ABSTRACT
This study was designed to assess genetic diversity of TLR7 gene and its expression profile in the Nigerian indigenous chickens and ISA Brown layer chicken. The study population comprises eight (8) groups (naked neck, normal and frizzled-feathered Nigerian indigenous chickens in rain forest and Guinea Savannah regions respectively, Fulani ecotype chicken and ISA Brown layer chicken). Five (5) blood samples were collected from each chicken group. Genomic DNA was isolated from each blood sample using the Zymo Quick-gDNATM Miniprep kit. The DNA sequencing of chTLR7 gene was done using the Sanger Sequencing Chemistry. Tissues from the thymus and the liver were aseptically collected from Two (2) clinically healthy chickens from each chicken group and were immediately transferred into separate 1.5 ml Eppendorf tubes containing 1 ml of RNALater solution. Total RNA was isolated using ISOLATE II RNA Mini kit. Complementary DNA (cDNA) was synthesized using SensiFASTTM cDNA synthesis kit. The expression of chTLR7 RNA was determined by qPCR assay; β-actin was used as the reference gene. 26 SNPs, two deletions and two insertions in the intronic region of TLR7 gene in the Nigerian indigenous chicken population and ISA Brown commercial layer chicken were found. Haplotype analysis revealed 13 haplotypes out of which nine (9) were unique to the Nigerian indigenous chickens; three (3) haplotypes were shared between ISA Brown layer chicken and the Nigerian indigenous chickens, while one (1) haplotype was unique to the Red jungle fowl. Nucleotide diversity estimates ranged from 0 to 0.019, which were close to zero and suggest that the chicken populations were not genetically differentiated at TLR7 locus. Estimates of gene flow ranged from -0.096 to 0.400 and were close to zero. Genetic distance estimates ranged from 0.007 to 0.054 and were close to zero, which suggests that the chickens have a close ancestor. The estimates of nearest-neighbour statistic ranged from 0.227 to 0.714, which showed that the chicken populations were part of the same panmictic population, hence were not genetically differentiated (P>0.05) at the TLR7 locus. Phylogenetic analysis of TLR7 gene sequences of the genetic groups and the Red jungle fowl revealed very close relationship at the TLR7 locus, which suggests that the TLR7 locus is highly conserved. TLR7 expression in the liver and thymus was significantly different (P<0.01) among the eight chicken groups; the Nigerian indigenous chickens expressed more TLR7 gene than ISA Brown layer chicken. Rain forest naked neck chicken had significantly (P<0.01) highest TLR7 expression of 2.07±0.07 fold. However, expression of TLR7 gene in the liver of rain forest frizzle-feathered and normal chicken, Guinea savannah naked neck, frizzle-feathered and normal chicken, and Fulani ecotype chicken were similar (P>0.05). Gene expression analysis of TLR7 RNA suggests that the Nigerian indigenous chickens could have comparatively more antiviral immune response than ISA Brown commercial layer chicken, hence could be used to develop chickens lines with good antiviral response. Polymorphisms observed at TLR7 gene in the Nigerian indigenous chickens could be used in marker-assisted selection to produce chicken lines with good antiviral response.
