International Standardized Nomenclature for Outbred Stocks of Laboratory Animals^*

Preface by the Secretary-General of ICLA

In accordance with the «Aims of ICLA» (ICLA Bulletin No. 26, March 1970) the Governing Board established in 1969 a Working Party to prepare an International Nomenclature System for Outbred Animals.

The members were:

• Professor, Dr. A. Spiegel, Federal Republic of Germany, chairman.

• Dr. M. Festing, United Kingdom

• Dr. K. Kondo, Japan

• Dr. R. Loosli, Switzerland

• Mr. S. Poiley, U.S.A.

The nomenclature rules, completed and approved by the ICLA Governing Board on 8 December 1971, are published herewith. I am convinced that this system will bring order out of the existing chaos. The system is an offer to the world laboratory animal science, particularly the breeders and users. Editors of scientific journals, catalogues, and indices all over the world are also encouraged to require and use animal stock identification by this system for outbred animals used in experimentation.

The ICLA Governing Board would have preferred to have seen an international centralization of symbol registration. However, the ICLA Secretariat has not got the capacity necessary for such a task and some practical solution to the registration problem will have to be found by the Governing Board. A final aim should then be for ICLA to publish a comprehensive world list of breeder symbols at intervals.

Oslo, January 1972

Stian Erichsen 

Secretary-General

INTRODUCTION

The rules for the standardized nomenclature of inbred strains of mice (Staats 1952, 1968) have been outstandingly successful, making it possible for scientists to compare the results of experiments using such strains with a considerable degree of confidence.

The need for a nomenclature for outbred stocks¹ of laboratory animals has been widely recognised and some proposals were published by Poiley (1970). These proposals, however, did not incorporate guidelines for the optimum methods of maintaining such stocks.

The nomenclature system proposed below recommends a set of rules based on principles of population genetics. No attempt is made to specify the degree of heterozygosity present in named stocks, since it can hardly be determined on theoretical grounds. It depends on the entire history of a stock, including all accidents and altered concepts which are bound to happen, and of which the records are often fragmentary. The main consideration of the present rules is therefore to minimize genetic changes in the course of propagating the stocks.

The historical origins of stocks have deliberately not been emphasized as subline divergence can be very much more rapid in outbred stocks than in inbred strains, and historical labels can be misleading in these circumstances. This applies particularly to stocks that have been separated through decades. If, on the other hand, different breeders are producing recently established sublines of the same stock, their products may be virtually equivalent. A common stock name to reflect the effective relationship between the stocks is then desirable.

It is proposed that the present rules should be put into effect at once. Lists of standard stocks can only be prepared after breeders and users have identified such stocks accordingly. We hope that periodic listings, e.g. «Animals for Research» (published by the U. S. National Academy of Sciences) and «International Index of Laboratory Animals» (published by the British Medical Research Council) will in the future publish annotated lists of outbred stocks. Cumulative indices of breeders, first prepared by the Committee on Standardized Nomenclature for Inbred Strains of Mice (1952), will probably continue to appear in these listings.

We hope that the present proposals for a standardized nomenclature of outbred laboratory animals will be accepted by users and producers of such animals. The authors appreciate it if inadequacies are called to their attention.

• A.  Guiding principles

  1. The major function of a system of nomenclature is to assist the biomedical investigator in choosing adequate research stocks and in communicating with breeders of such stocks.

  2. The use of standard names is only desirable for stocks that will stay relatively constant over several years. This implies that only closed colonies, maintained in sufficiently large numbers to minimize genetic drift and loss of heterozygosity, and not subjected to progressive selection should be included. Because of heterosis (hybrid vigour) and linkage disequilibrium, the colonies should have been closed for a number of generations before they are eligible for entering a stock list.

  3. Lists of outbred stocks should enable investigators to trace alternative sources of supply of any one stock, if such sources are available. Recently established sublines of common origin should therefore preferably share a stock code name.. (This may, unfortunately, be impracticable where commercial interests are involved.) Because of genetic subline divergence and husbandry differences, stocks bred at different laboratories must be clearly labelled as different sublines.

  4. Contrary to an American proposal (Poiley 1970), we do not encourage the inclusion of historical roots of stocks in the coded name, because such references are frequently misinterpreted as indicating effective genetic relationship. We recommend that the historical origin of a stock is described separately from its name in annotated catalogues in such a way that its true meaning can be explained and assessed. However, if a breeder has a reason to insist on a coded reference to stock origin, the use of the pertinent symbols as proposed by Poiley (1970) is compatible with the system presented here.

  5. Stock code names should be clearly different from the system in use for inbred strains.

• B.  Proposals

  1. A stock of any laboratory mammal species shall be eligible for standard naming after being bred as a closed colony for a minimum of 4 generations. Under this condition, heterozygous breeding populations are expected to reach an equilibrium and to produce stock of stabilized genetic composition.

     Derivations of formerly inbred strains may be included after 4 generations of closed outbreeding, provided continued outbreeding is intended.

