The nature of the influence of X-translocations on sex of progeny in Sciara coprophila

Summary

• 1. A reciprocal translocation between chromosomes X and II in S. coprophila was used to elucidate the occurrence of exceptional sons and daughters among the progeny of females heterozygous for any one of several X-translocations.

• 2. The studies support the view that the exceptions result from non-disjunction of sex chromosomes during oogenesis and not from genetic change induced by rearrangement on the X.

• 3. The percentage of exceptions was found to increase when chromosomal rearrangements were used to decrease synapsis of the sex chromosomes. Conversely, when the barrier to sex chromosome pairing was removed, by making the translocation homozygous, the exceptions did not occur.

• 4. The studies support the view that (1) the X′ influence on sex of progeny is one on sex chromosome elimination from the embryonic soma, and (2) sex of the individual fly is determined by the chromosome complement which remains in the soma after elimination.

• 5. Provided there are two sex chromosomes in the somatic complement, differentiation of the ovary is normal even when an extra sex chromosome is present in the germ line. Likewise, the testis differentiates normally provided the somatic complement is XO (maternal X) even when an extra autosome is present in the germ cells.

• 6. When an additional sex chromosome is transmitted to the zygote (i.e., 4 sex chromosomes present), errors may occur in the kind or the number of chromosomes eliminated from the embryonic soma. No errors in elimination of chromosomes from the germ line have been detected.