We do not select, nor are we choosy: reproductive biology of Strepsiptera (Insecta)

Polyandry and sperm competition in two traumatically inseminating species of Strepsiptera (Insecta)

Polyandry, the practice of females mating with multiple males, is a strategy found in many insect groups. Whether it increases the likelihood of receiving beneficial genes from male partners and other potential benefits for females is controversial. Strepsiptera are generally considered monandrous, but in a few species females have been observed copulating serially with multiple males. Here we show that the offspring of a single female can have multiple fathers in two Strepsiptera species: Stylops ovinae (Stylopidae) and Xenos vesparum (Xenidae). We studied female polyandry in natural populations of these two species by analysis of polymorphic microsatellite loci. Our results showed that several fathers can be involved in both species, in some cases up to four. Mating experiments with S. ovinae have shown that the first male to mates with a given female contributes to a higher percentage of the offspring than subsequent males. In X. vesparum, however, we found no significant correlation between mating duration and offspring contribution. The prolonged copulation observed in S. ovinae may have the advantage of reducing competition with sperm from other males. Our results show that monandry may not be the general pattern of reproduction in the insect order Strepsiptera.

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.

A generic classification of Xenidae (Strepsiptera) based on the morphology of the female cephalothorax and male cephalotheca with a preliminary checklist of species

The generic taxonomy and host specialization of Xenidae have been understood differently by previous authors. Although the recent generic classification has implied a specialization on the level of host families or subfamilies, the hypothesis that each xenid genus is specialized to a single host genus was also previously postulated. A critical evaluation of the classification of the genera of Xenidae is provided here based on morphology in accordance with results of recent molecular phylogenetic studies. External features of the female cephalothoraces and male cephalothecae were documented in detail with different techniques. Diagnoses and descriptions are presented for all 13 delimited genera. The earliest diverging genera are usually well characterized by unique features, whereas deeply nested genera are usually characterized by combinations of characters. Three new genera are described: Sphecixenosgen. nov., Tuberoxenosgen. nov., and Deltoxenosgen. nov. Five previously described genera are removed from synonymy: Tachytixenos Pierce, 1911, stat. res.; Brasixenos Kogan & Oliveira, 1966, stat. res.; Leionotoxenos Pierce, 1909, stat. res.; Eupathocera Pierce, 1908, stat. res.; and Macroxenos Schultze, 1925, stat. res. One former subgenus is elevated to generic rank: Nipponoxenos Kifune & Maeta, 1975, stat. res.Monobiaphila Pierce, 1909, syn. nov. and Montezumiaphila Brèthes, 1923, syn. nov. are recognized as junior synonyms of Leionotoxenos Pierce, 1909, stat. res.Ophthalmochlus Pierce, 1908, syn. nov., Homilops Pierce, 1908, syn. nov., Sceliphronechthrus Pierce, 1909, syn. nov., and Ophthalmochlus (Isodontiphila) Pierce, 1919, syn. nov. are recognized as junior synonyms of Eupathocera Pierce, 1908, stat. res. A preliminary checklist of 119 described species of Xenidae with information on their hosts and distribution is provided. The following 14 species are recognized as valid and restituted from synonymy: Tachytixenos indicus Pierce, 1911, stat. res.; Brasixenos acinctus Kogan & Oliveira, 1966, stat. res.; Brasixenos araujoi (Oliveira & Kogan, 1962), stat. res.; Brasixenos bahiensis Kogan & Oliveira, 1966, stat. res.; Brasixenos brasiliensis Kogan & Oliveira, 1966, stat. res.; Brasixenos fluminensis Kogan & Oliveria, 1966, stat. res.; Brasixenos myrapetrus Trois, 1988, stat. res.; Brasixenos zikani Kogan & Oliveira, 1966, stat. res.; Leionotoxenos hookeri Pierce, 1909, stat. res.; Leionotoxenos jonesi Pierce, 1909, stat. res.; Leionotoxenos louisianae Pierce, 1909, stat. res.; Eupathocera luctuosae Pierce, 1911, stat. res.; Eupathocera lugubris Pierce, 1909, stat. res.; Macroxenos piercei Schultze, 1925, stat. res. New generic combinations are proposed for 51 species: Leionotoxenos arvensidis (Pierce, 1911), comb. nov.; Leionotoxenos bishoppi (Pierce, 1909), comb. nov.; Leionotoxenos foraminati (Pierce, 1911), comb. nov.; Leionotoxenos fundati (Pierce, 1911), comb. nov.; Leionotoxenos huastecae (Székessy, 1965), comb. nov.; Leionotoxenos itatiaiae (Trois, 1984), comb. nov.; Leionotoxenos neomexicanus (Pierce, 1919), comb. nov.; Leionotoxenos prolificum (Teson & Remes Lenicov, 1979), comb. nov.; Leionotoxenos robertsoni (Pierce, 1911), comb. nov.; Leionotoxenos tigridis (Pierce, 1911), comb. nov.; Leionotoxenos vigili (Brèthes, 1923), comb. nov.; Eupathocera argentina (Brèthes, 1923), comb. nov.; Eupathocera auripedis (Pierce, 1911), comb. nov.; Eupathocera bucki (Trois, 1984), comb. nov.; Eupathocera duryi (Pierce, 1909), comb. nov.; Eupathocera erynnidis (Pierce, 1911), comb. nov.; Eupathocera fasciati (Pierce, 1909), comb. nov.; Eupathocera fuliginosi (Brèthes, 1923), comb. nov.; Eupathocera inclusa (Oliveira & Kogan, 1963), comb. nov.; Eupathocera insularis (Kifune, 1983), comb. nov.; Eupathocera mendozae (Brèthes, 1923), comb. nov.; Eupathocera piercei (Brèthes, 1923), comb. nov.; Eupathocera striati (Brèthes, 1923), comb. nov.; Eupathocera taschenbergi (Brèthes, 1923), comb. nov.; Eupathocera westwoodii (Templeton, 1841), comb. nov.; Macroxenos papuanus (Székessy, 1956), comb. nov.; Sphecixenos abbotti (Pierce, 1909), comb. nov.; Sphecixenos astrolabensis (Székessy, 1956), comb. nov.; Sphecixenos dorae (Luna de Carvalho, 1956), comb. nov.; Sphecixenos erimae (Székessy, 1956), comb. nov.; Sphecixenos esakii (Hirashima & Kifune, 1962), comb. nov.; Sphecixenos gigas (Pasteels, 1950), comb. nov.; Sphecixenos kurosawai (Kifune, 1984), comb. nov.; Sphecixenos laetum (Ogloblin, 1926), comb. nov.; Sphecixenos orientalis (Kifune, 1985), comb. nov.; Sphecixenos reticulatus (Luna de Carvalho, 1972), comb. nov.; Sphecixenos simplex (Székessy, 1956), comb. nov.; Sphecixenos vanderiisti (Pasteels, 1952), comb. nov.; Tuberoxenos altozambeziensis (Luna de Carvalho, 1959), comb. nov.; Tuberoxenos sinuatus (Pasteels, 1956), comb. nov.; Tuberoxenos sphecidarum (Siebold, 1839), comb. nov.; Tuberoxenos teres (Pasteels, 1950), comb. nov.; Tuberoxenos tibetanus (Yang, 1981), comb. nov.; Deltoxenos bequaerti (Luna de Carvalho, 1956), comb. nov.; Deltoxenos bidentatus (Pasteels, 1950), comb. nov.; Deltoxenos hirokoae (Kifune & Yamane, 1992), comb. nov.; Deltoxenos iwatai (Esaki, 1931), comb. nov.; Deltoxenos lusitanicus (Luna de Carvalho, 1960), comb. nov.; Deltoxenos minor (Kifune & Maeta, 1978), comb. nov.; Deltoxenos rueppelli (Kinzelbach, 1971a), comb. nov.; Xenos ropalidiae (Kinzelbach, 1975), comb. nov.Xenos minor Kinzelbach, 1971a, syn. nov. is recognized as a junior synonym of X. vesparum Rossi, 1793. Ophthalmochlus duryi Pierce, 1908, nomen nudum and Eupathocera lugubris Pierce, 1908, nomen nudum are recognized as nomina nuda and therefore unavailable in zoological nomenclature. The species diversity of Xenidae probably remains poorly known: the expected number of species is at least twice as high as the number presently described.

