In mig (
rh72; LG V) mutants, an extra half gonad, devoid of germ cells, is sometimes found in the head. These half gonads arise from a mesoblast, dubbed Z5, positioned dorsally between the nerve ring and the terminal bulb of the pharynx. When Z5 is present, it undergoes a lineage apparently identical to Z1 and Z4, the normal precursors to the somatic gonad. In hermaphrodites, Z5 generates 6 early descendants which comprise 1 distal tip cell, 1 anchor cell, and 4 blast cells which generate a uterus, a spermatheca, and an ovarian sheath in the late mitotic period (Kimble and Hirsh, 1979). In order to determine the embryonic origin of Z5, we examined newly hatched larvae for lineage abnormalities. Interestingly, certain embryonic cell migrations were found to be abnormal. The M mesoblast and its homolog, the right intestinal muscle (mu int R), were mispositioned or absent in a majority of animals. The M mesoblasts, which are more easily recognized, were found in positions ranging from the pharyngeal- intestinal valve to their normal position spanning QV5R. The division axes and lineages of the mispositioned M cells are often imperfect but body muscles, coelomocytes, and sex mesoblasts can be generated even when the M cell adjoins the pharynx. The migrations of the CAN neurons are also variably incomplete in
rh72 animals, creating the pale, thin tail phenotype described for
vab-8 (
e1017) (Sulston and Hodgkin, WBG 5( 1) p.19),
mia-2 (
rh17) (Hedgecock, WBG 8(3) p.53), and other genes ( Manser and Wood, WBG 9(2) p.63). We have not yet followed the embryonic origin of Z5, but an intriguing speculation is that it may be a variant fate for head mesodermal cell homolog. The head mesodermal cell and its homolog are sisters of Z4 and Z1, respectively (Sulston et al., 1983). These cells migrate circumferentially to the dorsal midline where they meet and align anterior-posteriorly. In the wild type, the anterior cell (hmc homolog) dies late in embryogenesis. Conceivably this cell survives in some
rh72 animals to become Z5. This would explain both the position of Z5 and why only 1 extra precursor has been found in any individual. Recently (actually yesterday), we found that
unc-39 (
e257! mutants also have variable defects in CAN, M, and mu int R migrations and frequently have a Z5 precursor. In
e257, in contrast to
rh72, the Z5 precursor usually migrates posteriorly to join Z1Z4. in hermaphrodites, the S-cell organ primordium generates three armed gonad fused at the uterus. Only one anchor cell is made and the vulva is normal. In a fraction of the
e257 animals, Z5 stops short of reaching the normal primordium. In such animals, it generates a second anchor cell and induces an incomplete vulva anterior to the normal vulva. We strongly suspect that
rh72 will prove allelic to
unc-39 (
e257).
unc-39 has been shown to map between
sma-1 and
sqt-3 on LG v. Interesting,
vab-8 (
e1017) has also been mapped to this interval by James Manser. It is possible that these two genes, both of which affect the CAN migrations, are more than chance neighbors. {Figure 1}