TABLE
OF CONTENTS
Cover
Page i
Declaration ii
Certification iii
Dedication iv
Acknowledgement v
Table
of Contents vi
List
of Tables x
List
of figures xi
List
of Plates xii
Abstract xiii
CHAPTER 1: INTRODUCTION 1
1.1 Background Information on the Nigerian
Indigenous Chickens 1
1.2 Brief Background on the ISA Brown
Commercial Layer Chicken 4
1.3
Toll-like Receptor Seven (TLR7) Gene 5
1.4
Statement of Problem 7
1.5
Objectives of the Study 8
1.6
Justification 8
CHAPTER 2: LITERATURE REVIEW 10
2.1
Origin of the Domestic Chicken 10
2.2
Genetic Diversity of the Domestic Chicken 10
2.3
Assessment of Genetic Diversity
within and Between Chicken
Populations 11
2.4
Genetic Markers for Assessing
Genetic Diversity 12
2.4.1
Random amplified polymorphic DNA (RAPD)
markers 13
2.4.2
Restricted fragment-length polymorphism
(RFLP) markers 14
2.4.3
Amplified fragment length polymorphism
(AFLP) 15
2.4.4
Microsatellites 15
2.4.5
Mitochondrial DNA D-loop 16
2.4.6
Single nucleotide polymorphism (SNP) 16
2.4.6.1 Single nucleotide polymorphisms
(SNPs) analyses in different chicken populations 18
2.5 Ecology and Genetic Profile of the
Nigerian Indigenous Chicken
Populations 21
2.6 Phenotypic Characteristics of the Nigerian Indigenous Chickens 22
2.7 Genetic Analysis of the Nigerian Indigenous Chickens Using
Physiological and Biochemical Markers 30
2.8
Molecular (DNA-based) Analyses of
the Nigerian Indigenous Chickens 35
2.8.1 Molecular genetic analyses of the Nigerian indigenous chickens
using microsatellite markers 36
2.8.2 Molecular genetic analyses of the Nigerian indigenous chickens at
mitochondrial DNA D-loop region 37
2.8.3 Molecular genetics analyses of the Nigerian indigenous chickens
using single nucleotide polymorphism (SNP) markers 38
2.9
Toll-like Receptors (TLRS) 39
2.9.1 The chicken toll-like receptor repertoire 40
2.9.2
Molecular structure of the chicken TLRs 41
2.9.3
The chicken toll-like receptor genes
and their roles in pathogen
recognition 42
2.9.4
Molecular variants of TLR genes in
avian species 44
2.9.5
The chicken toll-like receptor 7
(chTLR7) gene and its expression profile 47
2.9.6
Polymorphisms of chTLR7, and its
association with viral diseases 50
CHAPTER 3: MATERIALS AND METHODS 54
3.1
Experimental Populations 54
3.2
Blood Sample Collection 55
3.3 Laboratory Analysis 55
3.4 Experiment 1: Analysis of Genetic Diversity at TLR7 Gene in the
Nigerian Indigenous Chicken Populations and ISA Brown Commercial Layer Chicken
Using DNA Sequencing 55
3.4.1 DNA extraction and protocol 55
3.4.2
DNA quantification and integrity 56
3.4.3
Polymerase chain reaction (PCR) and primers 56
3.4.4
Preparation of agarose gel 57
3.4.5
Electrophoresis of PCR products 57
3.4.6
Visualization of PCR products
(amplicons) 58
3.4.7
Cleaning of amplicons 58
3.4.8
Agarose gel electrophoresis of cleaned
amplicons 59
3.4.9
Sequencing of PCR products 59
3.4.10
Alignment and editing of sequences 59
3.4.11 Single
nucleotide polymorphism (SNP) identification and estimation of genetic
diversity indices 59
3.4.12 Genetic distance estimation 60
3.4.13
Phylogenetic analysis 60
3.5 Experiment 2: Assessment of Evolutionary Relationship of TLR7 Gene in the Nigerian
Indigenous Chickens, ISA Brown Layer Chicken and TLR7 Gene Sequences from Other
Poultry Species in Genebank 60
3.5.1 Retrieval of DNA sequences from NCBI
database 60
3.5.2
Multiple sequence alignment 61
3.5.3
Genetic distance estimation 61
3.5.4
Phylogenetic analysis 61
3.6
Experiment 3: Gene Expression Profile of TLR7 in Lymphoid Tissues of the
Nigerian Indigenous Chickens and ISA Brown Commercial Layer Chicken 61
3.6.1
Experimental birds and management 61
3.6.2
Tissue collection 62
3.6.3
RNA extraction 62
3.6.4 RNA quantification and integrity 63
3.6.5
Complementary DNA (cDNA) synthesis 64
3.6.6
Real-Time Polymerase Chain Reaction
(qPCR) 64
3.6.7
Assembling of qPCR data and statistical
analysis 65
CHAPTER 4: RESULTS AND
DISCUSSION 66
4.1
PCR Optimization of chTLR7 Gene 66
4.2
Genetic Diversity at TLR7 Gene of
Nigerian Indigenous Chickens
and ISA Brown Commercial Layer
Chicken 66
4.2.1 Single nucleotide polymorphisms (SNPs) and INDELs of TLR7 gene in
the Nigerian indigenous chickens and ISA brown commercial layer chickens 66
4.2.2 TLR7 haplotype variations, nucleotide diversity, genetic
differentiation and nearest-neighbour statistic 75
4.2.3 Genetic distance and relationship of TLR7 gene sequences in Nigerian
