Molecular Placement of an Outbreak-Causing Gall Wasp, Zapatella davisae (Hymenoptera: Cynipidae), with Comments on Phylogenetic Arrangements in the Tribe Cynipini

Gall wasps (Hymenoptera: Cynipidae) have fascinated researchers for centuries due to the elaborate diversity of charismatic galls they produce, the presence of unique reproductive systems (e.g., a form of cyclical parthenogenesis), the possible convergent evolution of semiparasitic gall wasp forms (i.e., “inquilines”), and their multitrophic interactions. While many classifications for gall wasps have been proposed, recent DNA sequence efforts combined with taxonomic revisions are beginning to clarify the evolutionary relationships of this group. To date, however, a well resolved phylogeny is lacking, complicating the study of outbreak-causing pest species. Outbreaks by one such species, the black oak gall wasp, Zapatella davisae Buffington & Melika (Hymenoptera: Cynipidae: Cynipini), have led to extensive damage and mortality of black oaks, Quercus velutina L. (Fagales: Fagaceae), in the northeastern United States. Here we sequenced fragments of the nuclear ribosomal gene 28S, and the nuclear protein coding gene long-wavelength opsin from samples of Z. davisae collected on Cape Cod, MA, and Long Island, NY. Using these sequences and sequences previously published from the mitochondrial locus cytochrome b, we performed Bayesian and maximum likelihood multilocus phylogenetic reconstructions based on a concatenated alignment including species of gall wasps in the tribe Cynipini from which all three loci were present in the GenBank database. Confirming morphological work, we find that Z. davisae is most closely related to species in the genera Callirhytis and Neuroterus, and appears to be a basal member of the “Quercus” section of the tribe Cynipini. We find that recent generic reclassifications within the Cynipini have made great progress towards clarifying the taxonomic relationships of species of gall-inducing wasps in this tribe, and we comment on several classifications that require additional research.

Midstory Removal Facilitates Growth but Reduces Competitiveness of Oak Reproduction Prior to and after Shelterwood Establishment Cutting

This study examined the influence of midstory removal applied 9 years prior to a shelterwood establishment cut on survival, growth, and competitive status of black oak (Quercus velutina Lam.), white oak (Quercus alba L.), and an oak competitor, red maple (Acer rubrum L.), immediately before and at 6 years following shelterwood cutting on intermediate productivity sites in the Central Hardwood Forest Region. We also examined height and density distributions of the regenerating cohort to quantify seedling establishment and to elucidate relative oak competitiveness at each time period. Although oak seedlings were taller 9 years after midstory removal than in undisturbed controls, the midstory removal treatment reduced the frequency of free-to-grow oaks (relative to understory competitors) and increased the size of red maple and relative density of large competitors. Height growth of all three species following establishment cutting was increased by prior midstory removal and dependent on seedling competitive position immediately before shelterwood establishment. Additionally, we observed increased seedling densities 6 years following shelterwood establishment cutting and greater white oak seedling establishment when midstory removal preceded the shelterwood treatment.

Development of microsatellite markers for an outbreaking species of oak gall wasp, Zapatella davisae (Hymenoptera: Cynipidae), in the northeastern United States

Populations of the recently described black oak gall wasp, Zapatella davisae Buffington (Hymenoptera: Cynipidae), have been identified as the cause of extensive tree damage and mortality to black oaks, Quercus velutina Lamarck (Fagales: Fagaceae), in the northeastern United States. Relatively little is known, however, about the distribution, phylogenetic placement, and lifecycle of this important tree pest. Therefore, we conducted next-generation sequencing using the Ion Torrent™ PGM (ThermoFisher Scientific, Inc.) platform to develop genomic resources for the study of Z. davisae and for other closely related species of oak gall wasps. Individual sequence reads were aligned, assembled into unique contigs, and the contigs were then utilized for the in silico isolation and development of microsatellite markers. In total, we screened 36 candidate microsatellite loci, of which 23 amplified consistently (five polymorphic and 18 monomorphic). We then examined whether the polymorphic loci could be used to infer whether populations of Z. davisae from Cape Cod and Nantucket are sexual or asexual by calculating several metrics of genetic diversity that might indicate the mode of reproduction. These included testing for statistical deviations from Hardy–Weinberg equilibrium (HWE) and for linkage disequilibrium (LD), observations for the presence of the Meselson effect, and by calculating the probability that clonal individuals are more prevalent than would be expected in a randomly mating population. While we found significant deviations from HWE and more clonal individuals than expected, our estimates of the Meselson effect were inconclusive due to limited sampling, and we found no evidence of LD. Therefore, the sexual/asexual status of Z. davisae populations remains uncertain.