Evolutionary aspects of the parasitoid life strategy, with a particular emphasis on fly–spider interactions

Parasitoids are significant ecological elements of terrestrial food webs and have evolved within seven insect orders. Interestingly, however, associations with spiders as hosts have evolved only in two insect orders, Diptera and Hymenoptera. Here, we summarize various aspects of host utilization by dipteran flies with an emphasis on associations with spiders. Our synthesis reveals that spider flies (family Acroceridae) have evolved a unique life strategy among all the parasitoid taxa associated with spiders, in which koinobiont small-headed flies utilize an indirect oviposition strategy. This indirect oviposition in spider flies is inherited from Nemestrinimorpha ancestors which appeared in the Late Triassic and is characterized by the evolution of planidial larvae. Further, we discuss the advantages and disadvantages of indirect oviposition in spider flies. On the one hand, indirect oviposition allows the fly to avoid contact/wrestling with spider hosts. On the other hand, larval survival is low because the planidium must actively seek out and infect a suitable host individually. The risk of failure to find a suitable spider host is offset by the fly’s extremely high fecundity.

Anchored phylogenomics unravels the evolution of spider flies (Diptera, Acroceridae) and reveals discordance between nucleotides and amino acids

The onset of phylogenomics has contributed to the resolution of numerous challenging evolutionary questions while offering new perspectives regarding biodiversity. However, in some instances, analyses of large genomic datasets can also result in conflicting estimates of phylogeny. Here, we present the first phylogenomic scale study of a dipteran parasitoid family, built upon anchored hybrid enrichment and transcriptomic data of 240 loci of 43 ingroup acrocerid taxa. A new hypothesis for the timing of spider fly evolution is proposed, wielding recent advances in divergence time dating, including the fossilized birth-death process to show that the origin of Acroceridae is younger than previously proposed. To test the robustness of our phylogenetic inferences, we analyzed our datasets using different phylogenetic estimation criteria, including supermatrix and coalescent-based approaches, maximum-likelihood and Bayesian methods, combined with other approaches such as permutations of the data, homogeneous versus heterogeneous models, and alternative data and taxon sets. Resulting topologies based on amino acids and nucleotides are both strongly supported but critically discordant, primarily in terms of the monophyly of Panopinae. Conflict was not resolved by controlling for compositional heterogeneity and saturation in third codon positions, which highlights the need for a better understanding of how different biases affect different data sources. In our study, results based on nucleotides were both more robust to alterations of the data and different analytical methods and more compatible with our current understanding of acrocerid morphology and patterns of host usage.

Jewelled spider flies of North America: a revision and phylogeny of Eulonchus Gerstaecker (Diptera, Acroceridae)

The spider fly genus Eulonchus Gerstaecker is found throughout the Nearctic Region. Six species are recognized and intraspecific morphological variation is documented in several species. A phylogeny of Eulonchus based on DNA sequence data of three molecular markers (COI, CAD, and 16S) is presented and relationships of species are discussed in the light of biogeography and host usage. All six species of Eulonchus are redescribed using natural language descriptions exported from a character matrix, and a key to species is presented. Lectotypes are designated for Eulonchus sapphirinus Osten Sacken, Eulonchus smaragdinus Gerstaecker, and Eulonchus tristis Loew.