Résumé
Anopheles gambiae, responsible for the majority of malaria deaths annually, is a complex of seven species and several chromosomal/molecular forms. The complexity of malaria epidemiology and control is due in part to An. gambiae's remarkable genetic plasticity, enabling its adaptation to a range of human-influenced habitats. This leads to rapid ecological speciation when reproductive isolation mechanisms develop 1, 2, 3, 4, 5, 6. Although reproductive isolation is essential for speciation, little is known about how it occurs in sympatric populations of incipient species [2]. We show that in such a population of “M” and “S” molecular forms, a novel mechanism of sexual recognition (male-female flight-tone matching 7, 8, 9) also confers the capability of mate recognition, an essential precursor to assortative mating; frequency matching occurs more consistently in same-form pairs than in mixed-form pairs (p = 0.001). Furthermore, the key to frequency matching is “difference tones” produced in the nonlinear vibrations of the antenna by the combined flight tones of a pair of mosquitoes and detected by the Johnston's organ. By altering their wing-beat frequencies to minimize these difference tones, mosquitoes can match flight-tone harmonic frequencies above their auditory range. This is the first description of close-range mating interactions in incipient An. gambiae species.