We have recently isolated a duplication of the left arm of LG IV ( mDp1) which we intend to use to refine the existing fine-structure map of ama-I (see Bullerjahn and Riddle: this issue). mDp1 was obtained in a screen designed to identify duplications and deficiencies of the dpy- 13 region. Briefly, N2 males that had been exposed to 1500 rads 7- radiation were mated to
unc-17(
e113)
dpy-13(
e184) hermaphrodites and the Fl generation was screened for the presence of rare wild-type length (
e184/+/+) or Dpy (
e184/mDf) individuals among the semi-Dpy (
e184/+) cross progeny. Six Wild and two Dpy worms were found after screening approximately 1500 Fl hermaphrodites. One of the Dpy mutants carried an apparent
dpy-13 allele whereas the other carried a small deficiency (mDf 10), which includes
let-278, the strains established from wild-type cross progeny have been characterized, and both appear to carry duplications (mDp1 and mDp2). mDp1 carries wild-type alleles of
dpy-9,
dpy-13, ng that the entire left arm of LG IV is duplicated. The
unc-8, e not included in mDp1, thereby placing the duplication breakpoint in the interval between unc-S and
unc-8. This duplication recombines with the normal fourth chromosome at a low frequency since we have found both Dpy and semi-Dpy Unc recombinants in an
unc-17(
e113)
dpy-13(
e184) IV; mDp1(lV:III) strain. In addition, mDp1 appears to be homozygous viable, albeit slow-growing. We have determined that mDp1 is linked to
unc-32 and to
unc-45 but not
unc-25, suggesting that this duplication is attached to the left arm of LG III. In these mapping crosses suppression of
dpy-13(
e184) is the phenotypic marker for the duplication (
e184/+; mDp1 are wild;
e184/e184; mDp1 are semi-Dpy). In contrast to mDp1, the mDp2 duplication appears to carry only a small region of LG IV. Of the 5 genes tested, only
dpy-13 and
ama-1 are included in mDp2 whereas
unc-17, We have not tested this duplication for linkage to any chromosome besides the X: however, mDp2 is most likely a free duplication since it is apparently lost at a high frequency. Below is a genetic map of LG IV showing the extent of both duplications. {Figure 1}