Worm Breeder's Gazette 9(3): 49

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More About X-Linked Gene Expression in X-Duplication and X-Dependent Dpy Strains

P. Meneely

I told you earlier that X-chromosome duplications suppress X-linked 
hypomorphs in regions they do not duplicate. For example, a 
duplication including unc-20+ will suppress lin-15 (n765), a hypomorph 
at least 20 map units away from the nearest gene duplicated. This 
suggests that X-chromosome expression is elevated by X-duplications in 
2X animals, a suggestion that has been directly confirmed by assaying 
transcription of cloned genes (Casson and Meyer, last newsletter). The 
suppression in 2X animals is both size and dose-dependent; large 
duplications suppress better in general than small duplications, and 
two copies of a duplication suppress better than one copy. With the 
possible exception of a duplication of dpy-22+, which suppresses 
better than expected based on its size, the suppression does not seem 
to be especially dependent on the region of the X duplicated; not all 
regions of the X have been tested. The same result is found in both 1X 
males and 1X hermaphrodites (produced by her-l(e1520)), and again the 
amount of suppression is dependent on size and dose (in one case, for 
the latter), but not on the region duplicated. However, XO 
hermaphrodites are not suppressed as well as XX hermaphrodites, 
suggesting that X expression is not elevated as much in XO as in XX 
animals. I'll assume that suppression of X-linked hypomorphs reflects 
elevated X expression; so far, the data support that.
The suppression is enhanced by the X-dependent dpy mutation dpy-21, 
which elevates overall X expression: a Dp/+; dpy-21 strain is a better 
suppressor than either the duplication or dpy-21 alone in both 1X and 
2X animals. This is probably true for two other X-dependent dpy's, dpy-
26 and dpy-27, but the data are not so complete. Some time ago, I 
found (with Bill Wood) that dpy-21 and certain large X duplications 
interacted to make 1X animals intersexual. The fact that dpy-21 and X-
duplications each elevate X expression in 1X animals leads to a 
hypothesis that the intersexuality of these strains is due to 
overexpressing some !one or many) particular X-linked gene products 
involved in sex determination. One contradictory result was that the 
duplication of dpy-22+, which has very high X expression, is not 
intersexual in dpy-21 strains. Another problem now arises. A dpy-26: 
l is a normal male, but with high X 
expression, as high or higher than mnDp25/+; dpy-21, which is (at 
least sometimes) intersexual. This argues against the hypothesis that 
X expression alone is responsible for the intersexuality of Dp/+: dpy-
21 XO animals, and suggests that the duplication affects sexual 
development in a way that dpy-21 and dpy-26 do not; perhaps the 
duplications affect genes that dpy-21 doesn't affect strongly, or 
perhaps the duplications provide some non-expressed elements that 
affect sexual development.
I have also been looking at maternal effects on X expression by 
using XO hermaphrodites. The data don't form a coherent whole yet, but 
the general conclusion is that the progeny of XO mothers do not 
suppress hypomorphs as well as the progeny of XX mothers do. The 
effect is especially pronounced in the XO progeny of XO mothers.