indigenous chickens and ISA brown layer chicken 83
4.2.4 Phylogenetic analysis of TLR7 gene in Nigerian indigenous chicken
populations and ISA brown commercial layer chicken 88
4.3 Evolutionary Relationship of TLR7 Gene in Nigerian Indigenous
Chickens, ISA Brown Commercial Layer Chicken and TLR7 Gene Sequence from Other
Poultry Species in Genebank 90
4.4 Expression Profile of TLR7 RNA in Lymphoid Tissues of the Nigerian
Indigenous Chickens and ISA Brown Commercial Layer Chicken 94
CHAPTER 5: CONCLUSION AND
RECOMMENDATIONS 98
5.1
Conclusion 98
5.2
Recommendations 99
References 100
Appendices 117
LIST OF TABLES
Page
2.1 Chicken TLR genes, their locations and
length 42
2.2
Polymorphism statistics at ten TLR
genes in Lesser Kestrels 47
2.3 Polymorphism statistics at ten TLR genes
in House Finches 48
2.4 Expression pattern of chTLR7 mRNA in
different tissues 51
3.1 Chicken populations 54
3.2 Primers and sequences for amplification
of TLR7 gene 57
3.3 Primers information for qPCR of TLR7 RNA 65
4.1 SNPs and INDELs of chTLR7 in the Nigerian
indigenous chickens and
ISA
Brown commercial layer chickens 72
4.2 TLR7 gene haplotypes and their sequences 78
4.3 TLR7 gene haplotype distribution among
the eight chicken genetic groups
and the red jungle fowl 79
4.4 Haplotype and nucleotide diversity of the
chTLR7 gene in the Nigerian
indigenous
chickens and ISA Brown commercial layer chicken 80
4.5 Estimates of gene flow between the
chicken populations 81
4.6 Nearest-neighbour statistic, Snn between
populations of Nigerian
indigenous
chickens and ISA Brown commercial layer chicken 82
4.7 Estimates of mean genetic distance within
Nigerian indigenous chicken
Populations
and ISA Brown commercial layer chicken 85
4.8 Estimates of
mean genetic distance between Nigerian indigenous chicken populations, ISA
Brown commercial layer chicken 86
4.9 Estimates of mean genetic distance
between the chicken genetic groups 87
4.10 Percent identity of chTLR7 gene sequence
retrieved from NCBI database
and
consensus sequence of Nigerian indigenous chickens and ISA Brown
layer 91
4.11 Genetic distance between the Nigerian
indigenous chickens, ISA Brown
commercial
layer chicken and the red jungle fowl (reference sequence) 92
LIST OF FIGURES
Page
4.1 Phylogenetic (Neighbour-joining) tree
constructed from TLR7 gene
sequences from the Nigerian indigenous chickens and ISA Brown
commercial layer chicken 89
4.2 Phylogenetic (Neighbour-joining) tree
constructed from TLR7 gene
sequences of the Nigerian indigenous chickens, ISA Brown
commercial
layer chickens, and TLR7 sequence belonging to the red
jungle fowl 93
4.3 Expression profile of TLR7 RNA in liver
tissues 96
4.4 Expression profile of TLR7 RNA in thymus
tissues 97
LIST OF PLATES
Page
1 PCR optimization of TLR7 gene in
Nigerian indigenous chickens 70
2 PCR optimization of TLR7 gene in ISA
Brown commercial layer chicken 71
CHAPTER 1
INTRODUCTION
1.1 BACKGROUND INFORMATION
ON THE NIGERIAN INDIGENOUS CHICKENS
Indigenous chickens are found in Nigeria and many parts of the
world. They are kept and managed mostly by resource poor farmers in rural
communities as excellent source of protein (egg and meat) and income. Their
meat and eggs are preferred over that of exotic chickens (Dessie and Ogle,
2001). The Nigerian indigenous chickens constitute about 80 per cent of the 120
million poultry found in Nigeria (Fayeye et
al., 2005). Almost every family in rural communities cheaply rears the
Nigerian indigenous chickens under extensive system of management across the
country. They are left to feed on household leftovers and often move about as
scavenging birds. In some places, they are provided with little or no shelter
at night. Despite their large number and economic importance as excellent
source of protein and income to poor rural farmers, the Nigerian indigenous
chickens have not been fully exploited for the purpose of genetic improvement.
Nigerian indigenous chickens are characterized by poor growth and
small body size, which are not desirable in a competitive economic situation
(Ibe, 1990; Ebang and Ibe, 1994). The indigenous chickens of Nigeria lay small
sized eggs, and their egg production is dependent on the management system
(Sonaiya and Olori, 1990; Ibe, 1990). Their exposure to extreme or harsh
weather conditions, diseases and parasites partly account for their poor
performance as evidenced by their small mature body weight and small egg size.
