Worm Breeder's Gazette 13(2): 43 (February 1, 1994)

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.

Molecular Identification of a Major Feminizing Site on the X Chromosome: fox-1

Jonathan Hodgkin, Jonathan Zellan, Donna Albertson

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(MRC-LMB, Hills Road, Cambridge CB2 2QH, England)

As reported at the 1993 International C. elegans Meeting (JH and DA, 1993 Meeting Abstracts p. 347), a duplication of the left end of the X chromosome, eDp26 (otherwise known as e2532 ,or 'xol brother' ) has a lethal and feminizing effect on XO animals, suggesting that it includes a duplication of a major numerator site for the assessment of X:A ratio. A slightly smaller duplication, mnDp73 ,has no Xol (XO Lethal) effect, nor does a duplication further to the right, mnDp57 .Therefore, it seemed possible that a major numerator is located in the region duplicated by eDp26 but not by these other two duplications, between lin-32 and unc-2 (see Figure). More precise mapping of the endpoints of these three duplications, by FISH probing with labeled YACs, defined a region of less than 300 kb which is uniquely duplicated by cDp26 .

Cosmid clones covering much of this region were kindly provided by Ratna Shownkeen, and injected into him-8 hermaphrodites together with the cotransformation marker, pRF4 .Transmitting Roller lines, containing multicopy extrachromosomal arrays of each tested cosmid, were established and examined for segregation of Roller males. Most lines segregatcd Rollers of both sexes (usually 40 - 50% male). However, transgenic animals carrying one particular cosmid segregated many Roller hermaphrodites, less than 1% Roller males, and substantially increased numbers of dead eggs and L1 's,indicating a dominant Xol effect. This transgenic array, eEx28 ,was crossed into a dpy-26 ( nl99 )background, which should suppress lethality due to X chromosome underexpression. As expected, dpy-26 ; eEx28 XO animals are viable, and are mostly transformed into self-fertile hermaphrodites, comparable to dpy-26 ; eDp26 XO animals, which are also mostly hermaphrodites. The same result has been obtained with an overlapping cosmid. We conclude that these cosmids contain a major dose-sensitive feminizing site, which we are tentatively calling fox-1 (for 'Feminizing site On X').

These transgenic arrays are much more strongly feminizing than arrays carrying the 'feminizing elements' defined by McCoubrey et al (Science 242:1146,1988), which lead to some feminization of 3A;2X animals but have no effect on 2A;1X animals. Also, it is likely that eDp26 carries only one extra copy of fox-1 ,so a two-fold increase in fox-1 dosage seems sufficient to switch development into the hermaphrodite mode of sex and dosage compensation. It is therefore conceivable that fox-1 is the major primary sex determinant on the X chromosome. However, some XO animals with extra copies of fox-1 (from either eDp26 or eEx28 )are still intersexual or male in phenotype, indicating that fox-1 dosage is not the only factor involved. Probably there are additional minor or alternative numerator sites, elsewhere on the X chromosome.

Figure 1