Ebermast is "in" and Androstenon again a topic
Since individual countries in Europe have already banned the castration of piglets or are about to do so, the meat industry in Germany is also pressing for a more concrete approach to boar fattening. For farmers and slaughterhouses, attractive performance data of the boars in comparison to the Börgen are in the foreground. Consumers, however, may have some cause for concern because boars cause them a bad taste and smell.
This aspect is taken up in the summary work Genetic and metabolic aspects of androstenone and skatol deposition in pig adipose tissue: A review by R. ROBIC, C. LARZUL and M. BONNEAU according to the latest state of knowledge and with a view to breeding influence possibilities. The boar's unpleasant sex odor is made up of the two components androstenone and skatole, which accumulate primarily in fatty tissue.
Androstenone is a substance related to steroid hormones and, like them, is synthesized in the testicles. The urine-like odor is noticed by some consumers even in low concentrations, others cannot perceive this component at all.
Skatole belongs to the indole group of substances and is produced as a metabolite by the intestinal bacteria in the large intestine. Accordingly, the odor is feces-like and is perceived by practically all consumers as an unpleasant aroma component.
For both substances, breeding towards a low fat content is promising. The heritability is high for androstenone (h2 = 0,25 to 0,88), for skatole it is at least in the medium range (h2 = 0,19 to 0,54). There appears to be a positive genetic correlation between skatole and androstenone.
The authors' inventory is based on the results obtained so far for the characterization of "quantitative trait loci" (QTL) and of individual genes that are active in the metabolism of androstenone and skatole. First of all, QTL research only serves to determine certain chromosome sections that are related to quantitatively measurable characteristics.
A number of such sections (QTL) were actually found for both androstenone and skatole, which, however, can only explain a small proportion of the variance of the trait (usually less than 10%). A multigenetic inheritance can therefore be assumed. Only in one case is an androstenone QTL close to a skatole QTL.
When it comes to the search for individual influencing genes, it is of interest to look at the metabolism of the substances. Genes generally work by “switching on and off” enzymes, which in turn initiate the build-up and breakdown of substances. This perspective is particularly important for androstenone because it makes it immediately obvious how closely this substance is linked to the metabolism of androgens (especially testosterone) and estrogens as well as progesterone. This means: if the metabolism of androstenone is "twisted", there is a risk that the reproductive performance of both the boar and the sows could be affected.
At least two ways seem to be able to avoid this problem. The first leads via cytochrome oxidase b5, for which the underlying genetic information is already known due to the successful sequencing of the mRNA. This enzyme is relatively specifically involved in the construction of androstenone and could therefore be considered as a target gene (target gene). However, it has not yet been identified as part of a QTL.
Two other enzymes suggest the way to break down the androstenone. The first of these groups is a hydroxysteroid dehydrogenase (3ß-HSD), for which the liver variant is primarily considered. Far removed from the actual sexual metabolism, this enzyme regulates the deposit of androstenone in body fat. The other enzyme is a hydroxysteroid sulfotransferase that selectively inactivates androstenone in the testes.
For both enzymes, the localization of the underlying gene has been completed, but again there are no relationships to neighboring QTL.
A look at the metabolism is also promising for skatole. For understandable reasons, we are only concerned here with enzymes that are active in the liver: the conversion of a substrate produced in the intestine takes place first in the liver. The two enzyme systems involved are different cytochrome P450 enzymes and a sulfotransferase.
It is nevertheless clear that the P450 cytochromes initiate the first steps of skatole degradation in the liver and that defects in this system are associated with increased skatole levels in body fat. The nucleotide sequences involved are already known for individual enzymes, but in general no relationships between these metabolic-related individual gene loci and the QTL for skatole have been found. The situation surrounding sulfotransferase seems to be even further removed from a satisfactory explanation.
At least it is known that there are gene polymorphisms for this enzyme that are associated with different skatole conversion and high or low skatole levels. However, a link to the known QTL for skatole cannot be established here either.
In summary, the authors apparently judge the current state of research in such a way that further building blocks are still missing for using the results in concrete breeding work. The genetics of androstenone in particular have proven to be much more complex and extensive than expected. In the near future, the use of QTL will probably only be possible to a very limited extent, because the identification of the actually effective individual genes in the QTL sections encounters unexpected difficulties. The molecular-genetic approach, the article concludes, may not be the only solution as far as boar taint is concerned, and in any case will not lead to rapid success.
The practice information was published in the Meat Research Kulmbach bulletin (2009) 48, No. 184 - p. 95 - 96.
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Source: Kulmbach [ Branscheid ]
