Genetic and Molecular Regulation of Beak Tip (Rhinotheca) Shape in Layer Hens

The beak is a complex organ that varies considerably in shape and size. Chicken beaks are formed of two bones covered by a layer of keratin (protein that makes up hair, nails, feathers, claws, and beaks). The beak provides laying hens with the means to eat, drink, and explore their environment. It is also capable of inflicting injury on other hens, a harmful behaviour known as injurious pecking. Injurious pecking poses a serious challenge to hen welfare and production, particularly in non-cage flocks where its prevalence can reach 80%. Outbreaks of injurious pecking can result in cannibalism and death, with 50% mortality or more in extreme cases. Despite the advancements in management made in recent decades, pecking outbreaks still cannot be reliably controlled or prevented. Beak trimming is the most effective method of reducing pecking-related damage; however, the practice raises its own welfare issues. This has led to beak trimming bans in numerous countries. In the UK, a ban was considered in 2016 and declined; however, DEFRA aims "to stop routine beak trimming as soon as reasonably possible. To achieve this, every effort is needed to reduce injurious pecking." To move towards a sustainable solution, a multifactorial approach is needed. There has been some success in improving bird management to minimise outbreaks in non-beak trimmed flocks. There has also been success in reducing the incidence of IP by genetically selecting laying hens that are less apt to engage in the behaviour. This work proposes a different direction to supplement these efforts. Recent research suggests that the beak itself can be a tool to reduce pecking-related damage and improve the chances of successfully housing non-beak treated laying hens. Our research group has developed and published methods to identify beak shapes and found large variation within and between genetic layer lines, manifesting as pointedness, angle, and overall beak size. Beyond these external measures of shape and size, using the beak as a tool to reduce damage requires a detailed understanding of how the beak develops within the embryo and how its shape is determined. Presently, it is not fully understood how the distal beak tip (rhinotheca) develops. Namely, are the cells that make up the rhinotheca specialised with respect to cellular fate, molecular identity, and protein composition? Our overall goal is to enable beak shape modification at a commercial scale through genetic interventions including selection and gene editing. We aim to do this by establishing two groups of laying hens with opposing beak shapes. Birds will be placed into groups after their beaks are imaged and categorised using machine learning. Breeding pairs from each group will be used for embryo production. The embryos from these groups will undergo histology and in situ hybridisation to detail changes of cell differentiation that precede rhinotheca keratinisation. Using specialised avian lines from the Roslin Institute, fate maps (used to study embryonic origins of tissues) will be created to establish where the cells that contribute to the beak tip originate from. Finally, understanding where the rhinotheca cells originate and using the breeding group embryos, we will create an in vitro keratinocyte model to explore how perturbations, genetic and delivered, alter these specialised cells' protein production. This will enable testing of genetic variants relevant to beak development and composition. Success in this endeavour requires understanding the genetic, molecular, and cellular determinants of rhinotheca shape to avoid compromising beak function and other pleiotropic effects. By understanding factors that control beak shape at the molecular level, we gain knowledge and tools that may lead to being able to predict and/or manipulate how changes in the genome alter chicken beak shape. In the future, new breeding technologies could capitalise on this knowledge.

Injurious pecking is one of the most observed abnormal behaviours in laying hens. Injurious pecking results not only in pain and mortality, but in increased feed costs and poor feed efficiency. Beak trimming limits pecking-related damage; however, the practice raises its own welfare concerns. Balancing these concerns and the economic pressures to maintain productivity, the poultry industry requires sustainable alternatives to beak trimming that reduce the welfare impact and financial losses incurred by pecking-related bird mortality and feather loss. One such alternative is to use the beak itself as a means to reduce pecking-related damage. However, to do this, a detailed understanding of the genetic, molecular, and cellular determinants of beak shape is needed. Our aim with the proposed work is to fill this gap in the scientific literature by 1) establishing two groups of phenotypically opposed breeding populations of layer hens for embryo production, 2) analysing the pre-hatch embryos from these groups using histology and in situ hybridisation to detail the changes in placode size, progenitor organisation, and cell differentiation that precede beak tip keratinisation, and 3) generating an in vitro epidermal keratinocyte model using chicken primordial germ cells (PGCs) to enable testing of genetic variants that are potentially relevant to beak shape development and composition.