TMF breeding colonies are established with mice from approved vendors or mice produced in the TMF. Mice from any other source must undergo rederivation by embryo transfer before they can be housed in TMF mouse rooms (see our rederivation service).
All mouse transfers into and out of the TMF must be approved by UCI’s Clinical Veterinarian and the TMF, and must be performed by ULAR staff.
Breeders can be produced from frozen germplasm. Establishing a colony using frozen embryos from outside sources requires our rederivation service. Establishing a colony from frozen sperm requires our IVF (in vitro fertilization) service.
Some important “normal” mouse breeding characteristics (all of which can be affected by background and genotype) include
- 19-21 day gestation period.
- Sexual maturity at roughly 5-8 weeks of age (females tend to mature sooner than males).
- 6-8 pups per litter, on average.
- Pups must be weaned at 3-4 weeks of age to prevent unplanned matings.
- Coat color can be distinguished starting at about 8-9 days of age.
- Female fertility starts to decline at about 6-8 months of age, while males usually remain fertile for at least a year.
- Females can become pregnant within a day of giving birth (postpartum estrus).
- Multiple females can be mated with a single male, but not vice versa.
- Males used for breeding must not be housed with other males.
The service request form has a section for describing your mouse needs. Efficient use of cage space requires that you state as clearly as possible the number of mice needed (expressed as either a total number or number per unit of time), when and how often you want mice transferred to your room, and the desired ages, sexes, and genotypes. Offspring that need to be genotyped will be distal toe-clipped (or ear-notched and tail-cut) by TMF staff. Normally the client will extract DNA from the tissue biopsies and perform the genotyping, but the TMF does offer a PCR genotyping service, using protocols and primers supplied by the client. The TMF is also able to develop qPCR genotyping assays for clients.
Many factors contribute to the time required to establish a breeding colony and produce the desired number of mice. Please convey any information you may have about your strain’s breeding characteristics to the TMF. This could include average litter size, effective breeding lifespan (both sexes), lethal genotypes such as homozygous knock-outs, and phenotypes that affect pup development, nursing behavior, aggression, and fertility.
Due to the large number of factors affecting breeding performance of a given strain of mice, we cannot offer any guarantees in terms of output or numbers of cages required. However, we can offer general guidelines as to the number of cages and mice required for a particular project.
The TMF will perform all normal breeding functions, including
- Setting up matings.
- Checking for pregnancies and newborn pups.
- Recording number of pups and DOB (date of birth).
- Distal toe-biopsy, and genotyping if necessary.
- Arranging for transfers to the client’s animal room.
All cages assigned to a particular client’s breeding project will be recharged to the client’s account as a per diem (cost/cage/day). These include both breeding cages and cages of weaned pups. Thus, where appropriate, it is to the client’s advantage to genotype tissue samples as quickly as possible, so desired mice can be transferred and excess mice euthanized.
When genotyping is necessary, pups will be ID'ed by toe-clipping (or ear-notching and tail-cutting). Pups will be identified by a unique toe-clip number and date of birth (DOB). Both ID number and DOB are required for unambiguous identification, and should be included when communicating your genotyping results.
Tissue biopsy records will be supplied with each set of biopsies. These will show the date of collection, the DOB of each litter, and the ID number, sex, and coat color of each pup. The ID number is a unique identifier.
Mouse Strain Considerations
When mice (or frozen embryos or sperm) are acquired from another institution, it is important to obtain as much information as possible about the strain background and breeding history. Many genetically modified strains are on a mixed background, which is often poorly defined. It is the client’s responsibility to decide which strain will be used as mates for the acquired mice (or for their offspring), but we will gladly offer advice about this.
The fundamental consideration in breeding genetically modified mice is that any given transgene, mutation, or other genetic modification can interact with a (possibly unique) mixture of other alleles in a given mouse to produce a specific phenotype. Thus, in order to minimize mouse-to-mouse variations in phenotype, the genetic background of a colony should be as uniform as possible. Therefore, it is often advantageous to move the desired genotype(s) from a mixed background to an inbred background through the process of backcrossing. Backcrossing is simply the process of mating offspring of one background to the desired background at each generation. After 10 generations of backcrossing (including a backcross to a male and a female of the new strain during the backcrossing process), the resulting line is termed “congenic”. It is important to remember that congenic strains still have a small amount of genetic material from the original background, surrounding the genetic locus that was selected at each generation.
