A Reciprocal Translocation in Cochliomyia Hominivorax (Diptera: Calliphoridae). Genetic and Cytological Evidence for Preferential Segregation in Males

Abstract

HE mutant strain of the screw-worm fly (Cochliomyia hominivorax) known as Brc (black R-cell) was discovered in our laboratory almost two years ago, and has been maintained for over 20 generations. Earlier studies indicated that a dominant autosomal factor was involved. Appropriate tests indicated that penetrance of the mutation was excellent, but continued selection did not result in the establishment of a pure-breeding strain. Preliminary studies of the offspring from Brc x Brc indicated that the homozygous condition might render them viable but sterile. Further studies proved this hypothesis to be false and showed that the mutant phenotype was associated with a reciprocal translocation, that the homozygous condition was lethal, and that the aberrant segregation ratios were due to the occurrence of nonrandom segregation of the chromosomes at first meiotic metaphase in translocation heterozygotes. The genetic and cytological studies leading to these conclusions are herein presented. MATERIALS AND METHODSCochliomyia hominivorax (=Callitroga hominivorax) is an obligate parasite of warm-blooded animals. The biology and laboratory rearing procedures were discussed in several publications (MELVIN and BUSHLAND 1941; BUSHLAND 1960a, b). All flies used in these studies were reared on artificial media, allowed to pupate in sand or sawdust, and the adults were kept in wire mesh cages with honey and water as food. Colonies were maintained at 80°F. Other experimental procedures are described in connection with the tests in which they were employed.The term "Brc flies" denotes adults which have the mutant phenotype due to a dominant factor, but use of the term implies no distinction between heterozygotes and possible homozygotes. RESULTS Origin and phenotype of the Brc mutation:The mutation is characterized by having the entire R-cell in the wing completely blackened by dense pigmentation (Figure 1). The mutant flies have excellent vigor and appear normal in all other respects. The mutant strain was derived from a single male found among the progeny of irradiated flies. The mutant male was crossed to wild-type females