Worm Breeder's Gazette 8(2): 35
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.
Genes are being mapped on the embryonic metaphase chromosomes of C. elegans by in situ hybridization using probe DNAs nick translated to incorporate biotin-labeled dUTP. Initially, morphologically distinct chromosomal rearrangements were used to identify a linkage group (LG), and in this way the ribosomal genes were mapped to LG I. To facilitate the mapping of genes in C. elegans a strain carrying let( e2000);eDp20(I;II);mnT12(IV;X) has been constructed in which all linkage groups can be identified, either morphologically or by the hybridization of reiterated gene probes (ribosomal or 5S genes) to the chromosomes. The mutation let(e2000), formerly called let-209 appears to be a deletion of some ribosomal genes and so gives an abnormally small hybridization signal compared to wild type. In contrast, eDp20( I;II) a translocation of the ribosomal genes to the right arm of LG II causes LG II chromosomes to display a large hybridization signal with the ribosomal probe. Thus, using this C. elegans strain, LG I and II can be labeled by the distinctive hybridization signals from the ribosomal probe (let(e2000), small signal; eDp20(I;II), large signal) and LGs IV and X are together distinguishable morphologically. Having identified the linkage group to which a probe DNA hybridizes, the site of hybridization is assigned a position on the chromosome by drawing a series of straight lines down the center of the chromosome and measuring the distance from one end of the chromosome to the site of hybridization. Since the chromosome spreads vary considerably in the degree of condensation the site of hybridization is expressed as a percent length from one end of the chromosome. Thus, the 5S genes, which have been mapped to the right of dpy-11 on linkage group V by restriction fragment length polymorphism (Nelson and Honda, personal communication) were mapped by in situ hybridization to a position 15- 25% from one end of the chromosome (presumably the right) where a characteristic Hoechst dark band is also seen. Suitable markers for LG III and mnT12(IV;X) are still required, because a marker not only identifies the linkage group but should provide the left-right orientation of these otherwise featureless chromosomes. Several genes have now been mapped by in situ hybridization. A probe for the actin gene cluster on LG V was mapped to a position 50- 70% from the left end of LG V. An actin gene IV specific probe ( kindly provided by M. Krause) has been tentatively assigned to LG II. The myosin gene 3 (myo-3) which codes for the minor body wall myosin, myosin A (Miller, Stockdale and Karn, personal communication) has been mapped to the same site as the actin cluster, that is 50-70% from the left end of LG V. The coincident map position of the actin genes and the myo-3 gene raises the possibility that the sup-3 locus, which may play some role in the regulation of myosin A production might be linked to the structural gene for myosin A. David Miller is currently exploring the relationship between sup-3 and myo-3 by probing Southern blots of sup-3 deficiency DNA with the myo-3 cosmids. Since the chromosomal position of genes mapped by in situ hybridization covers a broad range, chromosomal rearrangements are being used to define more precisely the region to which myo-3 hybridizes. The myo-3 and 5S probes were hybridized to chromosomes from embryos carrying the reciprocal translocation, eT1(III;V) which breaks LG V in the interval between dpy-11 and unc-42 (Rosenbluth and Baillie, Genetics 99, 415 (1981)). Both probes hybridized to the same chromosome, suggesting that myo-3 maps to the right of the translocation break point on LG V. Another reciprocal translocation, nT1(IV;V) that may have a break point to the right of unc-76(V) ( Ferguson and Horvitz, WBG 6 (1)) unlinks the myo-3 genes and the 5S genes.