Data from: Speciation in Nearctic oak gall wasps is frequently correlated with changes in host plant, host organ, or both

Quantifying the frequency of shifts to new host plants within diverse
clades of specialist herbivorous insects is critically important to
understand whether and how host shifts contribute to the origin of
species. Oak gall wasps (Hymenoptera: Cynipidae: Cynipini) comprise a
tribe of ~1000 species of phytophagous insects that induce gall formation
on various organs of trees in the family Fagacae —primarily the oaks
(genus Quercus; ~435 sp). The association of oak gall wasps with oaks is
ancient (~50 my), and most oak species are galled by one or more gall wasp
species. Despite the diversity of both gall wasp species and their plant
associations, previous phylogenetic work has not identified the strong
signal of host plant shifting among oak gall wasps that has been found in
other phytophagous insect systems. However, most emphasis has been on the
Western Palearctic and not the Nearctic where both oaks and oak gall wasps
are considerably more species rich. We collected 86 species of Nearctic
oak gall wasps from 10 of the 14 major clades of Nearctic oaks and
sequenced >1000 Ultra Conserved Elements (UCEs) and flanking
sequences to infer wasp phylogenies. We assessed the relationships of
Nearctic gall wasps to one another and, by leveraging previously published
UCE data, to the Palearctic fauna. We then used phylogenies to infer
historical patterns of shifts among host tree species and tree organs. Our
results indicate that oak gall wasps have moved between the Palearctic and
Nearctic at least four times, that some Palearctic wasp clades have their
proximate origin in the Nearctic, and that gall wasps have shifted within
and between oak tree sections, subsections, and organs considerably more
often than previous data have suggested. Given that host shifts have been
demonstrated to drive reproductive isolation between host-associated
populations in other phytophagous insects, our analyses of Nearctic gall
wasps suggest that host shifts are key drivers of speciation in this
clade, especially in hotspots of oak diversity. Though formal assessment
of this hypothesis requires further study, two putatively oligophagous
gall wasp species in our dataset show signals of host-associated genetic
differentiation unconfounded by geographic distance, suggestive of
barriers to gene flow associated with the use of alternative host plants.