It is sometimes desirable to maintain a mixed background. However, if mice of a mixed background are simply mated with each other, and this is continued for many generations, in effect this will produce a new strain of mice, with possibly a different phenotype from the original (mixed) background. Two ways to maintain a well-defined mixed background are to backcross to a hybrid background or to an outbred background.
To maintain a defined background, whether inbred, outbred, or hybrid, mice of this background should be obtained periodically from an approved vendor and used as breeders. If this is not done, over time the colony can undergo genetic drift and unwanted genetic mutations may become fixed in the population.
Many genetically modified strains are on a C57BL/6 background, or a mixture of C57BL/6 and another strain. In these cases, it is helpful to know which vendor the C57BL/6 component came from originally (Jackson Laboratory, Charles River, Harlan (Envigo), or Taconic) because the various vendors’ colonies have been genetically isolated long enough to be considered substrains. The various substrains are designated as follows:
- Jackson Laboratory – C57BL/6J, C57BL/6NJ
- Charles River - C57BL/6NCrl
- Harlan (Envigo) - C57BL/6NHsd
- Taconic - C57BL/6NTac
Breeding Transgenic Founders
(This section also appears under our DNA microinjection service description.)
When microinjected into pronuclei, transgenes are integrated at random into the genome. Thus, each founder will have a different site of integration. The number of copies of the transgene possessed by each founder may also be different. For these reasons, each founder should be treated as a separate line and bred independently of other founders carrying the same transgene.
Offspring must be obtained from each founder to test for transmission and expression of the transgene. Due to position effects and different copy numbers, each founder line can have a different level of expression. However, we cannot guarantee that any expression of the transgene will be obtained.
The transgene will not necessarily be transmitted in a Mendelian fashion by a given founder. Founders can be mosaic for the transgene, if integration occurs after the first cell division. Mosaicism can result in a frequency of inheritance of less than 50% in the first generation offspring. In some founders, the transgene may integrate into more than one locus, resulting in a frequency of inheritance of more than 50%. In this case, the expression levels among the first generation offspring may vary, depending on which integration site they inherit.
A more uncommon problem is loss of the transgene altogether, which may be caused by meiotic recombination. Occasionally, transgene expression can be shut off in subsequent generations due to methylation, which may be connected with the fact that most transgenes are inserted as head-to-tail concatemers containing multiple copies of the transgene. We now offer another method of making transgenic mice, using homologous recombination at the ROSA26 or H11 loci, that eliminates the problems associated with randomly inserted transgenes. See our Targeted Transgenesis service.
If an inbred line of egg donors is used (e.g.C57BL/6NJ), then founders should be bred to the same background to maintain the inbred status. For founders from the hybrid B6SJLF2/J strain, the client must decide whether to backcross to an inbred line, or to maintain a mixed background.
Mice that have the transgene on one chromosome are termed "hemizygous" because they do not have a corresponding allele on the other chromosome. It is possible to produce homozygous transgenics, and these may have a higher level of expression than the corresponding hemizygous mice, but distinguishing homozygotes from hemizygotes can be difficult. Some kind of quantitative genotyping assay must be used (e.g., quantitative PCR or Southern blotting). Alternatively, suspected homozygotes can be crossed to wildtype mice and all offspring tested to see if they are hemizygous. The TMF can develop qPCR assays to discriminate hemizygous from homozygous transgenic animals.
(This section also appears under our ES cell injection service description.)
The contribution to the germline by the ES cell lineage in any given chimera is unknown. On average, weaker chimeras (those with less agouti fur) are less likely to have ES cell-derived gametes. Nevertheless, a high degree of chimerism is not a guarantee of germline transmission. Therefore, unless the number of chimeras is very large, it is generally in the client’s best interests to breed all of the chimeric males. We suggest producing at least 40 pups from any given chimera before concluding that it will not go germline, and it may be necessary to produce many more than that before the first agouti pup is obtained.
When deciding which strain of mice to use as mates for your chimeras, consider the strain from which the ES cells were derived, and the genetic background that you eventually want to achieve. Crossing the chimeras with the parental strain of the ES cells allows one to obtain a pure inbred line in one generation. However, 129 lines are often difficult to breed and not all 129 lines are true inbreds. If you want to move your mutation to a background other than the parental strain of the ES cells, it will be necessary to backcross to the desired background for 10 generations to achieve a true congenic.