Health – Roslin Institute
BY SERAP GONEN AND ROSS HOUSTON
Making a marker A
DNA genotyping technologies are helping fi ght disease by selecting the best parents
quaculture producti on has risen by approximately 90 per cent in the last 60 years and, for many established species, farming conditi ons are now well stand-
ardised to maintain fi sh welfare while enabling a profi table industry. However, producti on effi ciency can sti ll be
negati vely impacted by factors such as natural disease outbreaks, which can cause high levels of mortality and reduced market value of disease survivors. Disease outbreaks can result in major economic blows to the industry and decreased welfare of farmed fi sh populati ons. Although many bacterial diseases are well controlled with vaccinati on strategies, viral diseases sti ll present a major threat to the in- dustry, since the development of fully eff ecti ve vaccines is more challenging. In these cases, alternati ve disease control strategies need to be implemented.
One eff ecti ve and widely adopted approach is to identi fy families which are most resistant to the disease af- ter viral exposure, and
to use individuals from these families as parents of the next generati on. An alternati ve and more accurate method of selecti on is to incorpo-
rate the use of geneti c markers into selecti ve breeding programmes to improve resistance. The use of geneti c markers for selecti ve breeding has many welfare and
economic advantages, including the ability to identi fy resistant individu- als without viral exposure and reduced levels of mortality in the case of natural disease outbreaks. Importantly, this approach uti lises the natural geneti c variati on for
resistance by selecti ng parents carrying favourable versions of disease resistance genes for breeding. As such, their off spring have improved resistance to the disease. To implement this disease control strategy on farms, the geneti c mark- ers which are the best predictors of an individual’s resistance status must be identi fi ed. In modern selecti ve breeding programmes, geneti c markers known as single nucleoti de polymorphisms (SNPs) are most commonly used.
SNPs are natural variati ons in genomic DNA between individuals in a populati on, and can infl uence an individual’s biological response to viral infecti on. SNPs are bi-allelic in nature in that they exist as two variant DNA forms. For example, in some individuals, the SNP positi on in the genomic DNA may be ‘A’ (termed as the ‘A allele’), whereas in another individual, the SNP positi on may contain a ‘T’ allele.
Stati sti cal geneti c analyses enable the associati on of one of these SNP alleles to either resistance or suscepti bility to a parti cular disease by identi fying the allele most frequently represented among resistant or suscepti ble individuals.
DNA genotyping technologies can then be applied to determine whether a parti cular individual has the resistant allele even in the absence of infecti on, and these individuals can then be used as parents to pass on this resistant allele to future generati ons.
This method of selecti ve breeding is being successfully applied on Atlanti c salmon farms for improving resistance to the viral disease infecti ous
pancreati c necrosis.
In 2008, scienti sts at the Roslin Insti tute were able to identi fy SNP markers signifi cantly associated with high resistance to infecti ous pancreati c necrosis in collaborati on with the salmon breeding company Landcatch and the University of Sti rling.
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Above : Breeding programmes have resulted in improved resistance to some diseases
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