Maintenance of inbred strains

The aim is usually to keep the strain genetically constant with an easy and efficient breeding protocol. A self-supporting stem line colony of 10-40 breeding pairs, which are mated brother x sister, can provide breeding stock for one or more expansion colonies of several hundred breeding pairs. These may be mated at random for 2-3 generations without any-deleterious consequences, provided the strain is fully inbred to F >20.

Genetic quality control or genetic monitoring is essential as a single nonstrain mating can destroy an inbred strain. The following methods have proved useful, though the last method is likely to be most widely used in future.

Skin grafting

Skin grafts between members of the same strain should not be rejected. This provides a way of testing for isogenicity, and genetic contamination by a non-strain mating. However, it cannot easily be used to confirm that the strain is AKR rather than BALB/c (both of which are albino), and it takes 5-6 weeks to ensure that grafts are not rejected due to a weak incompatibility.

Protein polymorphism

Polymorphic loci, in which different alleles can be detected electrophoretically, can be used as genetic markers. For example, strain C57BL carries the Hbbs allele, and DBA/2 carries the Hbbd allele coding for the hemoglobin 3 chain. These give a single and a double band on the gel, respectively, following electrophoresis of a red blood cell lysate. The disadvantage is that these biochemical methods require considerable skill, and will not normally distinguish between congenic strains which differ, say, only at the major histocompatibility complex (MHC).

Serological methods

A strain-specific alloantiserum can be produced by injecting lymphocytes from 5-10 different strains with various lymphocyte alloantigens into animals of the strain to be monitored in the future. Such an antiserum will not react with lymphocytes from the host strain, but will usually react with lymphocytes from other strains. Similar serological methods can be used with monoclonal antibodies to detect the presence of polymorphic alloantigens (e.g. on lymphocytes), which can be compared with published data for the strain.

DNA polymorphisms

DNA fingerprinting, DNA probes for minisatellite loci and restriction fragment polymorphisms offer powerful methods for genetic quality control. However, 'microsatellite' markers appear to be even more convenient for most purposes. More than 6000 microsatellite markers identifying specific locations on all chromosomes have been described and mapped in the mouse, and a similar number will be available soon in the rat. These are highly polymorphic in the number of repeats, and can be detected rapidly using the appropriate primers and the polymerase chain reaction followed by gel electrophoresis. These provide a powerful tool for genetic quality control, requiring only very small quantities of DNA. Data on microsatellites are available by e-mail (see Further reading).

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