GENETIC IMPROVEMENT IN POULTRY
A major objective of the primary breeding sector of the poultry industry is to genetically improve the efficiency of production, the quality of eggs and meat and the health and welfare of poultry. Efforts are focused on economic traits required by the consumer and producer as well as traits related to environmental issues in poultry production. The impact of biotechnology on poultry genetics and breeding will therefore apply to the layer and meat (broiler, turkey, duck etc.) segments of the industry.
We are all aware of the tremendous genetic improvements that have been achieved in many economic traits in egg and meat strains including egg production, growth rate, livability, disease resistance, feed efficiency and egg and meat quality in the last 2-3 decades. Today, it is feasible for many commercial layers to achieve 314 eggs per HH, 19.20 Kg egg mass, 2.00 FCR and 94% livability from 17-72 wk of age. In broilers, live weight, FCR, eviscerated yield, de-boned breast meat to 42 and 49 days of 2300 & 2770 g, 1.82 & 1.96, 67.9 & 68.8%, and 16.0 & 16.5%, respectively are typical. Similar high performance levels are observed in turkeys. These genetic improvements have been accomplished using what is usually referred to as “traditional” or “conventional” breeding techniques. Further progress in genetic improvement of poultry through the current improved traditional breeding methods is expected to continue in the future.
In the May 2000 issue of the Monthly Newsletter of the International Egg Commission, the United Egg Producers issued the following statement regarding genetically modified (GM) foods and eggs. “Eggs are not a genetically modified food. This includes shell eggs and eggs used for processed egg products. Only traditional breeding techniques are used to produce laying hens in the U.S.; neither chickens nor eggs are modified by genetic engineering. Even when a laying hen eats genetically engineered feed, any product unique to genetic engineering are destroyed by the digestive process of the hen. Scientific research has confirmed that none of the genetically engineered materials are passed into the egg.”
Thus, the discussion of the impact of Biotechnology on Poultry Genetics and Breeding at this year’s Cornell Poultry Conference falls into the debate on GM foods.
It is well established scientifically that genetic improvement of any trait in an animal requires a modification in the genetic constitution of the animal with respect to the genes that influence the trait. In poultry, some of the traits that require continued genetic improvement include stress resistance, disease resistance, immune competence (immune balance), behavior, nutrient utilization efficiency and reproductive performance. In addition, layers must have high rate and persistency of egg production, better feed efficiency, egg size, egg quality (internal and external) and livability. In meat birds growth rate, muscle mass, carcass quality, feed efficiency and livability are important. As these traits are improved genetically, the birds under selection are thus genetically modified. It is therefore important to examine what traditional or conventional breeding methods are and what biotechnology entails with respect to genetic improvement of performance in poultry.
TRADITIONAL (CONVENTIONAL) POULTRY BREEDING
Traditional poultry breeding practices incorporate quantitative and qualitative genetics, reproductive physiology, statistics, computer science and poultry husbandry in a highly interactive fashion to maximize the genetic improvement. The initial ingredient in this process is the accurate measurement of the trait (phenotype). For example, egg number, feed consumption, egg weight, shell quality & mortality for layers; juvenile body weight, amount of fat, breast meat yield etc. for meat birds. The geneticist then analyses these traits for genetic variability at the genome level on the basis of quantitative genetics theory using the “infinitesimal model”. The infinitesimal model assumes that each trait is influenced by many genes, each with a small effect on the trait. Advanced statistical techniques, e.g. Restricted Maximum Likelihood (REML) and Best Linear Unbiased Predictor (BLUP) are used respectively to analyze for genetic variation and estimate the breeding value of the individual chicken or turkey. Selection is based on complex indexes that incorporate, optimally individual and family information for a range of traits with the aim of maximizing overall economic response in the commercial product. It must be noted that quantitative genetic theory is based on the assumption of the organization of the genome, the size and distribution of genetic effects and the linearity of the genetic effects. Essentially, these are just estimates and a complete knowledge of the actual genes involved and all their functions is not yet available. A better understanding of the biological and molecular aspects of growth, development and reproduction will enhance the efficiency of the traditional selection process.
BIOTECHNOLOGY AND ITS MEANINGS
In many agricultural industries including Poultry, the “new” science of Biotechnology is considered by many people to be the pathway to bring improved changes in the efficiency of production to feed an expanding human population. The question is: what can biotechnology achieve that traditional breeding techniques cannot? Alternatively one may ask: What can biotechnology add to conventional breeding methods to accelerate poultry genetic improvement?
