Strong variation in frequencies of male and female determiners between neighboring housefly populations

Sex‐determination mechanisms evolve rapidly and vary between species. Occasionally, polymorphic systems are found, like in the housefly. Studying the dynamics and stability of such systems can provide a better understanding of the evolution of sex‐determination systems. In the housefly, dominant male‐determining loci (M) can lie not only on the Y chromosome (M(Y) ), but also on autosomes (M(A) ) or the X chromosome (M(X) ). M enforces male development by inhibiting the female‐determining gene transformer (tra). A mutant tra allele, tra(D) , is insensitive to M and is a dominant female determiner. M(Y) prevails at high latitudes and polymorphic M loci together with tra(D) at low latitudes. To get more insight into the stability and frequencies of these sex determiners with mutually exclusive dominance, we investigated 5 regional Spanish populations. We found strong variation among populations. Two populations with hemizygous M(III) were found, 2 contained homozygous M(X) with additional hemizygous M(I) and M(II) in 1 population. One population contained homozygous and hemizygous M(X) with additionally hemizygous M(II) . All females in populations with homozygous M, had tra(D) , whereas no tra(D) was found in populations without homozygous M. Our results indicate locally stable systems may either harbor a single hemizygous M and no tra(D) , corresponding to a male heterogametic system, or homozygous and/or multiple M and heterozygous tra(D) , reminiscent of a female heterogametic system. They support that M loci can accumulate in the presence of a dominant female determiner. Limited migration between populations may contribute to the stability of these systems.