Divergent host-plant use promotes reproductive isolation among cynipid gall wasp populations

Abstract

Ecological speciation occurs when reproductive isolation evolves as a consequence of divergent natural selection among environments. A direct prediction of this process is that ecologically divergent pairs of populations will exhibit greater reproductive isolation than ecologically similar pairs of populations. By comparing allopatric populations of the cynipid gall wasp Belonocnema treatae infesting Quercus virginiana and Quercus geminata, we tested the role that divergent host use plays in generating ecological divergence and sexual isolation. We found differences in body size and gall structure associated with divergent host use, but no difference in neutral genetic divergence between populations on the same or different host plant. We observed significant assortative mating between populations from alternative host plants but not between allopatric populations on the same host plant. Thus, we provide evidence that divergent host use promotes speciation among gall wasp populations.

Parallel Patterns of Morphological and Behavioral Variation among Host-Associated Populations of Two Gall Wasp Species

“…For B. treatae , we compared the body size of asexual generation adults among populations distributed within and between host plant species. This analysis complements previous comparisons of the sexual generation [9]. For D. quercusvirens , we compared the body size of asexual generation adults among populations distributed within and between host plant species.…”

“…The two sister species, Q. virginiana and Q geminata , overlap geographically in the southeastern United States, where Q. geminata is restricted to this region, but Q. virginiana stretches further up the Atlantic coast to Virginia and further west and south along the Gulf coast into Texas and Mexico [29]. We have previously demonstrated host specificity among populations of B. treatae on Q. virginiana and Q. geminata , as measured by host preference [9], [10].…”

“…Belonocnema treatae (Mayr) and Disholcaspsis quercusvirens (Ashmead) (Hymenoptera: Cynipidae) are both ecologically specialized gall formers that attack certain species of ‘live oaks’ in the genus Quercus [9], [10], [30], [31], [36], [37]. The ‘live oaks’ (series Virentes ) are a lineage of five to seven oak species forming a nearly continuous distribution from the tropics (Costa Rica) to the temperate regions (southeastern United States) [29].…”

“…Thus far, the majority of studies addressing parallel ecological speciation have compared geographically separate populations of individual species in different versus similar environments [7], [9], [13], [22], [23]. These studies have suggested a central role for divergent selection in promoting speciation [4].…”

Description of a new species of Euderus Haliday from the southeastern United States (Hymenoptera, Chalcidoidea, Eulophidae): the crypt-keeper wasp

“…There are six to eight different cynipid gall wasps that are highly specialized and form galls on this same live oak complex (Egan et al 2013). We have reared out many of the parasitoids from this large community, including clearly documenting the community from another gall former, Belonocnema treatae (Forbes et al 2016), but this is the first time we have observed a member of the genus Euderus in this system after two decades of work (Lund et al 1996, Egan and Ott 2007, Egan et al 2012, Egan et al 2013, Egan, unpubl. data).…”