VARIATION OF DOMINANCE OF NEWLY ARISEN ADAPTIVE GENES

Newly arisen adaptive alleles such as insecticide resistance genes rep resent a good opportunity to investigate the theories put forth to exp lain the molecular basis of dominance and its possible evolution. Domi nance levels of insecticide resistance conferred by insensitive allele s of the acetylcholinesterase gene were analyzed in five resistant str ains of the mosquito Culex pipiens. Dominance levels were found to dif fer between strains, varying from partial recessivity to complete domi nance. This variation was not explained by differences in catalytic pr operties of the enzyme, since four of the five resistant strains had i dentical inhibition properties for die insensitive acetylcholinesteras e. Among these four laboratory strains and in individuals collected fr om natural populations, we found a correlation between increased acety lcholinesterase activities and higher dominance levels. We propose a m olecular explanation for how variation in acetylcholinesterase activit y may result in variation of dominance level. We also conjecture that the four resistant strains did not differ in their amino acid sequence in the catalytically active regions of acetylcholinesterase, but that the expression of the gene was regulated by either neighboring or dis tant sites, thereby modifying the dominance level. Under this interpre tation, dominance levels may evolve in this system, since heritable va riation in acetylcholinesterase activity was found.