The usefulness of a genetic balancer depends on characteristics in addition to the stability and genetic extent of the balancing activity. These characteristics can be grouped into two categories of experimental method: (1) ways to determine which animals carry the balancer and (2) ways to move the balancer between genetic backgrounds. The potential uses of good balancers are many, but different needs often require different balancer variants. For example, males homozygous for phenotypically wild-type balancers often are able to mate successfully, and the absence of marker mutations minimizes possible interactions with mutations to be balanced. However, balancer homozygotes and heterozygotes may not be distinguishable from each other. On the other hand, balancers marked with recessive morphological mutations are easily followed during stock maintenance, but the homozygous mutant balancer males often cannot mate. Recessive lethal or sterile variants are useful for balancing mutations that are only marginally viable as heterozygotes, since the balancer homozygotes cannot overgrow the population. Finally, dominantly marked balancers allow unequivocal identification of animals that carry the balancer without progeny testing, but render it difficult to transfer the balancer through the male. In addition to standard balancing, multiply-marked and wild-type balancers have been useful in determining balancer structure (Zetka and Rose, Genetics 131, 1992). We have isolated all these variant forms of mC6 in an effort to maximize its utility. This balancer virtually eliminates recombination in heterozygotes over the 13-25 map units covering the gene cluster on chromosome II between
lin-31 and
rol-1. The original isolate is marked with the recessive mutation
dpy-10(
e128). One variant is marked with both
dpy-10 and
unc-4 mutations, another is marked with only
unc-4, and a third is not marked. Each of these variants was shown to suppress recombination over the same interval and to the same degree as the original isolate. In addition, a novel mismatch PCR-RFLP technique was used to show that the putative wild-type mC6 no longer carries the
e128 mutation. A spontaneous lethal mutation in mC6 was obtained during passaging of a balanced lethal. Finally, gamma-irradiation integration experiments yielded an apparent insertion of a GFP construct expressed from early embryo through the adult stage (gift from Andy Fire and Kelly Liu) into mC6[
dpy-10]. This dominant marker is transferred efficiently through males, and allows easy identification of balancer heterozygotes without the potential complications of a dominant behavioral or morphological mutation. Tests are underway to demonstrate that the latter two variants retain balancing activity. Preliminary three-factor mapping experiments indicate that mC6 is an inversion, i.e. the relative order of
dpy-10 and
unc-4 is reversed relative to outside markers. These mC6 variants constitute a rich set of useful, characterized balancers for the most mutation-dense parts of chromosome II in C. elegans. This work is supported by the NIH National Center for Research Resources (NCRR).