1 Introduction

Genetic diversity (level of biodiversity), refers to the total number of genetic characteristics in the genetic make-up of a species (Wikipedia, 2013). Poultry biodiversity refers to the genetic variants available within and among poultry species (chicken, turkey, quail, duck, goose, guinea fowl, pheasant).

Local poultry breeds make up most of the world poultry genetic diversity and are very important in the developing world like Nigeria, where they represent up to 65% of the total poultry production. In Nigeria, local poultry species of economic importance include the indigenous chicken, guinea fowl and duck. These local breeds are well adapted to extensive husbandry systems or backyard poultry production, unlike the exotic breeds. They are results of millennia of years of evolution and are therefore adapted to the local environment.

As breeds continue to undergo diversity and variation, most of the desirable genetic materials are lost. In 2010, the number of avian breeds classified at high risk of loss and under threat of extinction increased to 30% ( Semik and Krawczyk, 2011). Conservation principles help to preserve the genetic constituent of species which are necessary for species to evolve. Species that have less genetic variation are at a greater risk. Laikre (2010) observed that the importance of genetic variation for maintaining biological diversity and evolutionary processes has been recognized by researchers for decades. This realization, the author opined, prompted agreements by world leaders to conserve genetic diversity. However, very little action has been taken to protect genetic diversity on a global scale. In Nigeria, genetic diversity conservation has remained largely a mere academic exercise. Outside the academia very little has been done in practical management and in national policy implementation.

This paper seeks to highlight the relevance of conservation of genetic diversity to poultry production and to review the principles involved in the conservation of genetic diversity in poultry.

2 Causes of Loss of Genetic Diversity in Breeds of Poultry

Decline in poultry genetic diversity and genetic variation are caused by the following factors.

2.1 Replacement of locally adapted breeds by exotic commercial breeds

There has been a tremendous decline in the number of indigenous poultry breeds over the last three decades. This is occasioned by their replacement with modern commercial breeds in small farms and in the villages. These indigenous breeds consist of locally adapted breeds which have gone through millennia of years of evolution.

2.2 Global amalgamation of primary breeding companies

A lot of closures, mergers and buy over of many poultry breeding companies have been witnessed globally since the past decade. These are due to poor profit margins and inclement business environments. The resultant effect is an overall decrease in total number of purelines by these multinationals marketing breeding stocks globally. Thiyagasundaram (2013) noted that only five major poultry breeding companies supply almost all the egg layer stocks globally. The author affirmed that in the broiler market, only six major primary breeders holding 30 to 40 pure lines control and supply 400 billion broilers globally. These stocks are designed and genetically selected over many generations for economic traits of growth and production. They require specialized management, nutrition, disease prevention and environmental conditions for optimal performance to ensure profitability. They lack adequate heterozygosity to sustain evolutionary changes and adaptation because of uniformity of the genotypic base.

2.3 Consumers preference for certain varieties and breeds

Owing to the consumers preference and demand for certain specifications in poultry products, there has been an intense genetic selection over many decades in favour of the preferred breeds and lines. This leads to loss of genetic variation in industrial stocks. Stocks with less genetic variability find it difficult to cope with most environmental challenges like disease outbreaks and climatic change. Success in commercial animal breeding is dependent on the genetic composition of the populations and reduced variability results in a limited flexibility to reach new breeding goals (Weigend et al., 2013; Crawford, 1990).

2.4 Uncontrolled crossbreeding of the village chicken

In developing countries village chickens represent a diverse gene pool that comprise unique genetic features. Due to their development in a given environment they are better adapted to survive under harsh environmental and management conditions. Crossbreeding has been utilized extensively in the developing countries to improve the productivity of the indigenous livestock. But uncontrolled crossbreeding is one of the major causes of loss of genetic diversity especially among the rural poultry species and breeds. This menace may lead to a replacement of local genotypes with those of commercial hybrids, which have a higher production potential based on high nutrient requirements but are not selected for survival in such an environment (Weigend et al., 2013).

3 Relevance of Conservation of Poultry Genetic Diversity

The relevance of genetic diversity conservation in poultry production cannot be overemphasized because genes play a major role in formation of breeds and species. Genetic resources are the building blocks for poultry development. It enables farmers and breeders to respond to changing environmental conditions and to meet consumers demand. Conservation of genetic diversity helps to reduce the dependency on external inputs as the improvement of indigenous breeds is encouraged.

Conservation helps to preserve desirable traits and produce birds with high quality grades. It also encourages natural selection of only the fittest individuals which are better able to cope with diverse environmental challenges. Wikipedia (2013) noted that if a population of a species has a very diverse gene pool, then there will be more variability in the traits of the individuals in that population and consequently more traits for natural selection to act upon to select the fittest individuals to survive.

