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Selective Breeding Case Study Chickens With Feathered

In each of the study regions two types of data collection were applied. Firstly, individual farmers were interviewed and a list of detailed information was obtained. Secondly, and based on the results of the individual interviews, farmers were asked to discuss in groups on what they considered as most important regarding selection decisions and market value.

Data collection and analysis

The interview was designed to collect two sets of data. The first set covered general information on household characteristics and poultry holdings. The second set included data on more specific aspects of village poultry production such as socio-management characteristics, production objectives, population structure, breed choice and trait preferences, market preferences of specific traits, and farmers’ selection practices. A total of 225 households (45 households from each Woreda) were interviewed. The interview data were analyzed using descriptive statistics, and the percentage of respondents was reported for each parameter.

The subsequent participatory farmers’ discussions were designed to involve stakeholders in identifying the breeding objective “traits” and deriving their relative importance in the different production environments. In total seven independent groups of farmers were formed in each region, where each group comprised of five to seven members. The groups consisted of neighboring farmers following a transect walk in the villages. In order to address the variations in the opinions of farmers in different agro-ecological regions, the production system was classified into two “sub-systems”: low altitude and high altitude systems. Three regions were selected to represent the two “sub-systems” (Mandura for the low altitude and, Farta and Horro for the high altitude production “sub-system”).

As point of departure for the discussions, the results of the individual interviews were summarized according to (1) identified overall objectives of keeping chickens (egg or meat production, income generation, cultural/religious roles), (2) “traits” affecting consumer preferences in purchasing and/or selling chickens (live weight, plumage color, comb type), (3) “traits” farmers desired to be considered in improving village chickens (adaptation, growth, egg production, plumage color, “qumena,” comb type, reproduction). The “traits” were defined in composite terms such as “adaptation” (comprising disease and stress tolerance, flightiness/ability to escape predators, scavenging vigor), “live weight/growth” (weight gain, live weight at market age/adulthood), “egg production” (annual egg number, persistency of egg laying), “reproduction” (broodiness, hatchability of eggs), and “qumena” (conformation/erectness, visual attraction/color, size). Farmers who had adopted exotic chickens (i.e., modern, genetically improved chickens, mainly Rhode Island Red) were asked to rate their opinions on the comparative production, reproduction, and behavioral performance of indigenous chickens with respect to modern ones.

The discussions were aimed at coming to consensus regarding the ranking of the traits in the three categories, and in some cases, on the preference for indigenous or exotic chickens. Per category, a list of the different functions of chickens and “traits” identified in the interviews was prepared into separate flip charts and presented to each group for rating them according to their order of importance. The ratings were carried out by assigning different weights, ranging from 1 to 4 for the different functions of chickens and “traits” affecting market preferences and, weights 1–5 and 1–7, respectively, to rate the relative importance of the “traits” farmers desired to be improved in males and females (the highest weight = most important, the lowest weight = least important). Each group discussed thoroughly and assigned relative weights, on consensus or majority vote otherwise, with the aid of a facilitator. Averages of the relative weights assigned by the groups in each region were finally ranked and compared using Wilcoxon signed ranks test.

To get an impression on the viability of the populations, the effective population size was determined (Falconer and MacKay 1996):

and the increase in inbreeding per generation as

where Ne is the effective population size, Nm the number of breeding males, Nf the number of breeding females, and ∆F the inbreeding coefficient.

