Extended Data Figure 10: A general model for mating-type determination in P. aurelia species.

In the three species examined, mating type E depends on expression of the mtA protein during sexual reactivity. mtA transcription in turn requires the mtB and mtC gene products (the requirement for mtC in P. septaurelia, and for both genes in P. octaurelia, remains to be verified). In P. tetraurelia and P. octaurelia, mating type O is determined during MAC development by excision of the mtA promoter as an IES, preventing expression of the gene. In P. septaurelia, mating type O is determined by the excision of segments of the mtB coding sequence as IESs, which similarly prevents mtA expression.