     «Synthetic» heterozygous populations, consisting of F₁ — hybrids, mosaic groups assembled from different F₁ — hybrids, or segregating F₂ — hybrids, respectively, may be included if their production relies on crossing standard parent stocks or strains.

  2. In order to minimize changes due to inbreeding and genetic drift, the population should be maintained in such numbers as to give less than 1 % inbreeding per generation. Population size and breeding system are an integral part of the stock characteristics, and recommended breeding systems and their influence on outbreeding efficiency are listed in  Appendix I.

     If the breeding basis of a stock is temporarily narrowed by disease, stock transfer or similar interference, the stock code name may continue to appear in the list. An account of the situation should be communicated promptly, however, and shall be incorporated in the stock history.

  3. The stock code name shall consist of 2 to 4 capital letters. Example: NMRI.

     Outbred stocks containing a specified mutation are given a regular stock code followed by a hyphen and the gene symbol. Heterozygosis at the mutant locus may be indicated by a + preceding the gene symbol. Example: NAK-N, NAK-+N, (naked mouse stock).

     Following Staats (1968), hybrids from crossing standard stocks or strains are designated by indicating first the maternal, second the paternal partner in the cross, the two symbols linked by «x» to symbolize the crossing. This compound symbol is bracketed and followed by the generation number (usually F₁ or F₂) of the resulting hybrids. Abbreviated hybrid symbols are acceptable.

     Examples: (C57BL/6x DBA/2)F₁ = B6D2F₁(NMRIxLAC)F₂

     Hybrids from backcrosses, threeway- and fourway crosses, respectively, are characterized on the same basis, i.e. by spelling first maternal and then paternal code in brackets, connected by «x» and followed by the generation number.

     Example: [(AKRxBALB/c)FixDBA/2]F₁ = (AKCFlxD2)F₁

     The code letters of the breeder, consisting of a capital and 2 to 3 lower case letters, shall precede the stock designation, a colon separating the two code segments.

     Example: Tif:MAG.

     Stocks transferred to different breeders will be recognized as new sublines by the appearance of a new breeder’s symbol.

     Code letters of previous breeders may continue to appear and thereby present a brief history of a stock. A breeder shall use the same breeder code letters for all strains, stocks and species of animals of his production.

     Brackets and a symbol to indicate microflora status may be added to the stock designation if relevant. ( Appendix II.)

     The prefixed position of the breeder code, and the separating colon, serve as distinguishing characteristics of outbred stock symbols.

     All other components of the code name do deliberately correspond with the rules for inbred strains (Staats 1968) as much as possible. We hope thereby to facilitate a fluent reading of coded animal definitions by the scientists involved.

  4. Stocks may be legitimately selected for maintaining a given level of reproductive performance, but no progressive selection to develop other characteristics may be practiced (see  Appendix I). Stocks with a previous selection history as a closed colony may only be named as standard outbred stocks after selection has been suspended for 4 generations. The same applies to stocks developed by outbreeding formerly inbred strains.

  5. An example of the proposed stock description appears as follows:

     Stock: Lac:LACAf[SPF]

     Genet: cc

     Origin: From Carworth Farms, USA, 1963 30 ♀ /30 ♂ germfree CFW. Previous history of inbreeding (more than 20 generations b x s)

     Breeding: Foundation stock of 30 breeding pairs.

     Selection for reproductive performance.

     Maintained by maximum avoidance of inbreeding system.

     Character: Males aggressive. Excellent breeding performance.

     Not histocompatible. Good general purpose stock.

  6. Stock designations must not be the same as the letter sequences reserved for inbred strains of the same species. As an exception, a stock derived by outbreeding a formerly inbred strain may continue to use the origin symbol. In this case, the preceding letters of the breeder code characterize the new outbred stock.

     Example: F344/Tif vs. Tif: F344.

     New designations should be cleared with the list editor before being used in sales lists or publications. Code names are allocated on a priority basis, with the assistance of the ICLA Secretariat in argued cases. Breeders are asked to continue with their present code names, pending the implementation of these rules by ICLA.

  7. If established stocks are already widely known under a code name not conforming with the present rules, the old code name may continue to be used.

  8. Stock lists shall periodically be published in representative journals devoted to biomedical research, and thus bring accessible stocks to the attention of interested investigators.

APPENDIX I

Breeding Systems

The primary objective in the maintenance of an outbred stock is to ensure that the stock remains constant in all characteristics for as many generations as possible.

For the stock to stay constant, it must be maintained as a closed colony (rule 1), without selection (rule 4), and in such a way as to give less than 1 % inbreeding per generation (rule 2). The purpose of this  appendix is to clarify the relationship between the breeding method and the rate of inbreeding. The rate of inbreeding is dependent on three factors:

1. The size of the breeding colony.

2. The method of choosing breeding animals for the next generation.

3. The method of mating the animals that are chosen.

In general, the larger the colony, the lower the rate of inbreeding. For a given size of colony, carefully attentions to points 2 and 3 above will halve the rate of inbreeding. Halving the rate of inbreeding is of great importance in small colonies with a relatively high rate of inbreeding, but may be of little importance in large colonies (over 100 breeding pairs), since in such colonies the actual rate of inbreeding is already low.