However, the performance of Nigerian indigenous chickens can be enhanced by improving
them genetically using various schemes of selection and improving their
production environment. The Nigerian indigenous
fowls possess great potentials for genetic improvement through breeding
programmes (Ikeobi et al., 1996).
Nigerian indigenous chickens constitute an indispensable Animal
Genetic Resource (AnGR) to the country. The uncontrolled distribution of exotic
chickens in different parts of Nigeria, coupled with uncontrolled crossbreeding
of exotic chickens with Nigerian indigenous chickens led to the dilution of the
indigenous chicken genetic stock. If it is not controlled, the Nigerian
indigenous chicken gene pool could be lost in the future before they are fully
characterized genetically. This is supported with the FAO report (FAO, 1999) that
animal genetic resources (AnGR) in developing countries are being eroded.
The Nigerian indigenous chickens are widely distributed in different
geographical/ecological zones as classified by vegetation types such as
rainforest zones of South-east, South-west and South-south; Savannah zones of
North-central, North-east and North-western Nigeria. The Nigerian indigenous
chickens found in each of the geographical zones are believed to constitute
different genetic populations with limited inter-population gene flow, which
could be attributed to long distances separating those (Ukwu et al., 2017b). These indigenous
chickens may have evolved pronounced adaptabilities in response to combined
influence of locally prevailing environmental conditions, uncontrolled breeding
as well as forces of natural selection, mutation and random genetic drift. It
is based on this background that some researchers have come up with phenotypic
classification of Nigerian indigenous chickens ecologically into different
ecotypes, for example Fulani ecotype, Yoruba ecotype, heavy and light ecotypes,
Tiv ecotypes etc. Categorically, Sonaiya and Olori (1990) characterized
Nigerian indigenous chickens based on ecotypes. Momoh et al. (2010) grouped Nigerian local chickens based on body size and
body weight into heavy ecotype (HE) and light ecotype (LE). The heavy ecotype
represents indigenous chickens from dry savannah (Guinea and Sahel savannah),
Montane region and cattle kraals of the north with mature body weights range of
0.9 – 2.5kg (Momoh et al., 2010). The
light ecotype represents indigenous chickens found in the swamp, rainforest and
derived savannah zones with mature body weights of between 0.68 and 1.50kg.
However, characterization of Nigerian indigenous
chickens based on their mean phenotypic values provides only a crude estimate
of the average effects of the functional variants of genes possessed by the
local chicken genetic resource. A better approach to characterization of
Nigerian local chicken populations is the use of more informative methods of
genetic analysis that employ DNA-based markers in high throughput genetic
analysis of chicken populations. This is necessary in order to precisely
delineate the Nigerian indigenous chicken genetic resource. Such techniques
involving the use of microsatellites, single nucleotide polymorphisms (SNPs)
and other DNA markers are more reliable and more informative than phenotypic
measurements in genetic diversity studies. There is, therefore, need to perform
further characterization of the Nigerian indigenous chicken genetic resource
for information on genetic diversity, relatedness and phylogeny of the
indigenous chicken populations as well as exotic broiler and layer chickens
available in commercial farms in Nigeria using molecular genetics approach. Our
central hypothesis is that genetic analysis of Nigerian indigenous chicken
populations using molecular genetics techniques could uncover any DNA based
polymorphism existing among the indigenous chicken populations across different
regions in Nigeria. Polymorphisms at the level of the DNA can be markers of
choice for molecular characterization of local chickens. If such polymorphisms
at the level of the gene or DNA are uncovered, they can be exploited in
marker-assisted selection (MAS) and gene-assisted selection (GAS) for the
purpose of genetic improvement of Nigerian indigenous chickens. Genetic
characterization is also useful to help conserve the valuable genetic variants
inherent in the local chicken genetic resource.
Some researchers have attempted genetic diversity study
of Nigerian indigenous chickens using molecular genetics approach. For example,
Adeleke et al. (2011) carried out a
preliminary screening of genetic lineage of Nigerian indigenous chickens based
on blood protein polymorphisms. Ohwojakpor et
al. (2012) have also carried out the use of microsatellite markers in
genetic diversity study of Nigerian local chickens. Although randomly amplified
polymorphic DNA (RAPD) markers are obsolete in genetic diversity studies, they
had been used to investigate genetic lineage and relatedness of indigenous chickens
in different countries (Rahimi et al.,
2005; Olowofeso et al., 2006; Mollah et al., 2009; Choy and Kumaran, 2011).
