Cadieu ET Al. Literature Reviews Example

Literature Review

Abstract
Why
The genetic basis of dog coat growth pattern, length, and curl is not as well-characterized as coat color.
What/How
Genome-wide association studies (GWAS) were performed on >1000 domestic dogs from 108 breeds to determine which genes direct or influence dog fur phenotypes.

Major Results

Distinct mutations in three genes (RSPO2, FGF5, and KRT71) were identified to have combinatorial effects on coat phenotypes in purebred dogs in the US.

Concise

The abstract contained all necessary information without being lengthy. Scientific jargon was kept to a minimum: enough to be understood by readers with sufficient knowledge on the topic but without sacrificing the technicalities of the performed research.

Introduction

Importance/Relevance
Elucidating the mechanisms of dog coat variation can lead to further understanding mammalian form and function evolution in the light of artificial selection.

Prior Research

Mutations in the FGF5 gene were associated with an atypical long-haired phenotype in Welsh corgis and excess hair growth in mice and cats.

Mutations in the KRT71 gene have been identified in mice with curly coats.

Relatively little research has been done to determine the genetic basis of coat (pelage) phenotype across multiple domestic dog breeds.
Current Research Objective
Materials and Methods
Animal Subjects
The research sampled a total of at least 1736 domesticated dogs (Canis lupus familiaris) from 108 breeds recognized by the American Kennel Club (AKC). Bias was eliminated by sampling individual breeds extending across more than 100 breeds with varied coat phenotypes. The relatively large sampling size also statistically validates the results of the study.

Phenotyping

Coat characteristics were determined through visual identification, as guided by the standards of the AKC. A Dachshund subgroup displayed three coat varieties: smooth-coated, wire-haired with furnishings, and long-haired without furnishings. APortuguese water dog (PWD) subgroup had two coat types: curly-haired and wavy-haired. The remaining dogs were of varying phenotypes.

Genotyping, Analysis, Equipment and Software

Affymetrix 2.0 canine single-nucleotide polymorphism (SNP) chip was used to obtain three data sets involving the following: (1) Dachshund subgroup (for furnishings and hair length); (2) PWD subgroup (for curl); and (3) multi-breed subgroup (CanMap data set).

All three traits (furnishings, length, and curl) were mapped using a similar strategy:

GWAS within a single breed to determine most strongly associated locus.
GWAS within the CanMap data set to eliminate false-positives.
Fine-mapping and sequencing for identification of the putative causative mutations.
Validation (genotyping) in at least 661 dogs from 108 breeds to confirm causation.
Results
Furnishings
The presence of furnishings was associated with a 167-bp insertion in the 3'UTR of the RSPO2 gene, encoding the R-spondin–2 protein.

Furnishings were observed to be dominantly inherited and expressed.

The insertion was associated with a threefold increase in transcript expression levels.
Hair length
A Cys95Phe mutation within the first exon of the FGF5 gene(encodingfibroblast growth factor-5) was associated with hair length.
The presence of the mutated allele in long-, short-, and medium- haired dogs suggested a recessive mode of inheritance.
A number of breeds with very long hair did not exhibit the mutation in the FGF5 gene or any association with the locus.

Curl

A non-synonymous Arg151Trp mutation within the second exon of the KRT71 gene (encodingkeratin-71) was associated with curly coat.
Findings suggest a recessive mode of inheritance.
Pelage phenotype
Combinations of the identified mutations in the three genes were concordant with at least seven dog coat phenotypes:
short: ancestral state (no mutations) of genes
wire and furnishings: RSPO2 insertion
wire and curly: RSPO2 and KRT71 mutations
long: FGF5 mutation
curly: FGF5 and KRT71 mutations
curly with furnishings: variant forms of the genes

Discussion/Conclusion

Importance
Mechanisms of other traits can be deduced from understanding the genetic basis of pelage characteristics, which may provide further insight into the evolution of form and function in animals.

Interpretations

RSPO2 is involved in the Wntpathway that is important in establishing hair follicles, which makes it most likely to be involved in pelage furnishings.
FGF5 mutations have been associated with hair length in Welsh corgis as well as in mice and cats. This study successfully replicated the previous results.

KRT71 encodes keratin, which is an obvious candidate for pelage variations.

Combinations of the genotypes could explain the phenotypic variation of dog coats across multiple breeds.
Findings suggest that mutations in the ancestral state genes occurred and gave rise to the variant traits. These were then transferred to different breeds through hybridization and selective breeding.

Future Directions

Hair length was observed to be influenced by other loci, which may be then further investigated to determine if influenced by other putative mutations or cross-talk among pathways.
Similar strategies (GWAS and trait mapping) can be employed to other seemingly complex phenotypes of dogs, mammals, or other species to determine relatedness of traits in the light of evolution.

Impact and Application

The research study exemplified that diversity in phenotypes can be rapidly obtained from simple genetic variations.
It may help direct breeders in selectively choosing for desirable traits.
Findings can also be used in determining the ancestry of pedigrees, e.g. determining the emergence of certain genetic variations.

Critical Analysis and Evaluation of Research

Overall the research made use of a sound scientific framework that addressed all the possible limitations of the study. Genome-wide association studies are appropriate for determining gene candidates that are causative of the observed phenotypes. Identifying the genes involved in the development of a particular phenotype can be daunting without a good starting point. By employing a specific to general approach, the researchers were able to determine haplotypes that they fine-mapped and which eventually led them to the specific genes and regions that are associated to the phenotypes in question. The presentation of the findings was concise and focused greatly on the main objective with supporting data not overshadowing the results (studies such as this can be confusing with the multitude of data generated). Finally, using a large sample with varied characteristics but still within an encompassing group (i.e. different breeds of Canis lupus familiaris with varying phenotypes) ensured that the findings were valid and applicable to the entire sample population.

Work Cited

Cadieu, E. et al. "Coat Variation in the Domestic Dog is Governed by Variants in Three Genes." Science 326.5949 (2009): 150-153.