Choice of a suitable breeding system will depend on the size of the breeding colony. Recommended breeding systems for colonies of various sizes are given in the remainder of this  appendix.

Maximum Avoidance of Inbreeding Systems

These breeding systems are essential for very small breeding colonies if the level of inbreeding is to be below 1 % per generation. Pedigree records must be kept. The systems are fully efficient only if all the matings are made up within a short period so that there is no serious over-lap between generations.

The basis is that each breeding male contributes one male, and each breeding female contributes on female to the next breeding generation, and the breeding stock is matched so as to avoid the mating of close relatives. Several different mating systems are available, each of which gives slightly different results (Kimura and Crow 1963). However, the mating system devised by Robertson (Falconer 1967) is simple to apply, and is theoretically good, so only this system will be described.

Supposing the colony consists of 16 breeding pairs, with cages labelled 1, 2,......16. Each generation the following matings are made (see Table 1).

In order to get less than 1 % inbreeding per generation at least 13 breeding pairs or 10 breeding trios must be mated strictly according to the above schedule every generation.

With some species (e.g. dogs, cats, and rabbits), it may not be practicable to follow such a rigid mating pattern. However, provided one replacement breeding male is chosen from each breeding male, and one replacement breeding female is chosen from each breeding female, and there is no mating of close relatives (i.e. full or half sibs), the rate of inbreeding will be considerably reduced. Closed colonies of this type must be maintained with at least ten males and twenty females per generation, but in practice it is advisable to have more than the minimum in order to allow for infertility and poor breeding performance in some breeders.

Rotational Mating System

A series of rotational mating schemes suitable for medium to large sized colonies was first described by Poiley (1960). The aim is to avoid the mating of close relatives, and to ensure that the next generation of breeding stock comes from a broader spectrum of parents than would occur by chance. The schemes can be applied where there is continuous replacement of breeding stock with no clearcut distinction between generations.

As an alternative to Poiley’s system, Falconer’s (1967) system may be used as follows:

The colony is sub-divided into a number of groups, and matings are arranged between groups in a systematic manner. A colony consisting of 48 breeding units, (e.g. pairs, trios or polygamous groups) could be divided into eight groups of six cages and mated as follows:

where group 1 consists of cage 1—6, group 2 consists of cage 7—12 etc., and the young are taken at random from any of the six cages in each group.

In general, in smaller breeding colonies, more groups should be used, so that with a very small colony a group size of one cage would be equal to the maximum avoidance of inbreeding system.

Chance Mating

This system is only recommended for large colonies (over 100 breeding units), where it may be impractical to follow a rotational breeding system. Breeding stocks are chosen at random from the whole colony, and they are mated by chance, without regard to their characteristics or relationships. The disadvantage is that some mating of close relatives may occur, though with a large colony this will not lead to a high over-all level of inbreeding.

An alternative with colonies of this size is to replenish a production colony that is not genetically supervised with breeding stock from a smaller foundation colony in which effective outbreeding is practiced.

Note on Calculating the Rate of Inbreeding with Different Systems

With the maximum avoidance of inbreeding system the increase in inbreeding per generation will be approximately:

With chance mating systems the rate of inbreeding will be approx.:

The rate of inbreeding in the rotational system will be between the above two rates.

APPENDIX II

Designation of hygienic status

The following recommendations are based on a report from the Gnotobiotic Association which appeared in INBRED STRAINS OF MICE 1969. It is recommended that the terms Gnotobiotic (GN) and Specified-pathogen-free (SPF) should be used. The definition of the latter term is controversial, but it implies relative freedom from disease. ICLA (ICLA Bulletin Annex I 1964) has defined SPF animals as «animals that are free of specified micro-organisms and parasites, but not necessarily free of the others not specified.» Colonies that are microbiologically monitored can therefore be labelled (SPF), whereas in the absence of periodic examinations only a reference to derivation by fostering or hand-rearing is permissible.

The proposed order of symbols is as follows:

1. Breeder code according to the preceding set of rules, followed by a colon.

2. Stock name according to the preceding rules.

3. f indicating foster nursed, h indicating hand-reared, e for transplanted ova, and o for transplanted ovaries.Steps 4 through 7, where applicable, follow in brackets [ ] :

4. Hygienic state, i.e. GN or SPF, respectively.

5. A number to indicate the year in which the stock was GN derived. (Years of transferring gnotobiotes into an SPF unit are to be specified in annotated catalogues and publications).

6. If necessary, an added lower case letter to differentiate stocks originated by the same person or laboratory in the same year.

7. Initials of the person or laboratory establishing the original GN stock.

Examples:

Lac: LACAf [SPFCaw]

The LACA stock produced by Laboratory Animals Centre under SPF conditions, transferred into the SPF from Carworth Farms which produced the gnotobiotic parents.

Rbs: BHRh [GN67Cds]

The Black Hooded Rat bred by the Rat Breeding Centre, originating from stock which the Caesarean Derivation Specialists originated in 1967 by hand-raising.

Footnotes

Literature