UCI > Research > TMF > Breeding

Breeding

• Introduction
• Ordering
• Turnaround Times
• Performance Guarantees
• Service Description
• Mouse Strain Considerations
• Breeding Trangenic Founders
• Breeding Chimeras

Introduction:

This service is available to UCI clients only. Our facility has a limited amount of cage space that can be used for breeding mice. Colonies may be established using SPF (specific pathogen free) mice from other UCI vivarium rooms, UCI’s quarantine room, the TMF’s rederivation room, or from approved vendors. Mice from non-approved vendors or other non-commercial institutions must either pass through UCI’s quarantine room or be rederived by embryo transfer (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.

The ULAR website has instructions for importing mice from other institutions, and for requesting the transfer of mice between UCI holding rooms.

Mouse strains are often acquired as frozen embryos or frozen sperm. The use of frozen embryos requires a separate service (see our embryo thawing service). Establishing a colony using frozen embryos from non-approved vendors will usually be treated as a rederivation (see 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

Ordering:

To order this service, go to our on-line order form. After filling out the on-line Service Request Form, print a hard copy, have it signed by the Principle Investigator, and mail or fax it to Tom Fielder.

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 ear-notched and tail-cut by TMF staff. Normally the client will extract DNA from the tail biopsies and perform the genotyping, but the TMF does offer a PCR genotyping service, using protocols and primers supplied by the client.

Turnaround Times:

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. These 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.

Performance Guarantees:

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 are happy to offer general guidelines as to the number of cages and mice required for a particular project.

Service Description:

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)
• Tail-cutting and ear-notching pups, if necessary
• Weaning
• 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 at a rate determined by the current ULAR per diem (cost per cage per day) and the current TMF breeding surcharge (see the service request form). These include both breeding cages and cages of weaned pups. Thus, it is to the client’s advantage to genotype tail samples as quickly as possible, so desired mice can be transferred and excess mice euthanized.

When genotyping is necessary, pups will be marked by ear-notch and tail-cut at about 2 weeks of age. Pups will be identified by cage number, ear-notch number, and date of birth. All 3 numbers are required for unambiguous identification, and should be included when communicating your genotyping results.

Tail-cutting records will be supplied with each set of tail 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 consists of the cage number and the ear-notch number, expressed as a decimal (e.g., 12.5, where 12 is the cage number and 5 is the ear-notch number).

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) onto 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, 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.

On the other hand, 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 create 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.

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 Lab, Charles River, Harlan, 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
• Charles River Laboratories - C57BL/6NCrl
• Harlan Sprague-Dawley - C57BL/6NHsd
• Taconic - C57BL/6NTac

Breeding Transgenic Founders:

(This section also appears under our DNA microinjection service description.)

Transgenes are integrated at random into the genome. Thus, each transgenic 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 with 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. This may be caused by meiotic recombination, as in a double-crossover event.

If an inbred line of egg donors is used (e.g., FVB/N or C57BL/6J), then founders should be bred to the same background to maintain the inbred status. For founders from the hybrid CB6F1 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.

Breeding Chimeras:

(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 20 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.

 

University of California, Irvine
Transgenic Mouse Facility
B123 Gillespie Bldg. • Irvine, CA 92697-1140
© 2008 The Regents of the University of California, All Rights Reserved.
Last Updated: October 9, 2008

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