The word biotechnology is a composite of 2 Greek words: “BIOS” - meaning “life”, “living being” or “organic nature” and “Technologia” - translated as “systematic treatment”. According to the Convention on Biological Diversity, biotechnology means: “any technological application that uses biological systems, living organisms or derivatives, to make or modify products or processes for specific use.” In this broad sense, biotechnology has been part of poultry research and improvement as well as production systems for many years, although the term is rarely used in conjunction with traditional poultry breeding. For example, strain-cross breeding to exploit heterosis (hybrid vigor), artificial insemination, vaccination, chick sexing, diagnosis to identify carriers of egg transmitted diseases may all be termed traditional biotechnologies. In recent years the word “biotechnology” has been used in a narrow sense to describe a myriad of new technologies that enable us to study, change or manipulate living cells or their components. Terms used to describe some of these novel technologies which are components of biotechnology include “molecular biology”, “molecular genetics”, “genetic engineering”, “gene cloning”, “recombinant DNA”, “gene transfer”, "genomics”, “proteomics” “transcriptomics” and “bio-informatics”. The common denominator of these technologies is that they allow us to manipulate both the reproductive process and the genome (genetic material). These technologies have demonstrated possibilities of introducing within living organisms dramatically rapid morphological and physiological changes that were until recently, only described by science fiction. Our understanding of the molecular structure and function of the DNA and our ability to manipulate it has expanded greatly through recent advances in molecular biology and related novel technologies. We now understand how the genetic information is stored in a cell, how the information is duplicated, and how it is passed from cell to cell, and from generation to generation. Poultry genetic improvement is based on the understanding of the inheritance of economic traits of interest.
With biotechnology, the initial ingredient required for an effective selection is the Gene(s) affecting the trait. The application of biotechnology in poultry breeding will be a move from measuring the Phenotype to measuring the Gene(s). The use of biotechnology should, therefore, be viewed as the logical extension to traditional poultry breeding. The novel aspect of biotechnology is the ability to directly manipulate or change some of the genes that influence traits of interest.
However, to be able to apply the tools of biotechnology or add them to current traditional methods, the breeder must identify the genes that influence the economic traits referred to as Quantitative trait loci (QTL) or Economic trait loci (ETL) or find markers closely linked to the genes. We must know in detail the genetic architecture (biological mechanisms/pathways) of the traits, the causal relationships between traits and the development of analytical tools for selection of genotypes with predictable performance.
The search for genes and markers have been conducted by researchers around the world for sometime and tremendous progress has been made in chickens and to a limited extent in turkeys. The chicken genome is now estimated to have between 50 to 80,000 genes on the 39 pairs of chromosomes. The rough draft of the chicken genome map has been completed through the collaborative efforts of scientists in the USA, the European Union and many other countries. The chicken genome map now has over 900 markers while in turkeys, the BUT/BUTA map is reported to have about 100+ markers.
With such tools, Geneticists will be able to use Marker Assisted Selection (MAS) to select for genes that control specific traits of commercial importance. Marker Assisted Selection, in general, is not new in poultry improvement. It has been used to improve productivity traits and resistance to Marek’s disease. What is new is the power of molecular techniques to use DNA markers. Marker Assisted Selection could be used to augment selection for traits like egg production that can be measured only in one sex (sex - limited trait), to select at a very early age, even at the embryonic stage when the trait is not expressed phenotypically, to type traits that are expensive to measure, and to select for resistance to specific diseases without direct challenge of pureline breeders. The Genetic testing and selection process could be accomplished more quickly. Furthermore, the accuracy of picking the best breeders to meet targeted genetic gains would be improved.
Alternatively, genes or markers may be isolated and inserted (transgenesis) into the germ line of different birds. This technology will also give us the ability to combine cells of different genotypes to obtain poultry that produce unique products. Research efforts toward developing routine and robust gene transfer techniques to produce transgenic chickens have been underway for many years. Much progress has been made and today, there are reports of transgenic birds developed to produce specific proteins. However, techniques to produce transgenic birds for commercial breeding of egg and meat strains are not yet finalized. When these techniques become routine and robust, they may have significant impact on the genetic improvement of many economic traits in poultry.
Although rapid advances are being made in poultry biotechnology, much more remains to be discovered about the chicken genome itself. For example, the delicate interactions of genetic background with genes of major effects is still an issue of great importance in selection. Various improvements in biotechnology will be achieved in steps and eventually biotechnology and conventional methods will be integrated to become standard tools for genetic improvement of poultry. It will be a marriage of necessity with the cultures of both technologies supporting each other. I hope that at that time we could readily say to any audience that genetic improvement in poultry is by the application of traditional (conventional) methods and tools of biotechnology.
Speaker at Cornell's Poultry Conference: George Ansah, ISA Breeders, Inc., Ithaca, NY.
Published on Cornell Poultry Pointers newsletter by the Cornell University Cooperative Extension