A female determining factor (F), epistatic to M factors, was used to map autosomal male determining factors (AM) of the housefly. On the basis of meiotic recombination frequency occurring in the sex-reversed females caused by F, three IM factors and two IIIM factors of different geographic origins were mapped in the close vicinity of the by and the pw locus, respectively. These results suggest that AM factors occupy a definite site on the respective chromosomes. As measured along the IM and the IIIM chromosomes, the distribution of recombination in the sex-reversed females was strikingly different from that observed in the male. It is proposed that AM factors are located in the centric heterochromatin.
A dominant wing-shape mutation, Bx2 (Beadex2), occurring on the IIIMchromosome was found in a wild-type housefly strain named Nagai. By backcrossing hybrid males between the Bx2 male and the third chromosomal mutant bwb ge females to the homozygous recessive mutant females, a new type of sex-limited strain designated as "bwb ge/Bx2 M(Nagai)" was established. The strain usually kept producing only male progeny with Bx2 phenotype and the female progeny with bwb ye phenotype, but the recombinant type offspring also segregated although the frequency was very low. Further analyses of these recombinants indicated that some of them were the resultants of crossing-over in the male parents, but the others were not. By using the male crossing-over data, the location of the male determining factor on the third chromosome was estimated to be the right side of ge locus.
A procedure to locate autosomal male-determining factors (AM) of the housefly is described, which employs a female-determining factor (F) marked with a closely linked dominant gene. A total of 70 AM chromosomes extracted from four IIM strains, nine IIIM strains and three VM strains of different geographic origins were examined.On the basis of female recombination frequencies, two to three types of AM chromosomes were found in each of the linkage groups, suggesting that the AM chromosomes are polymorphic for chromosome aberrations.All the AM factors were mapped to a specific site or closely linked sites on the respective autosomes. From the comparison of the mitotic autosome figures and the female recombination maps of AM chromosomes, it is suggested that AM factors are located near the centromeres of autosomes. The validity of this technique and the possible mechanisms for the origin of M polymorphism are discussed.
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