1.2 BRIEF BACKGROUND ON
THE ISA BROWN LAYER CHICKEN
The ISA Brown chicken is a hybrid type of sex-link chicken. The
ISA Brown layer chicken Parent Stock (PS) is produced and globally distributed
by HENDRIX Genetics, a multi-species Animal Breeding, Genetics and Technology Company. The ISA Brown chicken is one among several
commercial layer chickens distributed across Nigeria by Ajanla (CHI) Farms
limited Ibadan, Oyo State, with exclusive franchise for HENDRIX Genetics
(Europe). This brown egg layer chicken
has proven thirty-five (35) years of excellent performance as the best brown
layer in the world and is capable of laying 420 high quality eggs for laying
period between 18 to 90 weeks (ISA, 2019). This strain of layer chicken is a
reliable and versatile layer with excellent feed conversion, which adapts to
different climates, management systems and housing systems (ISA, 2019). The ISA
Brown layer chicken is among different commercial chicken strains usually
preferred by farmers due to their high egg production and their innate
characteristics (Islam et al., 2015).
However, the breeding programme from which ISA Brown layer chicken was
developed is usually a guarded secret by the Breeding Company that developed it
in order to avoid competition.
1.3 TOLL-LIKE RECEPTOR
SEVEN (TLR7) GENE
One of the mechanisms by which the innate immune system of farm
animals and humans sense the invasion of pathogenic micro-organisms is through
the toll-like receptors (TLRs). Thus TLRs recognize specific molecular patterns
known as pathogen associated molecular patterns (PAMPs) that are present in
microbial components (Akira and Takeda, 2004). TLRs are membrane-bound
pattern-recognizing receptors (PRRs) that play key roles in the phagocytosis
and activation of pro-inflammatory signal transduction pathways (Underhill and
Ozinsky, 2002) and are also essential for initiating adaptive immunity in
vertebrates (Alcaide and Edwards, 2011). TLRs are proteins synthesized by
toll-like receptor genes. Since these genes are implicated in the immune system,
they could therefore be appealing candidates for examining the selection
processes shaping genetic diversity (Downing et al., 2009). The identification of immunity related genes, which
causes variations among individuals in a population, will provide a key element
in characterization and further studies of disease resistance (Bulumulla et al., 2011).
Recent gene-targeting studies have revealed that TLRs are capable of
sensing organisms such as bacteria, fungi, protozoa and viruses (Uematsu and
Akira, 2008). In response to pathogen associated molecular patterns (PAMPs),
which include various components of pathogens that are not expressed by hosts,
TLRs induce the production of reactive oxygen and nitrogen intermediates (ROI
and RNI), inflammatory cytokines and up regulates the expression of co-stimulatory
molecules, subsequently initiating the adaptive immunity (Kannaki et al., 2010). The stimulation of
different TLRs induces distinct patterns of gene expression, which not only
leads to the activation of innate immunity but also instructs the development
of antigen-specific acquired immunity (Akira and Takeda, 2004).
Toll-like receptor (TLR) genes are highly conserved
group of DNA molecules (Boyd et al.,
2007) present in a wide species of animals and plants (Temperley et al., 2008). The TLR multi-gene family
comprises a large and variable number of genes (10 – 15 genes) with substantial
differences within and between vertebrate groups (Roach et al., 2005; Werling et al.,
2009). Since TLR genes are proven to be associated with resistance to infectious
diseases, these genes can be exploited as molecular markers in genetic
selection to develop disease resistant lines (Kannaki et al., 2010). TLR genes can also be candidate genes for genetic
diversity studies of indigenous chickens to exploit SNPs that may be used in
gene-assisted selection (GAS) and marker-assisted selection (MAS).
