Research/PIC Project 175
From Poultry Hub
Project title: Understanding eggshell matrix protein genes for improvements in shell quality
Project Leader/s: Dr. Max Hincke
Organisation/s: University of Ottawa
Date: March, 2008.
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Project overview
As a flock ages, the older hens tend to lay larger eggs with weaker shells. This accounts for the well known decline in shell quality, on average, of eggs from older birds. The eggs crack more easily, and flaws in the eggshell permit increased microbial contamination of the egg contents. Therefore, the economic cost of eggs from older flocks is higher due to increased loss of cracked eggs and greater risk of foodborne illness. Shell quality is dependent on the structure of the shell. This structure is comprised of a matrix of proteins on which calcium carbonate mineral crystallizes into a pattern. Although the most important aspect of shell strength is the amount of mineral in the shell, the microstructure (pattern) formed by this mineral also influences shell strength. Although eggs with weaker shells are a problem of older flocks, individual birds within the flock vary in the extent to which age influences shell quality. This observation suggests that the genetics of individual birds affects how the type of shell being produced changes as birds get older. Dr. Max Hincke at the University of Ottawa has investigated the genetic link between the genes of certain proteins that influence eggshell calcification.Read full report
Research
By identifying genetic markers that are responsible physical traits (e.g. shell thickness), breeding programs can be developed that focus on the genes involved. That is, rather than breeding the birds that lay eggs with thicker shells, birds can be bred based on whether they have the genetics to lay thicker-shelled eggs. The availability of the chicken genome sequence and expressed sequence tag resources has made carrying out these analyses easier in pedigree breeding flocks; An Expressed Sequence Tag is a tiny portion of an entire gene that can be used to help identify unknown genes and to map their positions within a genome.
In this study, Hincke and his research team tested the idea that sequence differences between the genes of certain eggshell matrix proteins are related to the decline in shell strength in eggs from older hens. Characterization of the genes coding for these proteins allowed for testing of the idea that small genetic differences between different birds can influence eggshell properties. If this can be demonstrated, then DNA-based tests can be developed by breeders to evaluate birds at hatching to guide selection programs to develop flocks with improved eggshell characteristics. It is foreseen that the normal turnover schedule for laying flocks will allow resulting improvements in breeding stock to be quickly introduced into the marketplace.
The study focused on two important proteins that influence eggshell calcification, Osteopontin (OPN) and Ovocleidin-116 (OC116) that are coded by adjacent genes on chromosome 4. Multiple QTL (quantitative trait loci) studies have reported that chromosome 4 is a critical region of the chicken genome for regulation of phenotypic variation of multiple egg production traits, such as egg shell weight and eggshell strength.In a previous PIC-funded study, it was determined that that there was linkage between 2 OC116 alleles (termed A and B), and two OPN alleles (named 0 and 1), such that the majority of individual birds could be sorted into homozygotes AA00 and BB11, or the heterozygotes AB01. There were, however, a certain number of birds that possessed crossover within the OC116 gene, such that AB00 and AB11 individuals could be identified. In that study with a small number of birds, it was found that older hens exhibited phenotypic differences in certain eggshell parameters that correlated with specific OC116 genotypes. Specifically, eggs from BB birds had thicker shells. Therefore, there are differences in egg and eggshell characteristics between birds that are homozygous or heterozygous for the different OC116 alleles.
The aim of the current study was to confirm this observation in a large sample size, in order to validate the hypothesis that differences in OC116 genotype were associated with eggshell characteristics in older birds. In addition, methods to rapidly assess these differences were developed. Dr. Hincke worked with Dr. Al Kulenkamp and Dr. George Ansah, Shaver Poultry Breeding Farms. Blood samples from a large number of young birds were collected. Dr. Urs Kuhnlein at McDonald College, McGill University assisted with sample processing to isolate genomic DNA from individual blood samples. Analysis for OPN and OC116 alleles was performed at the Genome Quebec Innovation Centre, McGill University. The birds included in the study were individually caged and eggs collected for analysis at approximately 40, 60 and 80 weeks of age. Egg and eggshell properties were measured at the Shaver facilities with specialized equipment (specific gravity, shell thickness, shell breaking strength and shell deformation, egg weight, egg width, egg length). Monitoring egg characteristics over this extended laying period allowed us to assess the impact of OC116 and OPN genotype on these complex traits. It was found that eggs laid by birds that are homozygous for one of the OC116 alleles (BB) have thicker shell than those laid by AA birds, as opposed to the normal tendancy of older birds to lay eggs with thinner, weaker
shell. It was not found that that inclusion of OPN SNP results (alone or in combination with OC116) allowed greater discrimination of these effects.Read full report
The Bottom Line
This study examined a candidate gene hypothesis that sequence differences between the genes of certain eggshell matrix proteins are correlated with the age-related decline in shell strength. The results of this study confirm the previous observation that eggs laid by older birds with a certain ovocleidin-116 single nucleotide polymorphism (SNP) have thicker shells that tend to be stronger. This information suggests that a selective breeding program based on this information would improve overall eggshell thickness in aging flocks. Industrial breeders must consider many bird characteristics; the results of this study provide another parameter that can be introduced into their programs.Read full report
Source
Poultry Industry Council
- 483 Arkell Road, Guelph, Ontario,Canada, N1H 6H8.
- pic@poultryindustrycouncil.ca
- http://www.poultryindustrycouncil.ca
