics, as described in Hanlon et al. (2021) .…”

Understanding Transcriptomic and Serological Differences between Forced Molting and Natural Molting in Laying Hens

“…In humans, age-associated decreases in follicle number and oocyte quality decrease fecundity due to oxidative stress [ 14 ]. The hypothalamic-pituitary-gonadal axis has an obvious effect on prolonged laying time and improves the fecundity of laying hens [ 15 ]. Therefore, the gene expression level of hypothalamus and ovary can be used as an important basis for the response of FM to external environmental changes, such as aging or fasting.…”