bw[V1] vs. Pm

[FlyBase Project Members]

Dear Drosophilist:

This letter concerns a point raised by Steve Henikoff in response to
a comment of Alan Christensen's regarding Drosophila nomenclature:

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Steve Henikoff writes:

>Dr. Alan Christensen writes:

>>P.S. I'm not going to tackle Pm vs. bw[V1].

>Muller originally reported second chromosome eye-color dominants which he 
>called Plum mutations apparently before he realized that they were allelic to 
>brown (bw) recessives (Pm/bw- is more extreme than Pm/bw+). bw[V1] is
>the only surviving allele originally referred to as "Plum" for which this 
>designation persists, so it is commonly called Pm without an allele number.
>But it is only one of many dominant brown variegated alleles, some of which 
>are also in common use, such as bw[VDe2] and bw[D], so a special name for 
>this particular allele seems unnecessary. I think the reason the name has 
>stuck is that bw[V1] has two inversions with breaks near both tips and is 
>lethal, so therefore Plum can serve as a balancer for much of the second 
>chromosome. Being viable over better balancers carrying Curly, Plum is 
>especially useful because by crossing to Cy/Pm, all progeny are typically 
>balanced and marked with an obvious dominant marker, not just half. It seems
>most appropriate to me to use Plum or Pm in reference to the chromosome (such
>as in referring to the widely-used "Curly-Plum" stock) which carries the 
>bw[V1] allele.

I agree with Steve.  Plum should be viewed as a nickname for a chromosome,
not as a formally accepted genotype.  There is a long tradition of such
nicknames for chromosomes: balancers, special compound chromosomes and 
multiply-marked chromosomes useful for recombinational mapping are three
classes of example (e.g., Basc = Muller-5, CyO, yf:=, rucuca, X-ple).

As Steve notes, Plum is really a chromosome carrying two pericentric
inversions.  Indeed, only one of these (In(2LR)40F; 59D4-E1) breaks
within brown and presumably causes the bw[V1] phenotype.  The other
inversion could potentially be separated by double crossing over, and
by some criteria, ought to really be given a separate name.  Indeed, 
the current database design for FlyBase will likely require us to do so.

Bill Gelbart (for FlyBase)