Host specialization and species diversity in the genusStylops(Strepsiptera: Stylopidae), revealed by molecular phylogenetic analysis

Stylopization by Xenos spp. (Xenidae, Strepsiptera) in invasive alien hornet, Vespa velutina, in South Korea

The invasive hornet Vespa velutina Lepeletier, which first invaded South Korea in 2003, has spread throughout the country, significantly affecting apiaries, ecosystems, and human health. Xenos spp. (Xenidae, Strepsiptera) are primarily parasitic to social wasps, with V. analis being the only known host in Korea. Until recently, no parasites or parasitoids on V. velutina had been discovered. In 2020, strepsipteran parasites were discovered on 11 hornet workers in Andong City, South Korea. These parasites, comprising four larvae and seven pupae, were all male, except for one individual of an undetermined sex. Molecular analysis and morphological examination identified the parasites as Xenos moutoni (du Buysson, 1903) and X. oxyodontes Nakase & Kato, 2013. This marks the first recorded instance of strepsipteran parasites on V. velutina in regions invaded by this hornet. Although the exact infection rate of these parasites could not be determined, it appears that native strepsipteran parasites have adapted to a non-native Vespa species. Stylopization, the condition caused by these parasites, is known to negatively affect hornet colonies: infected workers do not contribute to nest activities, hindering nest development, and infected reproductive individuals (males and new queens) do not mate, which impedes the establishment of new colonies. However, due to the hornet's high reproductive rate and compensatory mechanisms, the overall control effect of the parasites is likely to be minor.

First record of Halictoxenos Pierce (Strepsiptera: Stylopidae) from Mexico with the description of a new species parasite of Lasioglossum exiguum (Smith) (Hymenoptera: Halictidae)

Halictoxenos is a genus of parasites with a mainly holarctic distribution and exclusive parasite of bees of the Halictinae subfamily. In this work, we describe a new species from Mexico, parasite of the halictid Lasioglossum exiguum and with a known distribution in locations of central and southern Veracruz. The delimitation of this species is supported by morphological and molecular evidence. A diagnosis for Halictoxenos and certain characters for species delimitation is also proposed. It is expected that this research may be a steppingstone to the continuity of the study of Halictoxenos and the Stylopidae family in Mexico and the American continent.

Phylogenomic analysis of Stylops reveals the evolutionary history of a Holarctic Strepsiptera radiation parasitizing wild bees

Holarctic Stylops is the largest genus of the enigmatic insect order Strepsiptera, twisted winged parasites. Members of Stylops are obligate endoparasites of Andrena mining bees and exhibit extreme sexual dimorphism typical of Strepsiptera. So far, molecular studies on Stylops have focused on questions on species delimitation. Here, we utilize the power of whole genome sequencing to infer the phylogeny of this morphologically challenging genus from thousands of loci. We use a species tree method, concatenated maximum likelihood analysis and Bayesian analysis with a relaxed clock model to reconstruct the phylogeny of 46 Stylops species, estimate divergence times, evaluate topological consistency across methods and infer the root position. Furthermore, the biogeographical history and coevolutionary patterns with host species are assessed. All methods recovered a well resolved topology with close to all nodes maximally supported and only a handful of minor topological variations. Based on the result, we find that included species can be divided into 12 species groups, seven of them including only Palaearctic species, three Nearctic and two were geographically mixed. We find a strongly supported root position between a clade formed by the spreta, thwaitesi and gwynanae species groups and the remaining species and that the sister group of Stylops is Eurystylops or Eurystylops + Kinzelbachus. Our results indicate that Stylops originated in the Western Palaearctic or Western Palaearctic and Nearctic in the early Neogene or late Paleogene, with four independent dispersal events to the Nearctic. Cophylogenetic analyses indicate that the diversification of Stylops has been shaped by both significant coevolution with the mining bee hosts and host-shifting. The well resolved and strongly supported phylogeny will provide a valuable phylogenetic basis for further studies into the fascinating world of Strepsipterans.

Anatomical, phenological and genetic aspects of the host-parasite relationship between Andrena vaga (Hymenoptera) and Stylops ater (Strepsiptera)

