SELECTION FOR TAIL-SPOTTING IN THE HOUSE MOUSE

E. F. BARROWS Oregon Xormul School, Illonmouth, Oregoii

ONE FIQURE

I n the house mouse, two principal genes for white-spotting have been recognized for some time: one for ‘piebald’ (blotches of white with relatively distinct edges), which is recessive and designated by the symbol s ; and one for ‘varie- gated’ (splashes of white with less distinct edges), which is dominant and designated by the symbol W (Dunn and Charles, ’37). The phenotypic effect of each gene is greatly modified by a ) a number of modifying genes, and b) non- genetic factors.

Certain of these modifying genes determine the location of the spotting on the animal: on the face as a ‘blaze;’ more or less encircling the middle of the body as a ‘belt;’ in the mid-ventral region as a ‘belly-spot;’ or on the tail as a ‘tail- spot. ’ Several investigators have reported on these sub- types, of which the ‘blaze’ (or ‘white-face’) and the ‘belt’ have been most thoroughly treated. The ‘belly-spot’ and the ‘ tail-spot ’ were reported by Little ( ’24)’ and the inheritance of ‘belly-spot’ in SSww animals was studied by Murray and Green (’33) and by Clark (’34). As the results in these two papers are not in agreement, it would seem evident that there are at least two types of SSww belly-spotting. Also Dunn ( ’37) described the production of a number of ‘belly-spot’ animals by the accumulation of plus modifiers of variegated and the subsequent elimination of the W gene.

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Since ‘ tail-spot ’ has not been so thoroughly investigated, the publication of the accompanying pedigree demonstrating its heritability, along with some minor points, would seem valuable.

The stock of mice appearing in this pedigree was secured in 1931 from George D. Snell, then at Brown University. Early generations shown in this pedigree contained genes for black, brown, agouti, non-agouti, piebald and non-picbald, but brown and piebald were eliminated early, and agouti last appeared in generation XII. Thus the last four generations were homo- zygous for black, non-agouti and non-piebald, as well as pre- sumably for most other genes, as a result of the continued inbreeding.

I n this pedigree of mouse tails, generations I1 and I11 show a typical miscellany of tail types as they originally occurred in this strain; the presence of s in many of them probably increased the amount of white on the tails of those carrying this gene. The black-and-white rect5ngles are intended to represent actual tails hung proximal-end-up, and to show as cxsctly as possible the actual distribution of white on these tails.

I n generation VI, 8 E5 and P E7 were found to be almost certainly free from s (as verification, no piebald appeared through the next ten generations), and from here on, selection was made toward a greater amount of white on the tail. The effect of selection is as plainly marked as it was in Dunn and Charles’s (’37) line of pied mice, and although not carried through as many generations as theirs, nevertheless shows definite resemblance in the tendency of the curve to flatten out (selection to become ineffective) after about six genera- tions. It seems highly probable that their explanation applies to this pedigree as well: i.e., the accumulation of homozy- gosity in several interacting genes which tend to produce white on the tail.

Very likely this tail-spotting complex of genes is not wholly identical with Dunn and Charles’s I<-complex ; it would be remarkable if both strains had started with the same hetero- zygous possibilities, and if so (or even if not) had succeeded

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Fig.1 The black and white rectangles represent actual tails hung proximal- end-up, and show as exactly as possible the actual distribution of white on these tails. Spotting pattern of hair and of skin scales was not identical, although usually so. The records follow scale-pattern (wherever there is a difference) as this is more conspicuous on the tail. The pattern is not quite as symmetrical as drawings would indicate. White is less apt to occur on the proximal half of the tail.

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iii preserving exactly the same ones through several genera- tions. Indeed, a number of other gene-complexes for tail- spotting could doubtless have been secured here by following others lines of descent from the same E5 and E7 in genera- tion VI. Nevertheless, these various complexes would in all probability have many genes in common with each other, and with the K-complex also.

It will be noticed that the white band, when present, usu- ally begins just below the mid-point of the tail. This suggests that there may be a gene, or small group of genes, affecting this particular area. I have tried unsuccessfully to isolate such a strain, but I still believe it might be done: I failed to realize, and I think most other investigators have done the same, the value of at least one reversal (extending f o r several generations) of selection during the process of breed- ing for a homozygous strain.'

The value of this procedure (of including reverse selection) depends on the wide-spread occurrence, now generally recog- nized, of lethal genes. Clark's ('34) article is a case in point. As it is plain that white-spotting depends on the presence of many genes, we might assume that certain of these (for instance, A, B and C ) are lethal in the homo- zygous condition; also that the alleles of certain other genes for spotting (t, u and v) are lethal in the homozygous con- dition. I f then, starting with a strain of mice heterozygous for all these genes (Aa, Bb, Cc, Tt, Uu, Vv), selection is carried on only toward the largest possible amount of spotting, the result will be, in the course of a few generations, a strain of genotype Aa, Bb, Cc, tt, uu, w ; and this strain will of course not breed true. I n fact, eight genotypes, and possibly as many phenotypes, could be produced from it. But then if selection is carried on in the reverse direction through several generations, complete homozygosity might be obtained: i.e., aa, bb, cc, tt, uu, V V . ~

'Lack of facilities for research at my present location made it necessary for

a Reversal of selection several times, if effective, would indicate frequent me to discontinue this strain of mice before making these tests.

mutation of the genes involved.

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This procedure, applied to spotting on the tail as well as on the body, will assist in the isolation of further genes from the large complex of modifiers of white-spotting in the mouse.

LITERATURE CITED

CLARK, F. H.

DUNN, L. C. 1937

DUNN, L. C., AND D. R. CHARLES

LITTLE, C. C. 1924 MURRAY, J. Af., AND C. V. GREEN

1934 The inheritance and linkage relations of a new recessive

Genetic analysis of yariegated spotting in the house mouse.

Analysis of quantitative variations in

spotting in the house mouse.

Genetics, vol. 23, pp. 43-64.

the pied spotting of the house mouse.

Genetics, vol. 19, pp. 365-392.

1937 Genetics, vol. 22, pp. 14-42.

Carnegie Institution of Washiiigton Pear Book, no. 23. 1933 Inheritance of ventral spotting in mice.

Genetics, rol. 18, pp. 48148G.