ABSTRACT

Chickens, as well as other animals, have the ability to change their behavior (behavioral plasticity) and physiology (physiological plasticity) based on the costs and benefits to fit their environment (adaptation). Through natural selection, the population preserves and accumulates traits that are beneficial and rejects those that are detrimental in their prevailing environments. The surviving populations are able to contribute more genes associated with beneficial traits for increased fitness to subsequent generations. Natural selection is slow but constant; working over multiple generations, the changes to the population often appear silent or undetectable at a given point in history. Chickens were domesticated from the wild red jungle fowl. The principle of domestication of chickens, as well as other farm animals, by humans is similar to that of natural selection: selecting the best animals with the highest survivability and reproducibility (artificial selection). Compared with natural selection, the process of artificial selection is motivated by human needs and acts more rapidly with more visible results over a short time period. This process has been further accelerated following the development of current breeding programs and the emergence of specialized breeding companies. A laying hen, for example, produces more than 300 hundred eggs a year, whereas a jungle fowl lays 4 to 6 eggs in a year. During the domestication process, chickens retained their capability to adapt to their housing environments, which is usually achieved by genetic changes occurring with each subsequent generation. Genes control the behavioral, physiological, immunological, and psychological responses of animals to stressors, including environmental stimulations. With advances in understanding of genetic mediation of animal physiology and behavior and the discovery of the genome sequences of many species, animal production breeding programs can be improved in both speed and efficiency. Modern chicken breeding programs have the potential to be operated successfully in the breeding of tomorrow’s chickens with high production efficiency and optimal welfare, resulting from resistance to stress, disease, or both.

THE ABILITY OF ANIMALS’ ADAPTATION

Change is defined as becoming different or modification. Change is the rule (i.e., our world is constantly changing). In nature, the changing environment puts selection pressure on all living things including animals for survival and reproduction (natural selection). In artificial or man-made environments (domestication), farm animals including chickens also constantly change to meet human needs (artificial selection). In fact, under selection pressure (natural selection, artificial selection, or both), an animal or species has the ability to alter its development, growth, physiology, and behavior to fit a particular environment through adaptation (Crespi and Denver, 2005). How well animals adapt to their environment affects their reproductive success, survival capability, and degree of well-being.

Adaptation, in a physiological context, is the basic phenomena encompassing various biological processes that ensure an organism becomes better suited to its current environment (plasticity). In an evolutionary context, adaptation refers to changes in the characteristics of populations or species resulting from natural selection. The apparent degree of adaptation could be significantly different among species and among individuals within a species based on their genetic make-up (hereditary); however, adaptation is a universal reaction of all living organisms. In general, adaptation can be classified as evolutionary adaptation (heritable changes over generations), individual adaptation (changes within an individual during its lifetime), and cultural adaptation (passing on individual changes to others in the group). In fact, there are overlaps between the different adaptations in animals to meet the requirements of their given environment. Commonly, animal populations are genetically modified in response to environmental challenge to shape their unique characteristics in an adaptive manner (Brommer, 2000; Horton, 2005). The genetically associated adaptation can be observed in changes in the physical, physiological, and behavioral expression (i.e., associations of molecular marker-biological traits; Gilbert, 2001; Nijhout, 2003; Crespi and Denver, 2005; Soller et al., 2006). The ability of animals to express plasticity at genetic and physiological characteristics provides a foundation for the breeding of tomorrow’s chickens to improve well-being and, at the same time, to meet human needs.

MECHANISMS OF ADAPTATION OF ANIMALS

Genetic Plasticity

Genes are sensitive to environmental change. Adaptation of animals can occur through modification of their chromatin structure, such as change of the gene sequence through genetic drift, mutation, and recombination (Soller et al., 2006). Genetic changes in a population of animals, as a function of selection pressure, lead to sustained differences in phenotypes (phenotypic plasticity). Each phenotype, with changed gene expression, will have uniquely observable biological characteristics (Price et al., 2003; Price, 2006). For example, natural variations in stress response in rats influence gene expression and stress response in their offspring (i.e., some of them adapt to stimulations better than others; McGowan et al., 2008). Genetic improvement of farm animals including chickens has been done through selecting varieties under the production conditions (phenotypes based on human needs). The better flitted animals were selected to breed (specific alleles) and, consequently, to pass on favorable characteristics (genes) to their offspring. Genetic diversity in chickens (Soller et al., 2006) and their different expressions in response to various stimuli, such as social stress and temperature (cold and hot stress), have been reported previously (Craig and Muir, 1996a,b; Hester et al., 1996; Cheng and Muir, 2005). Understanding the mechanisms underlying genetic plasticity and expression as well as their association with biological traits will provide new strategies for breeding tomorrow’s chickens to improve their well-being.

