GENETICAL AND EMBRYOLOGICAL COMPARISONS OF SEMILETHAL t -ALLELES FROM WILD MOUSE POPULATIONS

HE genetic variants at the complex T locus in the house mouse have been Tmuch studied (see DUNN 1964; BENNETT 1964; and LYON and MEREDITH 1964, for reviews) because of their uniquely varied and interesting effects on such diverse characters as embryonic development, sperm function, suppression of recombination in their vicinity, and high level of conversion to new and different genetic forms. There is in addition the surprising fact that such variants occur in most wild populations of house mice. The situation with respect to these alleles as heretofore described in the literature is as follows. A dominant gene T (Brachyury) in linkage group IX serves as a diagnostic marker for this locus. In heterozygous condition T produces a shorttailed phenotype, apparently by interference with the growth and maintenance of the notochord during embryonic development. T is lethal when homozygous, such embryos dying about halfway through gestation. In these the posterior part of the body is missing, a situation apparently also due to defects in the primitive streak or notochord. In addition, there exist at this locus a whole series of different recessive variants. These so-called t-alleles are recognized solely by their interaction with T , the interaction being such that the genotype TJt has a tailless phenotype. Thus the dominant gene T provides a means for detecting these interesting recessive alleles which would otherwise in practical terms be unrecognizable. The recessive alleles known at this locus have been garnered from two different sources. Many of them have been isolated from wild populations of mice or laboratory stocks in which they are apparently maintained as a natural polymorphism. Others have occurred as “mutations” to new and different form; of t-alleles which have already been characterized and are maintained as balanced inbred lines in the animal colony. Although these recessive t-alleles have one constant factor in common, that is, their tail-modifying effect when combined with T, they proved to be a heterogeneous group with respect to other effects mentioned above. In general, however,