Does Stellate Cause Meiotic Drive in Drosophila melanogaster?

Drosophila melanogaster males deficient for the crystal (cry) locus of the Y chromosome that carry between 15 and 60 copies of the X-linked Stellate (Ste) gene are semisterile, have elevated levels of nondisjunction, produce distorted sperm genotype ratios (meiotic drive), and evince hyperactive transcription of Ste in the testes. Ste seems to be the active element in this system, and it has been proposed that the ancestral Ste gene was “selfish” and increased in frequency because it caused meiotic drive. This hypothetical evolutionary history is based on the idea that Ste overexpression, and not the lack of cry, causes the meiotic drive of cry– males. To test whether this is true, we have constructed a Ste-deleted X chromosome and examined the phenotype of Ste–/cry– males. If hyperactivity of Ste were necessary for the transmission defects seen in cry– males, cry– males completely deficient for Ste would be normal. Although it is impossible to construct a completely Ste– genotype, we find that Ste–/cry– males have exactly the same phenotype as Ste+/cry– males. The deletion of all X chromosome Ste copies not only does not eliminate meiotic drive and nondisjunction, but it also does not even reduce them below the levels produced when the X carries 15 copies of Ste.