CROSSES WITH WILD RATS.
As a further test of the permanency of the modification effected by selection in the hooded pattern of rats, crosses have from time to time been made of the selected races with a pure wild stock,i. e., with ordinary wild animals caught in traps. In all cases the wild animals used were known to be homozygous as regards gray coat and self pattern, since when crossed with black-hooded animals they produced only gray self offspring. In such crosses the hooded pattern is recessive, the F₁ offspring being indistinguishable from ordinary wild gray rats except for the possession of a white patch of varying size upon the belly, but even this may be lacking. (SeePlate 2, ♂ 8000, 8018, and 8021.)
The grade of the hooded young extracted from a cross with wild animals corresponds in a general way with the grade of the hooded animal used in making the cross, as the following cases will show. (Compare also Plates2and3.)
A female of grade -1.87, belonging to generation 2½, minus series (compare Tables2and3), was crossed with a wild male. (SeePlate 2, ♀ 6176.) Among her F₂ descendants (cf.Plate 2, 8070 to 8078) occurred 62 hooded individuals, whose grade distribution is shown inTable 42, first row. Their mean grade is +0.31, although the uncrossed race of the same grade and generation gave offspring of mean grade about -1.20. The cross, therefore, had apparently increased the pigmentation of the extracted hooded recessives. This idea is supported by the result of a control mating of the particular female used in making the cross. When she was mated with a hooded male of the same grade as herself, she produced three young, all of grade -1.00. The extracted recessive grandchildren, as a group, show greatly increased pigmentation as compared with this, but vary greatly in the extent of the increase. Some show very little modification, others very much, the most extreme individual being of grade +3.50. It was undoubtedly out of just such modified recessive individuals as this that the material for our initial plus selections arose; to this point we shall return later.
The F₂ (or second generation) offspring, however, include about 1 hooded individual in 4. In a total of 962 F₂ young; 230 were hooded, or 24 per cent. This summary includes only those litters in which dominants as well as recessives were recorded. In many litters only the hooded young were recorded, as the special object of the investigation was to ascertain whether the extracted recessives were like the pure hooded race in grade or not. In the above summary also the hooded grandparent was in every case a female. The reciprocal cross is more difficult to obtain, but one wild female rat, caught in 1911, has bred quite regularly in captivity, though each time she has murdered her hooded mate prior to the birth of the young. Her F₂ grandchildren derived from matings with males of the minus series include 32 hooded and 96 non-hooded individuals, exactly 25 per cent hooded.
A second cross of selected animals of the minus series was made between a wild male and four females of grade -2 derived respectively from generations 5¼, 5½, 6½, and 7. As a group these mothers are more nearly comparable with generation 6,Table 21, than with any other uncrossed group. As the F₁ progeny of these four mothers by a wild male were matedinter se, it is possible to deal with their hooded grandchildren only as a group. The character of these is indicated in the second row ofTable 42. They number 48 individuals and have a mean grade of +0.25, showing a modification in a plus direction similar to that observed in the previous case.
A third cross in which the minus series is concerned was made between females of grade -2 and -2.25, generation 10, and wild males. The F₂ offspring include 91 hooded individuals classified as to grade in the third row ofTable 42. Their mean grade is +0.24, confirming fully the results obtained in the two previous experiments.
With these three cases we may compare three cases in which animals of the plus selection series were crossed with a wild male. (See the last three rows ofTable 42.) Females of grade +3.00, generation 3, were crossed with a wild male. From this mating resulted 21 hooded grandchildren, ranging in grade from +1.75 to +3.50, mean +2.56. These grandchildren, it will be observed, in no case are of minus grade, as are about half the grandchildren when the grandparent is of minus grade. There is also no clear evidence of modification of the hooded character by the cross in this case. The grade of the extracted hooded individual is just about what uncrossed individuals of grade +3.00 produced in the corresponding generation of the plus series.
In the next case two females of the plus series, belonging to generations 5 and 6, respectively, were crossed with a wild male and their children were bredinter se. There resulted 38 hooded grandchildren, as shown in the next to the last row ofTable 42. The range of the grades of these hooded grandchildren was similar to that of the grandchildren in the foregoing case, but their mean was somewhat higher, aswe should expect, since they are descended from more highly selected individuals; for the hooded grandparents in this case were of grade +3.25 (generation 5) and +3.50 (generation 6), whereas the grandparent in the foregoing case was of grade +3.00 (generation 3).
It is a noteworthy fact that in both these cases the wild cross does not seem to have increased the pigmentation in extracted hooded individuals, as it did when the minus series was crossed, but rather to have diminished it; yet the difference between observed and expected is not great. We might disregard it altogether, if a similar but more striking result were not observable in the third case as well as in another series of crosses presently to be described.
The third case (last row ofTable 42) involves a cross between a female of grade +4.25, generation 10, plus series, and a wild male. The F₂ offspring include 16 hooded individuals of mean grade +3.15. Animals of this grade in the uncrossed race in this generation produced young of mean grade +3.84.
Before leaving this subject it is important to observe the considerable difference between the extracted hooded grandchildren of the minus-series rats, as a group, and those of the plus series. The latter is unmistakably a plus-series group; the former is on the border line between the two series. (Cf. Plates2and3.)