Worm Breeder's Gazette 4(1): 36

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More on X Duplication

B. Herman, J. Madl, C. Kari

In the last C.  elegans Newsletter we included, in connection with 
the map, a figure indicating the extents of several X duplications.  
Here we discuss some of these duplications a bit further.  The extents 
indicated in the figure were determined by checking each duplication 
against the set of 16 X-linked markers also shown in the figure.  We 
note that of six free duplications, mnDp2, 
ot to overlap in extent mnDp30, 
mnDp31,normalises the 
possibility that the X has a diffuse centromere.  On the other hand, 
it is possible that extra breaks that went undetected genetically 
allowed three of the duplications to pick up centromeric material.  We 
would be interested in other evidence for a diffuse centromere.
mnDp10 and mnDp25 are translocated to LGI near unc-54.  Both can be 
recognized cytologically as chromosome satellites: two satellites 
associated with one bivalent are seen in homozygotes, which are 
fertile, and one satellite in heterozygotes.  If we sample broods 
produced by mnDp10/unc-54; unc-3 animals, we find that most have no 
Unc-3 non-Unc-54 recombinants, a small fraction have one recombinant, 
and 1-2% have a cluster (3 or more) of recombinants.  We conclude that 
the unc-3+ on mnDp10 (the same is true of mnDp25) can be lost 
mitotically.  And, as expected, the duplication homozygotes 
occasionally segregate duplication heterozygotes.  A similar 
phenomenon has been observed in Neurospora: in several quasiterminal 
duplication stocks mitotic reversion to an apparently normal euploid 
condition is accomplished by breakage of the translocated segment at 
the interchange point.  We find it interesting that the frequency of 
deletion of these duplications in Neurospora is enhanced by certain 
mutations that affect both meiosis and sensitivity to UV (Newmeyer & 
Galeazzi 1978.  Genetics 89:245).
At the last C.  elegans meeting we reported that 3A;2X animals, 
generated by crossing dpy-11 V; unc-3 X tetraploid hermaphrodites with 
wild-type diploid males, are male.  Furthermore, they are fertile.  
Nigon (1951) showed (and we agree) that 4A;3X animals are 
hermaphrodites, distinguishable from 4A;4X hermaphrodites in that they 
give many more male progeny.  Thus, if sex is determined in C.  
elegans, as it is in Drosophila, by the X to autosome ratio, then a 
ratio of 0.67 (in triploids) gives a male and a ratio of 0.75 (in 
tetraploids) gives a hermaphrodite.  We were therefore prompted to 
construct animals with intermediate X to autosome ratios by adding X 
duplications to a 3A;2X chromosome constitution.  It turns out that 
adding mnDp10(X;I) to 3A;2X generally (if not always) gives a 
hermaphrodite.  This implies that the dosage of a single locus on the 
X relative to the autosomal number is not sex determining since mnDp1O-
bearing XO diploids are male.  Furthermore, adding mnDp9(X;I) or 
mnDp25(X;I), which appear to be smaller than mnDp10, to 3A;2X gives 
males, hermaphrodites, and intersexes.  This effect seems analogous to 
what is seen with 3A;2X Drosophila: patches of male structures and 
patches of female structures are present even though all the cells 
have the same chromosome constitution.