Synergus Diversity

Ward, Anna K. G., Sheikh, Sofia I. & Forbes, Andrew A., 2020, Diversity, Host Ranges, and Potential Drivers of Speciation the Inquiline Enemies of Oak Gall Wasps (Hymenoptera: Cynipidae), Insect Systematics and Diversity 4 (6), No. 3, pp. 1-13 : 7

Nearctic Synergus Diversity andTaxonomy

One immediate lesson from this study is that the Nearctic Synergus harbor both cryptic and undiscovered species. For 5 of 11 previously named species in our collections, we find evidence of additional genetic, morphological, phenological ( Fig. 4 ), and/or ecological structure indicative of>1 species. Three other putative species apparently fit no previous description. Because this was a geographically limited survey, it thus seems very likely that many more undiscovered species exist in North America, both as members of morphologically cryptic assemblages, and as undescribed species. Indeed, when recent authors have focused on the Nearctic, they have consistently added new species (e.g., Lobato-Vila and Pujade-Villar 2017; Lobato-Villa et al. 2018, 2019). The incompleteness of the record is perhaps unsurprising as most taxonomic and phylogenetic work has emphasized the Synergus of the western Palearctic ( Penzes et al. 2012) while the most comprehensive list of Nearctic Synergus species is now>40 yr old ( Burks 1979).

A taxonomic revision of the Nearctic Synergus that incorporates molecular, morphological, and ecological data appears warranted, and, though such a revision is beyond the scope of this paper, it might be informed by some of our findings. Morphological characters historically used to sort Synergus into taxonomically relevant groups do not necessarily reflect characters shared due to common

evolutionary histories. The Palearctic Synergus were initially divided into two major morphological groups ( Mayr 1872): Section I, which are univoltine and usually not lethal to their associated galler, and Section II, which tend to be both bivoltine and lethal ( Wiebes-Rijks 1979). The two sections also differed in patterns of microscopic punctures on their metasomal tergites. These sections were more recently revealed to be paraphyletic ( Ács et al. 2010). One of the potential reasons for the disconnect between morphological and molecular data is that Synergus can be morphologically cryptic as well as exhibit variance in morphology within species ( Wiebes-Rijks 1979).

One morphological character previously suggested as useful for organizing the Nearctic fauna seems immediately relevant given these new results. Gillette (1896) split the genus into three ‘natural’ groups based on whether females of each species have 13, 14, or 15 segments in their flagella. Among our collections, we found only 14- and 15-segmented females (with S. lignicola being indeterminately 14-segmented), but all 15-segmented females (clades 12–15) grouped together on the mtCOI tree ( Fig. 2 ; Supp Figs. 47 and 52 [online only]), suggesting a common ancestry among these samples. While we acknowledge caveats about inferring too much from a single short mitochondrial sequence ( Funk and Omland 2003; and more on this below), this may be an early indication that antennal segment number is an informative character. To assist future efforts in this regard, we include supplemental profile and forewing pictures of wasps from each clade (Supp Figs. S1–S 46 [online only]) and have deposited examples of most clades into the collection of the University of Iowa Museum of Natural History.

Figures

Fig. 4. Collection and emergence dates for each of the 27 clades (putative species) of Synergus in this study. Dots indicate individual Synergus emergences; left-most margins of boxes demarcate the earliest gall collection that produced Synergus in each clade. In three cases, boxes with different border styles are used to indicate collection events from galls that occur at different times during the year, and which may indicate the use of temporally distributed gall hosts. Though galls were sometimes collected in different years, each collection was standardized to the year in which the gall first formed. Dates span>1 yr because some insects did not emerge from galls until more than a year after galls were collected.

Fig. 2. Overview of all data used in inferring Synergus species hypotheses for this study.Left:simplified mtCOI phylogeny of Synergus included in this study (see Supp Fig. 47 [online only] for full tree). Bold branches indicate support ≥0.9. ‘Clade’ describes putative species assignments based on the sum of information to the right of this column. Gray bars in ABGD (conservative [‘C’] and liberal [‘L’] partitions) and bPTP columns indicate assignments of individuals into groups by these respective algorithms. ‘Morphological ID’ refers to each collection’s similarity (or lack of similarity) to previously described species. ‘Gall host’, ‘oak section’,‘plant tissue(s)’, and ‘Host gall morphology’ refer to ecological characters for Synergus in each clade, and example photos of galls are shown in Fig. 6.

Fig. 1. Map of gall collections that yielded Synergus samples used in this study. For a list of samples, locations, and tree and gall associations, see SuppTable 2 (online only).