Worm Breeder's Gazette 10(2): 86

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Transformation Rescue of glp-1

A. Fire and J. Priess

This project started with a somewhat ill-fated attempt to identify 
the glp-1 gene within the 400kb Iin-12 to ep-7 contig.  The glp-1 gene 
had been mapped between lin-12 and ep-7 and we hoped to test a large 
number of cosmids within the contig to see which one(s) rescued glp-1 
mutations.  To assay for rescue we were originally using a ts allele (
e2142) of glp-1 which lays dead (or doomed) embryos at the non-
permissive temperature (25 C).  The e2142 animals to be injected were 
born and raised at the permissive temperature of 15 C and then shifted 
up to 25 C as larvae.  They were injected as late L4's or adults in 
the distal tip region of the gonad at 25 C and then maintained at that 
temperature.  Since the embryonic lethality in glp-1 is a maternal 
effect, expression of glp-1 in the maternal gonad should yield rescued 
progeny.  The choice of the distal tip region for injection was a 
result of studies with the sup-7 tRNA gene, which gives strikingly 
efficient maternal rescue of tra-3 amber alleles when injected into 
that region.  To make a long story short none of the cosmids tried 
gave significant rescue of e2142 in this assay.
It subsequently became clear from the data of J.  Austin, J.  Kimble,
J. Yochem, and I.  Greenwald that the glp-1 gene was likely to be 
partially or completely contained in the cosmid ZK506 and so we 
concentrated our subsequent efforts on the activity of that cosmid (
this cosmid lies to the right of lin-12, and does not overlap the lin-
12 gene).  A more conservative approach toward assaying for expression 
was tried.  For this, another allele of glp-1 was used (e2141).  This 
ts allele exhibits both the sterile phenotype and maternal effect 
lethal phenotypes associated with glp-1.  Like e2142, the e2141 stock 
will not grow at 25 C but unlike e2142 (for which many of the embryos 
fail to hatch even at 15 C) e2141 embryos almost all hatch at 15 C.  
Thus it was feasible to inject into e2141 animals raised at 15 C, and 
allow all the resulting F1 progeny to hatch at 15 C; expression of glp-
1 in the germ lines of the F1 progeny was then assayed by shifting to 
25 C as larvae and scoring for fertility.
This procedure resulted in recovery of four transformed lines (PD101-
PD104) from twenty injected adults.  No convincing examples of 
transient expression in the F1 germline without recovery of a 
transformed line were observed.  Thus the transformation signal with 
this gene is comparable to heritable transformation signals with other 
genes, but significantly below levels of transient expression of early 
zygotic genes such as unc-54 or mec-3 (WBG10#1 pp28&60).  The lack of 
a real burst of 'transient' rescuants in the F1 germline may turn out 
to hold true for all germline genes, and could result from the greater 
number of divisions for germline versus soma, which will eventually 
dilute injected DNA before it reaches the adult germline.
Although they are all viable at 25 C, the transformed lines are all 
somewhat odd.  For PD101 and PD102 the rescued (fertile) animals tend 
to be scrawny and unc, and some of the PD102 animals are Muv's.  No 
homozygote line has been derived for either of these.  For PD103 (
which is homozygote viable), the fertile adults look quite normal but 
most of the eggs fail to hatch at 25 C.
These injections were performed as co-injections of ZK506 with a 
plasmid (pShZ1) containing a fusion construct between a Drosophila 
heat shock promoter and E.  Coli  -galactosidase.  This was done in 
hopes of following the transforming DNA in individual animals.  
Animals from the transformed lines PD101 and PD102 have been stained 
for  -gal activity after heat shock.  Staining is in a pattern 
characteristic of the plasmid pShZ1, and presence of stain in the 
animals correlates to rescue of the glp-1 phenotype.  DNA from strains 
PD102-104 have also been analyzed on southern blots (by S.  Harrison) 
and shown to contain 2-12 copies of the injected cosmid.
John Yochem sent a lambda phage with most or all of a lin-12 homolog 
gene mapping in the glp-1 region.  This was injected into e2141 in 
order to select rescued lines as described above.  The phage gives a 
very low transformation frequency (one transformed line in ~50 
injected animals).  It is not clear whether the low frequency reflects 
something missing in this lambda clone or just reflects a difference 
in transformation frequency between linear lambda and circular cosmid 
DNAs.  The transformed line (PD110) expresses a co-injected HSP- gal 
fusion (confirming that transforming DNA is present).  The PD110 line 
grows well at 25 C as a heterozygote and exhibits a very slow and 
scrawny homozygous phenotype.
For the sake of future cloning of other maternal effect genes, we 
have gone back and repeated the distal tip injection experiments with 
ZK506.  In the e2141 strain, a small number of rescued progeny were 
observed from this experiment (~.25 per injected tip); no convincing 
rescue was obtained after injection of e2142.  These numbers (unlike 
those originally obtained for maternal rescue by sup-7) are 
insufficient for a really useful assay (since the rescued animals 
obtained from this sort of procedure would not be transformed.)