To learn if orthologous mutations are temperature-sensitive in related species, we studied four C. briggsae mutations orthologous to alleles of important C. elegans genes. Both
Cel-glp-4(
bn2) and
Cbr-glp-4(
v473) are temperature-sensitive, causing sterility at 25C. By contrast,
Cel-fog-1 (
q253) is strongly ts , but its ortholog
Cbr-fog-1(
v442) causes a loss-of-function at all temperatures. Finally, the C. elegans
glp-1 alleles
bn18 and
e2141 are ts sterile. However, their C. briggsae orthologs,
Cbr-glp-1(
v429) and
Cbr-glp-1(
v438) respectively, are wild-type at all temperatures. Thus, a ts mutation in one species provides clues about how to design ts alleles in another, but all theoretical outcomes are possible.