Re: non-occurance of polyploidy in hybridomas

An interesting question!  In some ways you already answered it.  When the 
cells are fused (usually by polyethylene glycol) the resulting fused cell 
does actually become tetraploid - but only for a short time.  The fused 
cells are actually quite unstable and begin to throw out chromosomes in 
the hope of restoring their diploid status.  Of course, sometimes they 
don't survive this process and they can also throw out the chromosome 
that has the rearranged Ig gene on it, resulting in the loss of antibody 
production.  So they may not be dipoid nor tetraploid but they will be 
somewhere between the two.....See 'Antibodies' by Harlow and Lane, an 
excellent source of all things antibody!