Worm Breeder's Gazette 4(1): 36
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.
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.