Ecological speciation describes the evolutionary process whereby divergent natural selection between environments generates reproductive isolation. Studying the magnitude of sequential reproductive barriers between ecologically divergent populations improves our understanding of the way these barriers evolve and how each contributes to the speciation process. Immigrant inviability describes the lower fitness of immigrants in non-native environments and is an important, but long underexplored, reproductive barrier. Here, we tested the role of immigrant inviability among different host-associated populations of the gall wasp Belonocnema treatae Mayr (Hymenoptera: Cynipini: Cynipidae) by measuring the ability of gall wasps to initiate and complete gall formation on closely related native and non-native live oaks. In 2017, we conducted a reciprocal transplant experiment with 3 populations of B. treatae from the host Quercus virginiana (Qv) and 3 populations from the host Q. geminata (Qg). For each B. treatae population, we deployed 5 saplings of the native and non-native sapling under trees heavily infested with B.treatae, let gall wasps select the host plant upon which to oviposit. In parallel, we conducted the manipulation experiment where we collected individuals from each B. treatae population, bagged them onto saplings of their native and non-native host oak. Combining natural colonization and manipulative experiments, we explore (1.) the generality and magnitude of immigrant inviability as a reproductive barrier between different host-associated populations of B. treatae, and (2.) explore the effect of host preference on the magnitude of immigrant inviability.
Results/Conclusions
Data shows that when Qv populations colonized Qg saplings, both the percentage of trees where galls were formed and the total number of galls formed are lower on non-native host Qg than on the control-native host Qv. When Qg populations colonized Qv saplings, the percentage of trees formed galls and the number of galls formed on non-native host Qv are not significantly different from they are on native host Qg. These suggest that strong fitness cost of switching host occurs in one direction: that is when gall wasps from Qv trees colonizing Qg saplings (both percentage of trees formed galls and number of galls formed are lower on non-native host Qg than on native host Qv), not when gall wasps from Qg trees colonizing Qv saplings. Further analysis will focus on whether the populations with the host choice will perform better on both native and non-native host.