Specialized Chromosomes and Their Uses in Caenorhabditis elegans - Caenorhabditis Elegans: Molecular Genetics and Development - page 42

Fig. 6 Schematic showing the genotype of the reference sDp3 strain. The strain is homozygous for
dpy-18 and unc-32 mutations and carries one copy of sDp3. sDp3 provides a WT copy of dpy-18. Animals
that retain the duplication display the unc-32 phenotype while progeny that have lost sDp3 are Dpy Unc.
mutant phenotype. The approach also applies to extrachromosomal SCs containing
transgenic DNA sequence. In C. elegans, cell mosaics have been useful for exam-
ining cell autonomy and determining the timing of gene expression. For example
Hunter et al. utilized eDp6 (III;f) to demonstrate the cell autonomy of tra-1, the gene
that determines somatic sex, indicating that the gene product is a cellular signaling
factor ( Hunter and Wood, 1990 ). Yuan et al. used the free duplication nDp3 (IV;V;f)
to show that the apoptotic genes, ced-3 and ced-4 function within dying cells to cause
cell death ( Yuan and Horvitz, 1990 ). Bucher et al. used qDp3 (III;f) to combine lethal
screens with mosaic analysis to investigate the role of essential genes in develop-
ment, and found that many essential zygotic genes encoded specific developmental
functions ( Bucher and Greenwald, 1991 ).
3. Advantages and Disadvantages
An advantage of using a duplication to balance a mutation is that once the
mutation is balanced, it can be maintained as a homozygote. This has advantages
both for genetic crosses as every outcross progeny will carry the mutation, and for
preparation of DNA as the yield is 2:1 from the balanced strain. The lethal
homozygote can be readily observed in the balancer background without requiring
additional outcrossing as for translocations. The disadvantage is that they may