With recent advances in
molecular biology and genomics, analyses of chicken expressed sequence tags
(ESTs) and genomic sequences have revealed TLR genes in chicken (Leveque et al., 2003; Iqbal et al., 2005). TLRs and their corresponding genes in the domestic
chicken (Gallus gallus domesticus)
have been well characterized (Fukui et al.,
2001; Lynn et al., 2003; Smith et al., 2004; Philbin et al., 2005; Iqbal et al., 2005; Yilmaz et al.,
2005). Chicken toll-like receptor (chTLR) genes have been reported to be
polymorphic among different avian breeds, suggesting a varied resistance across
numerous chicken breeds (Ruan et al.,
2012b). Toll-like receptor TLR7 gene is
a member of this multi-gene family, which is implicated in recognition of viral
PAMPs. The chicken toll-like receptor 7 (chTLR7) gene encodes 1047 amino acid
protein with 62 per cent identity to huTLR7 and a conserved pattern of
predicted leucine-rich repeats (Philbin et
al., 2005). Although TLR domains are highly conserved among species and
among different members of the multi-gene family because of their crucial
participation in signaling transduction (Barreiro et al., 2009), point mutation on their sequences can bring about
single nucleotide polymorphisms (SNPs) hence creating genetic variants among
populations of the same species. For example, four new single nucleotide
polymorphisms (SNPs) of TLR7 gene were discovered in Chinese ducks (Zhu et al., 2011). In addition, three
genotypes were all found in each mutation site on TLR7 gene in Chinese ducks
(Zhu et al., 2011). Recent reports
suggest that the ability of certain individuals to respond properly to TLR
ligands may be impaired by single nucleotide polymorphisms (SNPs) within TLR
genes, resulting in an altered susceptibility to, and course of infectious or
inflammatory disease (Schroder and Schumann, 2005). However, TLR7 has not been
investigated in the Nigerian local chicken populations. An important goal in studies of DNA sequence
variation is to identify loci that have been targets of natural selection and
thus contribute to differences in fitness between individuals in a population
(Akey et al., 2004).
Since TLR7 gene has been
characterized and sequenced (Yilmaz et al.,
2005; Bulumulla et al., 2011), it is
an appealing candidate gene for genetic diversity study of Nigerian indigenous
chicken populations as well as exotic broiler and commercial layer chickens in Nigeria.
Thus, polymorphisms at TLR7 locus could be used in selection to develop
chickens with excellent anti-viral response.
1.4 STATEMENT OF PROBLEM
The local chicken population in
Nigeria constitutes a valuable genetic resource, which has not been fully
harnessed. This valuable animal genetic resource (AnGR) is gradually being
eroded due to extensive introduction and distribution of exotic chicken breeds
into the local chicken gene pool. The use of molecular tools to assess the
genetic characteristics of the local chicken population before they are
completely eroded constitutes the problem to be addressed in this study.
1.5 OBJECTIVES OF STUDY
The objectives of this study are to:
·
Determine novel SNPs of TLR7
gene in Nigerian indigenous chicken populations and ISA Brown commercial layer
chicken using DNA sequencing,
·
the haplotype variations that
may exist among Nigerian indigenous chicken populations at TLR7 genes, and
·
the genetic distance and establish
a phylogenetic relationship of TLR7 gene in Nigerian indigenous chicken
populations and ISA Brown commercial layer chicken;
·
Compare the evolutionary
relationship of TLR7 gene in Nigerian indigenous chickens ISA Brown commercial layer
chicken and TLR7 gene sequences from other poultry species in Genebank, and
·
the expression profile
of TLR7 RNA in the liver and thymus tissues
from Nigerian Indigenous chickens, and ISA Brown commercial layer chicken using
real-time polymerase chain reaction (qPCR).
1.6 JUSTIFICATION
There is need to
analyze the genetic potentials of the Nigerian local chicken using molecular
genetics approach. Poultry, particularly local chickens are genetically
diverse, so it is important to analyze their populations in order to identify functional
variants, genotypes or individuals of particular merit. Report on phenotypic
assessment of Nigerian indigenous chicken populations revealed a lot of
phenotypic variations, which perhaps does not delimit the chickens into a
breed. However, these reports need to be properly verified using DNA-based
information in other to accurately delineate the genetic diversity status of
the indigenous chicken populations in Nigeria, and probably classify them into
a breed or different breeds. Chicken toll-like receptor (chTLR) genes have been
found polymorphic among different avian breeds, which suggest a varied
resistance across numerous breeds of chicken. Since toll-like receptors 7
(TLR7) gene is implicated in intracellular recognition of nucleic acids,
especially viral ribonucleic acid (RNA), and has been sequenced, therefore,
there is need to exploit this gene in the Nigerian indigenous chicken
populations and ISA Brown commercial layer chicken. This is necessary in order
to uncover hidden functional variants that may exist at TLR7 gene. Such
functional variants at the level of the DNA, if uncovered, could be exploited
in further studies of disease resistance and also used as molecular markers in
genetic selection for the purpose of improvement of quantitative traits in the
Nigerian local chickens.
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