Inheritance and evolution of male response to sex pheromone in Trichoplusia ni (Lepidoptera: Noctuidae)

The evolution of chemical communication in moths has implications for speciation because mate attraction and reproductive isolation are achieved primarily by unique blends of female-produced sex pheromones and the specificity of male responses to these signals. For most organisms, the genetics of speciation remains an open empirical question, but both major gene effects and additive genetic variation are likely to play roles in the diversification of chemical communication that accompanies speciation. In the cabbage looper, Trichoplusia ni (Hübner), a single gene mutation results in females that produce a drastically different signal. We conducted four wind-tunnel experiments to examine the potential role of additive genetic variation in the evolution of male T. ni response to this novel pheromone blend. The heritability of male response to both normal and mutant pheromone signals was tested in two experiments using a father-son regression design. Normal male response to the mutant blend showed a heritability of 0.25. The heritability of normal male response to the normal blend and the mutant male responses to both normal and mutant blends were at or near 0. In a third experiment, we imposed selection on normal male lines for increased response to the mutant pheromone blend. After three generations of selection, normal male response had increased by 24% as compared to only 5% in control lines, confirming the heritability value determined earlier. A fourth experiment showed that the change in the selected lines was due to an improved male response to both the normal and mutant signals.