Abstract
A transposable hobo element in the Notch locus of the Uc-1 X chromosome, which does not interfere with the normal expression of the locus, interacts with other hobo elements in the same X chromosome to produce Notch mutations. Almost all of these mutations are associated with deficiencies, inversions or other rearrangements, and hobo elements are present at each of the breakpoints. The Uc-1 X chromosome produces the Notch mutations at a rate of 4-8% in both sexes of flies in a strain that has been inbred for 96 generations. At least two-thirds of the mutations are produced in clusters suggesting that they have originated in mitotic (premeiotic) germ cells of the Uc-1 inbred strain. The interaction of hobo elements in the Uc-1 X chromosome can be repressed by at least two different mechanisms. One found in three inbred strains not related to the Uc-1 strain involves a maternal effect that is not attributable to the actions or products of hobo elements. Repression by this mechanism is manifested by a clear reciprocal cross effect so that the production of Notch mutations is repressed in the daughters of Uc-1 males, but not in the daughters of Uc-1 females. The other mechanism apparently requires genetic factors and/or hobo elements in a particular strain of Oregon-R; complete repression is present in both types of hybrids between Uc-1 and this strain.