Stylops ater is an endoparasite of the mining bee Andrena vaga with extreme sexual dimorphism and hypermetamorphosis. Its population structure, parasitization mode, genetic diversity and impact on host morphology were examined in nesting sites in Germany to better understand this highly specialized host–parasite interaction. The shift in host emergence due to stylopization was proven to be especially strong in A. vaga. Around 10% of bees hosted more than 1 Stylops, with at maximum 4. A trend in Stylops' preference for hosts of their own sex and a sex-specific position of extrusion from the host abdomen was found. Invasion of Andrena eggs by Stylops primary larvae was depicted for the first time. Cephalothoraces of female Stylops were smaller in male and pluristylopized hosts, likely due to lower nutrient supply. The genes H3, 18S and cytochrome c oxidase subunit 1 were highly conserved, revealing near-absence of local variation within Stylops. Ovaries of hosts with male Stylops contained poorly developed eggs while those of hosts with female Stylops were devoid of visible eggs, which might be due to a higher protein demand of female Stylops. Male Stylops, which might have a more energy-consuming development, led to a reduction in head width of their hosts. Host masculinization was present in the leaner shape of the metabasitarsus of stylopized females and is interpreted as a by-product of manipulation of the host's endocrine system to shift its emergence. Stylopization intensified tergal hairiness, most strongly in hosts with female Stylops, near the point of parasite extrusion, hinting towards substance-induced host manipulation.

The genome sequence of the common mini-mining bee Andrena minutula (Kirby, 1802)

We present a genome assembly from an individual female Andrena minutula (the common mini-mining bee; Arthropoda; Insecta; Hymenoptera; Andrenidae). The genome sequence is 380 megabases in span. Most of the assembly (92.19%) is scaffolded into seven chromosomal pseudomolecules. The mitochondrial genome has also been assembled and is 17.7 kilobases in length. Gene annotation of this assembly on Ensembl has identified 10,936 protein coding genes.

Have female twisted-wing parasites (Insecta: Strepsiptera) evolved tolerance traits as response to traumatic penetration?

Traumatic insemination describes an unusual form of mating during which a male penetrates the body wall of its female partner to inject sperm. Females unable to prevent traumatic insemination have been predicted to develop either traits of tolerance or of resistance, both reducing the fitness costs associated with the male-inflicted injury. The evolution of tolerance traits has previously been suggested for the bed bug. Here we present data suggesting that tolerance traits also evolved in females of the twisted-wing parasite species Stylops ovinae and Xenos vesparum. Using micro-indentation experiments and confocal laser scanning microscopy, we found that females of both investigated species possess a uniform resilin-rich integument that is notably thicker at penetration sites than at control sites. As the thickened cuticle does not seem to hamper penetration by males, we hypothesise that thickening of the cuticle resulted in reduced penetration damage and loss of haemolymph and in improved wound sealing. To evaluate the evolutionary relevance of the Stylops-specific paragenital organ and penis shape variation in the context of inter- and intraspecific competition, we conducted attraction and interspecific mating experiments, as well as a geometric-morphometric analysis of S. ovinae and X. vesparum penises. We found that S. ovinae females indeed attract sympatrically distributed congeneric males. However, only conspecific males were able to mate. In contrast, we did not observe any heterospecific male attraction by Xenos females. We therefore hypothesise that the paragenital organ in the genus Stylops represents a prezygotic mating barrier that prevents heterospecific matings.

Xenos yangi sp. nov.: A new twisted-wing parasite species (Strepsiptera, Xenidae) from Gaoligong Mountains, Southwest China

Here we report a new twisted-wing parasite species of the family Xenidae based on both morphological and molecular evidence. By using nearly complete mitogenomes, we confirmed the twisted-wing parasites on two wasps (Vespavelutina and Vespabicolor) (China: Yunnan) as the same species, and associated its neotenic females and alate males. Combining the mitogenomic data (COI) and morphological traits, this species was identified to be a new species of the genus Xenos, namely Xenosyangi Dong, Liu & Li, sp. nov. Detailed descriptions and illustrations are provided for the new species.

Molecular identification of Stylops advarians (Strepsiptera: Stylopidae) in western Canada

Strepsiptera are an enigmatic order of insects with extreme sexual dimorphism which makes it difficult to "match-up" free-living adult males with parasitic conspecific females of the Stylopidia, and free-living females of the Mengenillidae using morphological characters. Species identification is further complicated for the Stylopidia because adult females are endoparasitic and neotenic. Therefore, we used DNA sequencing of the mitochondrial cytochrome c oxidase subunit 1 gene (cox1) to confirm the species identity of adult strepsipterans that were morphologically identified as Stylops advarians. These specimens, collected from Saskatoon (Saskatchewan, Canada), included one adult male, and eight females, the latter of which had been collected from solitary bees (Andrena milwaukeensis). Also included in the analyses were three pools of first-instar larvae that had emerged from three of the females. The results of the molecular analyses revealed that all specimens had an identical cox1 sequence, and belonged to a clade, with total statistical support (bootstrap value of 100%), that contained specimens of S. advarians from New York and Maine (USA). Hence, the results were consistent with the morphological identification of S. advarians. This study demonstrates the usefulness of a molecular approach for the identification of endoparasitic adult female and larval strepsipterans, life cycle stages that lack significant morphological characters for species identification.

