Sympatric speciation in Yponomeuta: No evidence for host plant fidelity

According to sympatric speciation theory, adaptation to different host plants is expected to pleiotropically lead to assortative mating, an important factor in the reduction of gene flow between the diverging subpopulations. This scenario predicts mating on and oviposition preference for the respective hosts in both the diverging subpopulations and recently originated species. Here, we test both predictions in the oligophagous Yponomeuta padellus (L.) and the monophagous Yponomeuta cagnagellus (Hübner) (Lepidoptera: Yponomeutidae), two closely related small ermine moth species from the western European clade of Yponomeuta for which speciation in sympatry has been proposed. Mating location and adult host acceptance were evaluated under both semi-field (in a large outdoor cage with a choice of host and non-host plants) and field conditions. In the semi-field experiment, only Y. cagnagellus showed some preference for mating on its own host (16% of all mating pairs) over non-host plants (3% of all mating pairs). However, in both species, more than 80% of the mating pairs were not formed on a plant but instead on the cage itself. Further examination of the mating site of Y. cagnagellus in the field revealed no preference for host plants over non-host plants in the two consecutive years of observation. Yponomeuta padellus females, collected from and reared on Prunus spinosa L. (Rosaceae), showed an oviposition preference for the alternative host Crataegus monogyna Jacq. (Rosaceae) in the semi-field experiment. We thus found no evidence that host-plant fidelity (in terms of mating site) has been the driving force in the speciation process of these Yponomeuta species, nor did we find evidence of host race formation in the tested population of the oligophagous Y. padellus.