Sterile F1 adult hybrids can be obtained from crosses of C. briggsae males to C. remanei females. Depending on the strains used, all hybrids are females. The absence of male hybrids results from sexual transformation, not male-specific lethality, as both XX and XO females are observed. Variant strains of C. briggsae and C. remanei suppress this hybrid sexual transformation, demonstrating genetic variation within both species the affects sex determination in F1 hybrids. This variation has been used to map hybrid sex reversal loci. In C. remanei, an ill-defined region of the X chromosome is implicated. In C. briggsae, three loci linked to hybrid sex reversal have been identified. The strongest linkage is to allelic variants in
Cb-tra-2. A null mutation in
Cb-tra-2 (thanks Eric) completely suppresses the transformation of XO hybrids. Other C. briggsae loci linked to sex reversal variants include
Cb-daf-5 and
Cb-mab-9.