L. C. Dunn

Sex differences in recombination of linked genes in animals

Reports of sex differences in crossing-over in animals, published since Haldane in 1922 suggested that crossing-over should be less frequent in the heterogametic sex, have been reviewed and discussed. No general rule is discernible apart from the absence of crossing-over in males of the dipteran genera Drosophila and Phryne and in females of some lepidopteran species, due apparently to failure of chiasma formation in the heterogametic sex. In the majority of animal species examined crossing-over occurs in both sexes. While there is some tendency in mammals for crossover values in females to exceed those in males, it was of greater interest to find that marked sex-differences occur in the same species (data chiefly from the house mouse) in opposite directions in different chromosomes. The influence of factors acting locally in the chromosomes, such as those associated with hetero-chromatin, were indicated as promising subjects for the study of variations associated with sex.

Serological identification of sperm antigens specified by lethalt-alleles in the mouse

Cytotoxic antisera were prepared by immunizing wild-type recipient mice with sperm from donors carrying different recessive lethal alleles at theT locus (T/t0,T/tw 1,T/tw 5, andT/tw 32). After removal of sperm autoantibody by absorption with sperm of recipient type, each antiserum reacted only with sperm from males whose genotype contained at least one of the immunizing alleles. Cytotoxicity was high against sperm populations in which both immunizing alleles were represented and was lower when only one was present. Thus, each allele at theT locus which has so far been tested serologically is recognizable as a discrete antigen on the surface of sperm.

Transmission ratio distortion at the T-locus: Serological identification of two sperm populations int-heterozygotes

Certain recessive t - alleles in the house mouse are transmitted from male heterozygotes in proportions much higher than expected. Males of genotypes of T/tx (T=Brachyury, a dominant marker at the locus) or +/tx produce progeny of which 70 - 99% are themselves heterozygous for tx; the actual ratio is characteristic of a particular allele. T/+ males, however, give normal 50:50 ratios. This transmission ratio distortion is clearly sperm-dependent, since observations on pre-natal and postnatal litter size show that normal numbers of eggs are fertilized and survive in females mated with t-heterozygotes. Cytological studies of gametogenesis and of spermatozoa in t-heterozygotes have not provided evidence that sperm of the two possible haploid genotypes are produced in unequal numbers, although this possibility cannot be completely dismissed. In animals, there is little evidence for gene activity in male gametes after segregation (see Beatty 1970) since virtually all forms of genetic lesions are transmitted normally by sperm. Recently, and more germane, Lyon et al. (1971) have presented evidence that sperm deficient for the chromosome segment carrying the T-locus can successfully fertilize eggs, suggesting that a haploid expression at this locus is not essential for normal sperm function. Nevertheless, the simplest explanation for the ratio distortion effect of the t-alleles remains a post-meiotic phenomenon. The demonstration that cell surface components on sperm are specified by alleles of the T-locus allowed us to re-investigate the transmission ratio distortion in an

Lethal alleles near locus t in house mouse populations on the jutland peninsula, denmark.

Polymorphism for lethal or semi-lethal alleles near locus T (linkage group IX) appears to be the rule in North American populations of Mus musculus (Dunn, 1964). Since American populations of this commensal species are presumably descended from migrants from Europe, it is of interest to inquire concerning the state of the Tregion in European populations. Through the kindness of Professor Robert K. Selander we have been able to test some members of each of several Danish populations of Mus. These came from the populations which Selander et al. (1969) had found to be polymorphic for a number of different proteins. After their protein determinations had been made they sent to us (February 5, 1970) some 150 living specimens or descendants of mice captured in 1968 on farms in the Jutland peninsula and nearby islands. For details of the location and description of the Jutland Mus populations the publication of Selander et al. (1969) should be consulted. We were able to observe progenies from 41 males in test matings with females of our Brachy (T/?) stock. The results are shown in Table 1. Seven males produced tailless offspring in addition to the Brachy and normal offspring expected from matings of T/+ x +/+. These males each transmitted an allele interacting with T to produce a tailless phenotype. Such alleles (like the talleles in North American Mus populations) were transmitted by each male in high ratios rangingo from 0.81 to 1.00, with the

Maintenance of Gene Frequency of a Male Sterile, Semi-Lethal T-Allele in a Confined Population of Wild Mice

An allele (tw2) at the complex locus T was first noted in January, 1952, in a population of wild house mice which had been bred in confinement at The Rockefeller Institute since 1944 (Schneider, 1946). The population, descended from house mice captured in New York City and Philadelphia, had been maintained in the laboratory at a size averaging 60 to 70 individuals at a reproductive rate of about two generations a year. The allele was revealed when in test crosses with mice carrying the dominant marker Brachy (short tail) T/+, certain members of the population produced tailless offspring, T/tw2. These when bred together gave rise to a balanced tailless stock producing only tailless (T/tw2) and normal-tailed (tw2/tw2) offspring. The genotype T/T is known to be lethal, embryos dying at about 102 days post-fertilization. The normal-tailed female offspring were fertile but males of this genotype were entirely sterile, (Dunn and Morgan, 1953a). Recent tests have shown that only about 55% of the expected number of animals of this genotype were present at birth so the allele can be classified as malesterile and semi-lethal. The allele was nevertheless present in considerable frequency in all samples from this population tested from 1952 through 1959 (Levine and Dunn, 1956; Dunn and Levene, 1961). Its retention in the population was favored by its high transmission ratio in heterozygous males, estimated at .95 in 1952 and .85 in 1959 (Dunn and Levene, 1961). This advantage was opposed by the low viability of homozygotes and the sterility of homozygous males. Samples of 46 males and of 43 males taken from this population in 1963 and 1965, respectively, were tested by mating with Brachy (T/+) females. The results are given in Table 1. Males giving one or more tailless offspring from test crosses with Brachy females were classified as +/tw2, while those giving three or more Brachy offspring, but no tailless, when so tested, were classified as +/+. The latter result (3 Brachy offspring) has a probability of only .001 if the father is +/tw2 since from such males the probability of tw2 in sperm is .9 and of + .1; hence (.1)3 = .001. From Table 1 the male transmission ratio of tw2 can be estimated as the ratio of tailless (T/tw2) to all relevant tested gametes. In 1963 this was 126 . 161 12___ = .91; in 1965 it was = .91. 13+ 126 15+ 161