Ectocarpus Ec32 HMG mutants
In many multicellular Eukaryotes, genetic sex determination is not governed by theclassical XX/XY or ZW/ZZ sexual systems but by a specialized region located on the poorlystudied U (female) or V (male) sex chromosomes. Previous genetic analyses have hinted at theexistence of a dominant male-sex factor on the V-sex chromosome in brown algae (seaweeds), agroup of multicellular Eukaryotes that has been evolving independently from animals and plants.The nature of this sex-determining factor has remained elusive. Here, we demonstrate that anHMG-box gene, which we named MIN, acts as the master sex-determining factor in brown algae,mirroring the role HMG-box genes play in sex determination in animals. Our analyses reveal thatover a billion-year evolutionary timeline, these eukaryotic lineages have independently co-optedthe HMG-box for male sex determination, representing a remarkable paradigm for evolution'sability to recurrently use the same genetic 'toolkit' to accomplish similar biological tasks.