Worm Breeder's Gazette 9(3): 92
These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.
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}