Worm Breeder's Gazette 7(1): 82

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Informational Suppressors: A Cautionary Note

J. Hodgkin

The pleiotropic suppressors sup 5 III and sup 7 X (Waterston,1981) 
have been used by several worm breeders as a means of identifying null 
alleles ( = amorphs ), because they appear to act only on a subset of 
null alleles, and not on missense alleles.  Also, the extent and 
efficiency of suppression ( i.e., whether suppression is dominant or 
recessive) have been used as an indication of whether a particular 
gene product is needed in catalytic or stoichiometric amounts.
While these indications are probably fairly reliable in general, it 
is possible to find exceptions.  I have identified eight sup-7 
suppressible alleles of tra-1 III : one (e1781) was derived from a 
general screen for tra mutants and seven were obtained by reversion of 
the dominant feminizing allele e1575 ( 'her-2').  Six of these have 
similar properties: they appear to be null alleles causing XX animals 
to develop into phenotypic XX males instead of hermaphrodites ) and 
suppression is recessive.  The other two alleles are anomalous.  One, 
e1825, is a weaker allele (i.e., not a complete null) because e1825 XX 
animals often have abnormal male tails, and they are never fertile 
males, nor are e1825/e1781 XX heterozygotes.  The other anomalous 
allele, e1835, appears to be a null allele but it is unusual in that 
suppression is dominant rather than recessive.
Since six out of eight alleles exhibit a consistent pattern, it is 
reasonable to regard the other two as exceptional.  Precedents for 
such exceptions can be found among microbial amber mutations.  However,
the existence of the exceptions shows that, as usual, genetic data 
can sometimes be misleading.