Worm Breeder's Gazette 9(3): 92

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The Curious Origin of Z5

E. Hedgecock

Figure 1

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}

Figure 1