Worm Breeder's Gazette 9(1): 78
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.
At the recent CSH worm meeting we reported that mutations in the gene egl-16 1) can promote male development in XX animals, and 2) have phenotypes which are sensitive to X-chromosome dosage, affecting XX but not XO animals. The former assertion was based on the observations that a) egl-16(n485.Y4)/Df XX animals display an incompletely penetrant Tra phenotype, and b) in double mutant strains, egl-16(n485) enhances the sexual transformation phenotypes of weakly masculinizing mutations in the genes tra-2 and egl-41. The latter assertion was based on the fact that her-1(e1520);egl-16(n485) XX hermaphrodites are Egl and slightly short, while XO hermaphrodites are essentially wild-type. Likewise, tra-1(e1099.e1488); 85) XX animals are short pseudomales with abnormal bursae, while XO males are wild-type in length and have normal bursae. Since the meeting, we have extended our understanding of egl-16 in several ways: The sexual transformation phenotype of egl-16 is effected through the major sex determination pathway. XX animals homozygous for the weak allele tra-2(n1106) are not visibly transformed but are merely HSN Egl hermaphrodites. The vast majority of animals of the genotype tra-2(n1106)unc-4(e120);egl-16( n485), however, are either visibly sexually transformed or sterile; a homozygous strain of this genotype cannot be maintained. To test whether this enhancement of sexual transformation caused by the egl-16 mutation occurs through the major sex determination pathway, we constructed the triple mutant her-1(e1520);tra-2(n1106)unc-4(e120); 85). The her-1 mutation blocks sexual transformation in this self-fertile Egl hermaphrodite strain, indicating that the egl-16 mutation promotes masculinization directly through the major sex determination pathway rather than via some peripheral mechanism. Arguing from epistasis, egl-16 acts upstream of her-1, perhaps by modulating her-1 activity. egl-16(n485) disrupts dosage compensation in XX animals. The fact that egl-16(n485) affects XX but not XO animals regardless of sexual phenotype suggested that this mutation may disrupt dosage compensation, producing an elevated level of X-linked gene expression in XX animals. This hypothesis was tested using both the biochemical assay for X- specific mRNA levels (Meyer and Casson) and the morphological assay for lin-14 expression (Plenefisch and De Long) described in this issue. We have measured the levels of myo-2(X) transcripts present in egl- 16(n485) and N2 hermaphrodite RNA preparations. When the preps are normalized to myo-1(I) transcript levels, we find that the level of myo-2 mRNA in the egl-16 prep is indeed substantially increased over that seen in the N2 prep (quantitation is in progress). We are currently preparing to measure transcript levels in him-5(e1490);egl- 16(n485) males to determine whether the observed increase is specific for XX animals. egl-16(n485) also causes phenotypic suppression of the hypomorphic mutation lin-14(n179) in the double mutant. This result is consistent with an increase in the level of X-linked gene expression caused by egl-16. No such suppression is observed in XO animals, suggesting that the effect is limited to XX animals. Thus, both biochemical and genetic evidence indicate that egl-16( n485) disrupts dosage compensation in XX animals. egl-16(n485) differs from previously characterized sex determination and dosage compensation mutations in that it identifies a common function which unites the two pathways. Our data are consistent with a model in which the wild-type egl-16 activity is involved in transmitting information about the X:A ratio to both sex determination and dosage compensation pathways. By this model, a mutation in egl-16 results in an apparent underassessment of the X:A ratio in XX animals, shifting both sex determination and dosage compensation mechanisms toward their XO modes. We have begun to examine interactions between egl-16 and other components of the dosage compensation mechanism. A few results are summarized briefly for afficionados: 1. dpy-21(e428);egl-16(n485) is a viable Dpy odite strain. 2. him-5(e1490);egl-16(n485) XXX animals are inviable, or at least greatly reduced in viability. 3. dpy-28(y1);egl-16(n485) is inviable as a strain, even at 15 C ( permissive temperature for Y1); Y1/y1;n485 progeny from Y1/+;n485 mothers are sometimes self-fertile, but produce broods comprised entirely of arrested larvae and occasional wild-type males.