Physiological Plasticity

Physiological plasticity refers to changes in the biological processes of an animal in response to environmental stimuli. It permits the animal to adapt to a particular environment for successful survival or reproduction, or both. In animals, physiological changes could act at multiple levels, such as within the endocrine, metabolic, and nervous systems. Among these systems, the nervous system plays an important role in animal adaptation to a given environment.

In the central nervous system, neurons receive, identify, integrate, and interpret incoming sensory stimuli from the internal and external environments of the body, then produce electrochemical impulses that are transmitted to the effectors organs of the body (muscles and glands) to initiate appropriate responses to the stimuli (adaptation). The responses aim to satisfy physiological drives of an organism, such as survival, experience of positive or avoidance of negative emotions, and learning to improve performance. In response to changes in the environment, neurons have the ability to change cellular characteristics and functions (neuroplasticity).

Neuroplasticity refers to the ability of the brain to respond and adapt functionally and structurally (such as synaptic plasticity; i.e., activity-dependent modification of the strength of synaptic transmission) to a given environmental challenge, which in turn adjusts subsequent movement, thoughts, feelings, and behavior (Kasper and McEwen, 2008; Pittenger and Duman, 2008; Calabrese et al., 2009; Maleszka et al., 2009). There is evidence that suggests that the central nuclei of avians, at least in part, are morphofunctional homologous to the mammalian nuclei, such as the hypothalamus (Watkins, 1975), nucleus taeniae (homolog to the amygdala of mammals; Thompson et al., 1998; Soma et al., 1999), and Raphe nucleus (Challet et al., 1996), and exert a similar capability for plasticity in response to environmental stimulations (Lowndes and Stewart, 1994; Tramontin and Brenowitz, 2000; Meitzen et al., 2009; Wissman and Brenowitz, 2009).

The mechanisms underlying alterations in neuroplasticity are believed to relate to changes in neurotransmitters, hormones, and growth factors (Kasper and McEwen, 2008). Neurotransmitters are regulated by neuronal activity that is affected by changes in the morphological and physiological properties of the central nervous system neurons. Changes in the neurotransmitter system, including alterations in biosynthesis, densities of receptors, and gene expression, have been used as indicators of central neuronal plasticity in response to stimulations, including domestication, in mammals (Popova et al., 1997; Ferris et al., 1999). These changes functionally help the animal to adapt to its environment and are reflected as changes in behaviors. There is evidence in birds, as in mammals, that neurotransmitter systems exert similar functions in controlling behavioral adaptation (Barrett et al., 1994). Previous studies have shown that there are similar distributions of neurotransmitter receptors, including dopaminergic and serotonergic receptors, in avians as those found in mammals (Richfield et al., 1987; Walker et al., 1991). Different genotypic or phenotypic, or both, characteristics are associated with specific neurotransmitter systems (Popova et al., 1997; Siegel et al., 1999) and several neurotransmitters, such as serotonin (5-HT) and dopamine (DA), that control stress response and behavioral styles have been reported in animals including chickens (Bell and Hobson, 1994; Popova et al., 1997; Weiger, 1997; Cheng and Muir, 2005, 2007).

Behavioral Plasticity

Behavioral plasticity is defined by changes in the way that individuals respond and interact to a given environment (such as searching for food, mating, vocalization, and migration). Behavior of animals is controlled by both internal and external factors. Behavioral traits evolve as a function of other phenotypic and morphological traits, such as neural and endocrine factors (Hetts, 1991; Mehiborn and Rehkamper, 2009). For example, in the chickens, behaviors such as fear, feather pecking, and aggression can be reduced through selecting productivity or longevity, or both, in a group selection paradigm (Muir, 1996, 2003, 2005; Bolhuis et al., 2009).