Frozen Antarctic path for dispersal initiated parallel host-parasite evolution on different continents

After the break-up of Gondwana dispersal of organisms between America, Australia and Africa became more complicated. One of the possible remaining paths led through Antarctica, that was not yet glaciated and it remained habitable for many organisms. This favourable climate made Antarctica an important migration corridor for organisms with good dispersal ability, such as Aculeata (Hymenoptera), till the Oligocene cooling. Here we tested how cooling of Antarctica impacted global dispersal of Aculeata parasites (Strepsiptera: Xenidae). Our data set comprising six nuclear genes from a broad sample of Xenidae. Bayesian dating was used to estimate divergence times in phylogenetic reconstruction. Biogeography was investigated using event-based analytical methods: likelihood-based dispersal-extinction-cladogenesis and Bayesian models. The Bayesian model was used for reconstruction of ancestral host groups. Biogeographical methods indicate that multiple lineages were exchanged between the New World and the Old World + Australia until the Antarctica became completely frozen over. During the late Paleogene and Neogene periods, several lineages spread from the Afrotropics to other Old World regions and Australia. The original hosts of Xenidae were most likely social wasps. Within one lineage of solitary wasp parasites, parallel switch to digger wasps (Sphecidae) occurred independently in the New World and Old World regions. The biogeography and macroevolutionary history of Xenidae can be explained by the combination of dispersal, lineage extinction and climatic changes during the Cenozoic era. A habitable Antarctica and the presence of now-submerged islands and plateaus that acted as a connection between the New World and Old World + Australia provided the possibility for biotic exchanges of parasites along with their hymenopteran hosts. Although Xenidae are generally host specialists, there were significant host switches to unrelated but ecologically similar hosts during their evolution. There is little or no evidence for cophylogeny between strepsipteran parasites and hymenopteran lineages.

Strepsiptera of Canada

In Canada, the order Strepsiptera consists of 27 known species representing five families: Corioxenidae (1 species), Elenchidae (1 species), Halictophagidae (5 species), Stylopidae (15 species), and Xenidae (5 species). These totals represent an increase of 21 species since the 1979 assessment. Half of these species represent unpublished records recently discovered by study of stylopized hosts in museum collections and DNA barcoded species. It is estimated that as many as 19 more species will eventually be discovered in Canada. DNA barcode sequences are available for 4 Canadian species. The fauna of Canada is poorly surveyed and there is a need to fill knowledge gaps with increased examination of museum specimens for stylopized hosts, broader field surveys (including use of pheromone-baited traps), and more effort to obtain DNA samples.

Host species exploitation and discrimination by animal parasites

Parasite species often show differential fitness on different host species. We developed an equation-based model to explore conditions favouring host species exploitation and discrimination. In our model, diploid infective stages randomly encountered hosts of two species; the parasite's relative fitness in exploiting each host species, and its ability to discriminate between them, was determined by the parasite's genotype at two independent diallelic loci. Relative host species frequency determined allele frequencies at the exploitation locus, whereas differential fitness and combined host density determined frequency of discrimination alleles. The model predicts instances where populations contain mixes of discriminatory and non-discriminatory infective stages. Also, non-discriminatory parasites should evolve when differential fitness is low to moderate and when combined host densities are low, but not so low as to cause parasite extinction. A corollary is that parasite discrimination (and host-specificity) increases with higher combined host densities. Instances in nature where parasites fail to discriminate when differential fitness is extreme could be explained by one host species evolving resistance, following from earlier selection for parasite non-discrimination. Similar results overall were obtained for haploid extensions of the model. Our model emulates multi-host associations and has implications for understanding broadening of host species ranges by parasites.This article is part of the themed issue 'Opening the black box: re-examining the ecology and evolution of parasite transmission'.

Rediscovered parasitism of Andrena savignyi Spinola (Hymenoptera, Andrenidae) by Stylops (Strepsiptera, Stylopidae) and revised taxonomic status of the parasite

Parasitism of Andrena (Suandrena) savignyi Spinola (Hymenoptera: Andrenidae) by Stylops Kirby (Strepsiptera: Stylopidae) has been recorded only once, and from an individual collected in Egypt almost a century ago, with the parasite described as Stylops savignyi Hofeneder. The recent rediscovery of this Stylops from an individual of Andrena savignyi permits a reinterpretation of the species and its affinities among other Stylops. The bee was collected at flowers of Zilla spinosa (Turra) Prantl. (Brassicaceae) in Amariah, Riyadh, Kingdom of Saudi Arabia. Based on DNA barcode sequences from material sampled across Africa, Asia, and Europe, it is apparent that Stylops savignyi is conspecific with Stylops nassonowi Pierce, and we accordingly synonymize this name (syn. n.), with the latter representing the senior and valid name for the species. A differential diagnosis is provided for Stylops nassonowi and the morphology of the female is described, as well as the first instars.