Mapping of the spontaneous duplications against
him-1 mutations has been complex.
him-1 was originally mapped to the interval between unc- 38 and
dpy-5 (Hodgkin et al. 1979: Genetics 91: 61). Our duplication mapping with most duplications is in agreement with this result (see map in accompanying article). In contrast, two of the
unc-68(+); spontaneous duplications, hDp21 and hDp79, plus hDp21, which is probably spontaneous but this is not known for sure, were
him-1(-). These three duplications failed to complement the lethal phenotype of
him-1(
h134) and in addition, hDp21 failed to complement the Him phenotype of the viable allele
e879. To explain these results and be consistent with the original
him-1 position, we propose that in the formation of hDp21, hDp23 and hDp79, a new
him-1 mutation was induced on the duplication chromosome. The duplication that complements
him-1(
h134) with the right-most breakpoint is hDp76. In addition to hDp21, hDp23 and hDp79, hDp29 and hDp59 are also
him-1(-) despite having a breakpoint to the left of hDp76. A total of eight spontaneous duplications with a breakpoint to the left of hDp76 have been isolated and four of these are associated with a
him-1 mutation. All of the progenitor duplications were tested and found to be
him-1(+). There is either a very high mutation rate at the
him-1 locus when a duplication spontaneously shortens, or the
him-1 gene is inactivated by another mechanism, such as position effects, relating to the structure of the duplications. These observations and those described in the accompanying article indicate there are some complex changes during spontaneous duplication breakage.