There are variations in behavioral phenotypes within a species and within an individual over time. Behavioral traits have a strong hereditable component in much the same way as physiological characteristics [i.e., inherited patterns of behavior of individuals (innate behavior traits) can be modified in accordance with the habitat on an individual]. In addition, behavior of an animal is also determined by its memory and ability to learn new behaviors. Both instinct and learning affect inherited patterns in behavior.

Ability to Learn

Animals can learn new behaviors by trial and error. Wild animals live in a natural environment and possess both inborn behavior patterns (species-specialized) and learned behaviors from life experiences (individual-specialized) to survive and to produce successful offspring in competitive environments. Similarly, farm animals including chickens have the ability to learn new behaviors, which affects their ability to adapt to given environments (Martin et al., 2000; Gibbs et al., 2008). Change of spine and dendrite morphology (synaptic plasticity) as well as the strength of synaptic transmission (long-term potentiation) in the limbic system, especially in the hippocampus, has been found in animals following memory tasks (Martin et al., 2000). Ali et al. (2009) reported that a single exposure to an enriched environment for 1 h stimulates the activation of discrete neuronal populations in the mouse brain. Within the populations, animals’ memory and ability to learn are influenced by both heritable and nonheritable factors.

Physical Plasticity

Physical plasticity is the type of change in an animal affecting its actual structure (structural adaptation). It exhibits as changes of physical features of an animal, such as beak shape and body covering in birds, by which the animal can live in a particular environment and in a particular way. For example, in 1977, Peter and Rosemary Grant documented that during drought, finches can change their physical characteristics, increased body size and beak length, to meet drought-associated environmental demands (Grant, 1986). Honey bees can adjust their body development in response to their social environment (i.e., body development is slowed down in response to an isolated environment; Maleszka et al., 2009).

Taken together, animals including chickens have the ability to adapt to better fit their surrounding environments through functional and physical plasticity. However, the genetic-based varieties in plasticity have been found among the individuals of a population and species in response to natural selection, artificial selection, or both.

NATURAL SELECTION IN ANIMALS

Individuals of a population or populations can be changed by their environment through natural selection. Natural selection is defined as that “selection due to ongoing selection else effects of a past environment,” (i.e., an evolutionary process that leads to differential survival and reproductive success among individuals or groups; Wikipedia, 2009). Competing species have to constantly adapt to their environment to maintain their relative standing. Natural populations carry an immense amount of genetic variability (Dobzhansky, 1981). Some individuals within a population, with favorable phenotypes for a particular environment, are better suited to survive and to have more offspring than others, passing their genetic material, including the genetic variants that code for those favorable phenotypes, on to the next generation through natural selection.

Darwin (1859) was the first to recognize that natural selection causes the phenotypic diversity of evolution in natural populations. In nature, selection acts to provide animals with the ability to display the best behavior (flexible behavioral programs, also called behavioral plasticity) and physiology (flexible energy programs, also called physiological plasticity) based on the costs and benefits of each, with the largest net benefit under different conditions of life being selected for at any given point in history (Krebs and Davies, 1991; Arnold, 2004; Hendry, 2005; Arnold and Burke, 2006).

Natural selection randomly applies selection pressure (the blind force) on the individuals of a population or species. Selection pressures pull in different directions, phenotype as a whole, and the adaptation results in an elegant compromise. Competing species have to constantly adapt to maintain their relative standing. However, in nature, not all individuals and species of animals have equal capability to adapt to their environments or modify their behavioral and physiological characteristics in response to environmental challenges. Most animals die before becoming sexually mature. Only the individuals and populations with greatest adaptive capability survive to pass on their genetic material to future generations, and subsequent generations become better adapted to the environments. Diversification of populations is achieved through natural selection, beneficial traits in a given environment are acquired and maintained (in terms of survival and reproductive), whereas those detrimental in the same environment are rejected (Hendry, 2005). The process (phenotypic plasticity) is evidenced in almost every group of plant and animal (Via and Lande, 1985). However, it may take multiple generations before the changes are great enough to be observed.

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