Worm Breeder's Gazette 7(2): 25

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.

Studies at the zyg-11 Locus -- A New Look at an old Friend

N. Wolf, K. Kemphues, B. Wood, D. Hirsh

We are analyzing zgy-11 II (0.8 map units to the right of dpy-10; 
between tra-2 and vab-9 by deficiency mapping*).  This locus was first 
identified by tsb2, a maternal effect zygotic lethal.  Preliminary 
observations of tsb2 embryos suggested that zgy-11 plays a role in the 
establishment or maintenance of polarity in the early embryo.  In eggs 
from homozygous tsb2 at 25 C, the first cleavage furrow is often 
improperly positioned, giving rise to even cleavages and cleavages 
with reversed asymmetry.
Three new alleles of zgy-11 (tsb271, b272, tsb273) were isolated in 
complementation screens of 6000 heterozygous F1 progeny of tsb2 males 
and mutagenized dpy-10 hermaphrodites.  Two other alleles, mn40 (from 
R.  Hernan) and ct-1 (isolated by J.  Laufer) were identified by 
complementation tests.  mn40 is suppressible by sup-7 X (Newsletter, 
Vol.7, no.l, p.87).
Phenotypic analysis thus far has concentrated primarily on tsb2 and 
mn40 and has utilized Nomarski microscopic observations of early 
development in live worms, tcrit analysis using short pulses of high 
temperature in individual embryos, and staining of fixed worms and 
embryos with DAPI and/or anti-tubulin antibody (as per Albertson et al.
, Newsletter Vol.7, no.1, p.73).
All alleles of zgy-11 show similar defects but the expressivity and 
penetrance differ.  The first detectable abnormality occurs at 
metaphase II of meiosis.  Embryos cut from tsb2 and mn40 homozygotes 
and stained with DAPI show a three fold increase, relative to N2, in 
the proportion of one-cell embryos with metaphase II figures.  This 
observation is consistent with an increase in the proportion of 
metaphase II spindles observed in preps stained with anti-tubulin.  
Normal resolution of meiosis II seems to fail completely--with DAPI 
staining second polar bodies are seen with low frequency in tsb2 
embryos and have not been observed in mn40 embryos.  Furthermore, the 
time between the extrusion of the first polar body and the appearance 
of pronuclei is increased three to four-fold in tsb2 and mn40 
respectively.  During this extended period, large areas of cytoplasm 
form which exclude cytoplasmic granules, and extremely vigorous 
cytoplasmic movements occur.  When (and if) pronuclei form, they often 
appear abnormal in position, number, size, and morphology.  The effect 
of these early defects on the first cleavage is variable (even in mn40,
a presumed null allele).  Spindles can form with reversed or normal 
asymmetry or with near symmetry (as was first reported for tsb2).  
Often, however, spindles in one-cell embryos are quite abnormal (e.g.  
multipolar or disorganized).  Finally, in most cases, multiple nuclei 
form at the end of first cleavage, irrespective of the polarity of 
cleavage.  These nuclei often appear to contain fragmented chromosomes.

We have been able to elicit the mutant phenotype in tsb2 embryos 
with a temperature pulse of 25 C for 15 minutes.  Embryos are 
refractory to shifts up after pronuclear fusion and seem to be most 
sensitive around the time of meiosis.  We do not yet know to what 
extent the tcrit extends into oogenesis.
Based on our previous results with sup-7 suppression of mn40, zgy-11 
appears to encode a protein which is translated prior to the time of 
zygotic gene expression.  The more recent observations suggest that 
the time of action of the zgy-11 protein is near the time of meiosis 
and may be specific to a time between meiosis I and pronuclear 
formation.  Expression of zgy-11 does not seem to be required at any 
time other than oogenesis.  Spermatogenesis appears normal in mutant 
hermaphrodites, and in at least one non-conditional mutant, b272, male 
fertility is unaffected.  Most alleles are not rescued by mating to 
wild type males.  However, tsb273, the least severe zgy-11 mutant, can 
be rescued by mating with N2 males.  Rescue is not dependent on the 
wild type allele and cannot be effected by tsb273 males raised at high 
temperatures.  This suggests that the zyg-11 gene, while not essential 
for spermatogenesis, may be expressed and its products stored in sperm.

* Deficiencies generously provided by Chris Sigurdson.