Errors in the Elimination of X Chromosomes in SCIARA OCELLARIS.

It was previously assumed that the X-linked recessive mutation, sepia, induced errors in X-chromosome elimination during early embryogenesis of Sciara ocellaris. The results obtained in the present analysis corroborate this assumption and permit a further classification of the type of error this mutation induces. Among 85,244 individuals analyzed, three kinds of aberrant flies were identified: mosaics (0.01%), gynandromorphs (0.42%) and phenotypically exceptional individuals (0.25%). The origin of these abnormal flies could be ascribed to errors in selective elimination of X chromosomes that occur in male meiosis or during the early cleavages of the zygote nuclei. This last kind of error could be classified into three types: (a) error in number, (b) error in type, and (c) error in number and type of X chromosome eliminated. Evidence is provided indicating that sepia has no direct effect on the X chromosome; it has a maternal influence and exerts its effect only in the heterozygous condition.