Evolutionary Change within a Bipotential Switch Shaped the Sperm/Oocyte Decision in Hermaphroditic Nematodes

A subset of transcription factors like gli2 and oct1 are bipotential - they can activate or repress the same target,in response to changing signals from upstream genes. some previous studies implied that the sex-determination protein tra-1 might also be bipotential; here we confirm this hypothesis by identifying a co-factor,and use it to explore how the structure of a bipotential switch changes during evolution. first,null mutants reveal that c. briggsae trr-1 is required for spermatogenesis,rna interference implies that it works as part of the tip60 histone acetyl transferase complex,and rt-pcr data show that it promotes the expression of cbr-fog-3,a gene needed for spermatogenesis. second,epistasis tests reveal that trr-1 works through tra-1,both to activate cbr-fog-3 and to control the sperm/oocyte decision. since previous studies showed that tra-1 can repress fog-3 as well,these observations demonstrate that it is bipotential. third,trr-1 also regulates the development of the male tail. since cbr-tra-2 cbr-trr-1 double mutants resemble cbr-tra-1 null mutants,these two regulatory branches control all tra-1 activity. fourth,striking differences in the relationship between these two branches of the switch have arisen during recent evolution. c. briggsae trr-1 null mutants prevent hermaphrodite spermatogenesis,but not cbr-fem null mutants,which disrupt the other half of the switch. on the other hand,c. elegans fem null mutants prevent spermatogenesis,but not cel-trr-1 mutants. however,synthetic interactions confirm that both halves of the switch exist in each species. thus,the relationship between the two halves of a bipotential switch can shift rapidly during evolution,so that the same phenotype is produce by alternative,complementary mechanisms. © 2013 guo et al.