Sex pheromones and their potential role in the evolution of reproductive isolation in small ermine moths (Yponomeutidae)

Sex pheromone communication in the nine European species of small ermine moths (Yponomeuta) is reviewed in regard to the potential role of pheromones in the speciation process. Six of the nine species studied (viz., Y. evonymellus, Y. cagnagellus, Y. padellus, Y. irrorellus, Y. plumbellus, and Y. vigintipunctatus) use a mixture of (E)-11-and (Z)-11-tetradecenyl acetate in different ratios as primary pheromone components, with combinations of tetradecyl acetate, (Z)-9-tetradecenyl acetate, (Z)-11-hexadecenyl acetate and the corresponding alcohols of the acetates as additional pheromone components. Analysis of (Z)- to (E)-11-tetradecenyl acetate ratios produced by individual females of these species demonstrated significant variation among females of all species. However, the ranges of ratios produced by Y. cagnagellus, Y. irrorellus, and Y. plumbellus, sharing the same host-plant species, spindle tree, did not overlap. Niche separation of all six species mentioned required consideration of at least one additional pheromone component or of temporal aspects. The remaining three species, i.e. Y. malinellus, Y. mahalebellus and Y. rorellus, have pheromones that differ qualitatively. Biosynthetic routes to the pheromone components identified are proposed on the basis of fatty acid pheromone precursors found in the pheromone glands. A phylogenetic tree for the genus is constructed based on allozyme frequency data and changes in pheromone composition are superimposed on this tree. We suggest that the ancestral ermine moth pheromone is a mixture of (Z)-11- and (E)-11-tetradecenyl acetate and the corresponding alcohols, and a scenario of how present-day patterns evolved is outlined. The pheromone differences among the three species using spindle tree as their host-plant might have evolved through reproductive character displacement upon secondary contact between populations that had already diverged genetically in allopatry. Pheromone differences within the so-called padellus-complex (including Y. cagnagellus, Y. mahalebellus, Y. malinellus, Y. padellus, and Y. rorellus) in which species might have originated sympatrically, may have evolved by reinforcing selection as these species still hybridise and produce viable offspring when confined in cages. The role of pheromones in reproductive isolation among Yponomeuta species is emphasised by (1) the function of pheromone components of some of the species as behavioural antagonists to other species, (2) the cross-attraction under experimental conditions between allochronic species with similar pheromones, and (3) the formation of hybrids in the laboratory between species that are isolated in nature by pheromone differences