Worm Breeder's Gazette 11(2): 117

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.

Variable Expression from an Extrachromosomal fem-1 Gene: Possible Germ-line Confinement?

Hodgkin J.

A stable male/female strain (CB4628, or strain A) consisting of tra-
2;fem-1:xol-1 XX females and tra-2 xol-1 XX males 
was constructed by conventional means: in this strain sex is 
determined by the presence or absence of fem-1(+) on LGIV, so LGIV now 
behaves as a Y(dominant masculinizing) or an X (recessive feminizing). 
As expected, strain A consists of 50% fertile females and 50% fertile 
males, and hermaphrodites are never seen.
A variation on strain A was constructed, consisting of tra-2; 
xol-1 XX females and tra-2; 
xol-1: eEx14 XX males.  In this strain (CB4706, or 
strain B) sex is determined by the presence or absence of the 
extrachromosomal eEx14 element (from Andrew Spence) which carries a 
10kb fem-1(+) fragment.  Sex ratio in this strain is rather variable, 
owing to unstable transmission of eEx14, but has a mean of about 50% 
females, 50% males.  A few clearly intersexual animals are seen, 
presumably the result of somatic loss of eEx14.  Unexpectedly, strain 
B also segregated a few (~5%) self-fertile hermaphrodite worms.  These 
were used to found a separate strain, CB4707 or strain C.  Initially 
only about 1-10% of the animals in strain C were self-fertile, but 
with time this fraction increased.  The self-fertile animals have 
truncated tail spikes but are not otherwise strongly masculinized.  
They produce broods of up to 200 progeny, consisting approximately of 
20% hermaphrodites, 60% females, and 20% unhatched eggs (often 
misshapen).  Males or strongly masculinized animals are never seen.  
In strain B, eEx14 is expressing full fem-1(+) function in both germ 
line and soma; in strain C it is expressed in the germ line but not in 
the soma (or at drastically lower levels).  One possible explanation 
is that eEx14 has integrated on one of the chromosomes at a site that 
only permits expression in the germ line, and has disrupted a zygotic 
essential gene in the process, hence the dead eggs.  However, the fem 
activity and the lethality do not segregate in a Mendelian manner.  A 
second explanation is that 'eEx14C' has acquired a spontaneous 
mutation that selectively interferes with fem-1 somatic function.  
However, no such mutation has been found in previous work on fem-1, 
despite strong selections.  A third explanation is that the eEx14C 
extrachromosomal array has become sequestered, either physically or 
functionally, so that it is now only transmitted or expressed in the 
germ line.  Further molecular and cytological analysis should clarify 
what is going on in strain C.  Two preliminary and tentative 
conclusions can be drawn.  First, the germ-line function of fem-1 
seems to be autonomous to the germ line.  Second, expression from a 
transgene can be quite variable, and the presence of a functional gene 
in one tissue does not guarantee expression in all tissues.  Some 
workers have begun to use cloned fem-1(+) as a convenient selectable 
marker for cotransformation experiments, but these results suggest 
that its behavior might be treacherous.