Genetic architecture of host specialisation in Yponomeuta

Host shifts can trigger ecological speciation in herbivorous insects. The probability of a successful host shift is a function of the genetic complexity underlying the traits defining host use. Host use includes at least three components: adult and larval acceptance and larval performance.
European species of Yponomeuta are highly specialised on species of different plant families. One of these specialists, Yponomeuta cagnagellus, originated from an ancestor specialised on Rosaceae, but has shifted to Celastraceae. Its close relative, Y. padellus, is associated with Rosaceous hosts. We studied the inheritance of host use traits on their interspecific hybrids.
We conclude that adult host acceptance, larval host acceptance and larval performance are inherited independently. Larval survival involves multiple genes. It seems that the architecture of host use is defined by species-specific, co-adapted combinations of alleles.
Second generation hybrids survive when F1 is crossed back to one of the parental species, but only when fed the host plant of that species. Mortality on the host of the least contributing parent is high. We have ruled out that this is due to intrinsic hybrid breakdown or sterility. This means that there is an indirect selection pressure against the intermediate (heterozygous) genotype: Possessing alleles for use of both hosts, will increase the chance of producing offspring not fit to survive on any host. The choice of a partner that is carrying the same co-adapted alleles, will prevent this subsequent loss of fitness in the next generation, and promote evolution of assortative mating and ecological speciation.