Hybrid dysgenesis in<i>Drosophila melanogaster</i>: the evolution of mixed<i>P</i>and<i>M</i>populations maintained at high temperature

Abstract

SUMMARY The ability of hybrid dysgenesis P factors to survive and multiply under conditions of strong negative sterility selection was studied in mixed P and M laboratory cultures. Eight populations were initiated with varying proportions of P and M strains. Mixed populations and controls were maintained for seventeen generations at 27 °C, a temperature sufficiently high to induce maximum frequencies of sterility in dysgenic hybrids. The two components of dysgenesis, P factor activity and cytotype, were monitored every generation for the first ten generations and intermittently thereafter. With one exception, all the mixed populations evolved to the P type indicating that P factors can survive and multiply, despite low initial frequency and strong negative selection against dysgenic hybrids. However, the average level of P factor activity attained at equilibrium was considerably lower than that of the P strain control population maintained under the same conditions. It was also lower than the equilibrium level of P factor activity attained in a similar experiment carried out at a lower temperature, suggesting that selection favoured P factors with weak rather than strong sterility potential.