Some two locus models for the evolution of sex-determining mechanisms (Q579163)

Since 1958 several wild laboratory populations of the housefly (musca domestica) have been found which have single locus ``autosomal'' sex determination instead of the ``standard'' chromosomal mechanism. In western European populations of this species there is a north-south cline with respect to sex-determining mechanisms, from the XY-XX to the completely autosomal. In addition there is a similar cline with respect to altitude a.s.l. in Italy. This has stimulated a theoretical study of the action of selective forces on these polymorphisms. A series of two locus models have been considered. Assuming that the populations were originally all standard, we have attempted to construct possible scenarios for the establishment of this cline. In the absence of selection it is highly unlikely that such a cline would be established. On the other hand, in the presence of either meiotic drive or viability selection on genotypes, a population can shift from one sex-determining system to another, e.g. from male heterogamety to female heterogamety and to a two locus system. The sex ratio at some of the equilibria of the systems can be different from 1:1. The results are discussed with respect to the establishment of the above cline.