Genetic diversity and biodiversity are dependent upon each other. Diversity within a species is necessary to maintain diversity among species, and vice versa (NSF, 2006). The authors argued that if one type is removed from the system, the cycle can break down, and the community becomes dominated by a single species. The National Biological Information  Infrastructure (2008) asserted that the interdependency between genetic and biological diversity lead to changes in the environment, leading to adaptation of the remaining species; and changes in genetic diversity leads to a loss of biological diversity (Pullin, 2002). As breeds continue to undergo diversity and variation, most of the desirable genetic materials are lost. Conservation principles help to preserve the genetic constituent of species which are necessary for species to evolve. Species that have less genetic variation are at a greater risk.

4 Measuring Genetic Diversity

The maintenance of genetic diversity of poultry resources at adequate levels require systematic scientific application of conservation biology principles. This entails the integration of population genetics and molecular biology.

Breeds in many cases have been described based on a few phenotypic traits, not minding that large parts of the genomic make-up of these breeds might be in common. To set up efficient conservation measures, therefore, reliable information about genetic difference between individuals, populations and breeds are required. In the differentiation of poultry breeds emphasis have been shifted from morphological and feather coloring characteristics to differentiation based on measurements at the molecular level (Pym, 2013). Weigend et al. (2013) enumerated the major forces creating genetic differences between breeds and populations to include mutation and recombination, genetic drift, natural selection and migration.

5 Phenotypic Approach to Measurement of Genetic Diversity

5.1 Using phenotypic markers

This gives an initial insight into breed diversity by examining the differences in phenotypic traits. Phenotypic markers may be subdivided into discrete traits (such as morphological characters and phenes) and continuous traits (like body measurements) that are applicable to assessing genetic variation and phylogenetic relationships between various breeds and populations. Several authors have used this approach to differentiate breeds of animals (Ebegbulem et al., 2011; Salako and Ngere, 2002; Moiseyeva et al., 1994; Nikiforov et al., 1998).

5.2 Using biochemical markers

Application of protein polymorphisms and physiological traits in the estimation of genetic variation within and between poultry (and other animals) populations have been reported by several authors (Ukwu and Ibe, 2012; Nosike et al., 2013; Oke et al., 2012 ; Romanov, 1994).

5.3 Using immunogenetic markers

Such as blood groups, is another class of polymorphic markers. They are characterized by genotyping complications and require an accurate locus and allele identification (Weigend et al., 2013).

6 Measuring Genetic Diversity Using Molecular Markers

The assessment of genetic variability at the DNA level has been enabled by the recent advances in molecular technology. Different classes of molecular markers such as restriction fragment length polymorphism (RFLP), endogenous avian virus loci, random amplified polymorphic DNA (RAPD) markers, amplified fragment length polymorphisms (AFLP), mitochondrial DNA (mtDNA) markers, microsatellites and minisatellites, as well as single nucleotide polymorphisms(SNP) markers have been used in genetic diversity studies (Weigend et al., 2013).

According to Weigend and Romanov (2001) the assessment of DNA marker polymorphism suggests that variability in DNA is a powerful tool for examining diversity within and among individuals, families and populations. The authors added that the major advantages of these highly polymorphic markers are their locus specificity, abundance and random distribution over the genome , and their co-dominant inheritance.

Studies have shown that microsatellite typing can be applied to pooled DNA samples for population studies (Semik and Krawczyk, 2011; Hillel et al., 2003). The biodiversity of 52 chicken populations was evaluated (Hillel et al., 2003) based on analysis of 22 microsatellite sequences. Results obtained were in agreement with earlier hypothesis that the red jungle fowl is the major progenitor of the domestic chicken. Using allele ladders for each microsatellite, scoring can be standardized between gels and even between laboratories (Weigend et al., 2013). The authors further argued that single nucleotide polymorphism (SNP) has been identified as a very promising class of molecular markers not only for biodiversity studies. Modern biotechnological technologies and the dramatic expansion of the sequence information within species will afford completely new possibilities for the use of genome variability in the next years to study diversity within and between breeds. SNPs will allow assessing variation within coding regions of genes, and may therefore provide higher sensitivity than microsatellites for identifying populations of greater conservation value, the authors noted. The authors however cautioned that the suitability of SNPs as molecular markers in biodiversity studies needs to be properly evaluated, since DNA polymorphism at the population level is affected by various constraints such as the position in the genome (neutral vs non-neutral), drift, selection and recombination.

AFLPs are reproducible biallelic markers that are distributed through the genome as restriction sites, and they allow estimates of genetic differentiation between breeds and related species (Ajmone-Marsan et al., 2002). AFLPs evaluate the presence or absence of amplification of specific fragments which correspond to insertion, deletion between restriction sites, or single nucleotide polymorphism at a site recognized by restriction enzymes (Semik and Krawczyk, 2011). Using AFLP markers, De Marchi et al.  (2006) determined the genetic diversity of local chicken breeds of north- eastern Italy.

7 Conclusion

It is important to collate research information on genetic diversity of and genetic distance between different populations and breeds of local poultry stock especially, as this is essential to the conservation of these resources. It is advocated that a database containing detailed information on the genetic diversity of Nigerian poultry breeds be developed as this will serve as a useful tool in the conservation of these breeds.

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