Frequent origins of traumatic insemination involve convergent shifts in sperm and genital morphology

Traumatic insemination is a mating behavior during which the (sperm) donor uses a traumatic intromittent organ to inject an ejaculate through the epidermis of the (sperm) recipient, thereby frequently circumventing the female genitalia. Traumatic insemination occurs widely across animals, but the frequency of its evolution, the intermediate stages via which it originates, and the morphological changes that such shifts involve remain poorly understood. Based on observations in 145 species of the free-living flatworm genus Macrostomum, we identify at least nine independent evolutionary origins of traumatic insemination from reciprocal copulation, but no clear indication of reversals. These origins involve convergent shifts in multivariate morphospace of male and female reproductive traits, suggesting that traumatic insemination has a canalizing effect on morphology. We also observed sperm in both the sperm receiving organ and within the body tissue of two species. These species had intermediate trait values indicating that traumatic insemination evolves through initial internal wounding during copulation. Finally, signatures of male-female coevolution of genitalia across the genus indicate that sexual selection and sexual conflict drive the evolution of traumatic insemination, because it allows donors to bypass postcopulatory control mechanisms of recipients.

The diversity and evolution of odorant receptors in beetles (Coleoptera)

The insect odorant receptors (ORs) are amongst the largest gene families in insect genomes and the primary means by which insects recognize volatile compounds. The evolution of ORs is thus instrumental in explaining the chemical ecology of insects and as a model of evolutionary biology. However, although ORs have been described from numerous insect species, their analysis within and amongst the insect orders has been hindered by a combination of limited genomic information and a tendency of the OR family toward rapid divergence, gain, and loss. We addressed these issues in the insect order Coleoptera through a targeted genomic annotation effort that included 1181 ORs from one species of the sister order Strepsiptera and 10 species representing the four coleopteran suborders. The numbers of ORs in each species varied from hundreds to fewer than 10, but coleopteran ORs could nevertheless be represented within a scheme of nine monophyletic subfamilies. We observed many radiations and losses of genes amongst OR subfamilies, and the diversity of ORs appeared to parallel the host breadth of the study species. However, some small lineages of ORs persisted amongst many coleopteran families, suggesting receptors of key function that underlie the olfactory ecology of beetles.

Differential expression of the adult specifier E93 in the strepsipteran Xenos vesparum Rossi suggests a role in female neoteny

Holometaboly is a key evolutionary innovation that has facilitated the spectacular radiation of insects. Despite the undeniable advantage of complete metamorphosis, the female of some holometabolous species have lost the typical holometabolous development through neoteny. In Xenos vesparum Rossi (Strepsiptera: Stylopidae), a derived species of the holometabolous endoparasitic order Strepsiptera, neotenic females reach sexual maturity without the pupal and the imaginal stages, thus retaining their larval morphology (with the exception of the anterior part of the body or cephalothorax), while males undergo normal pupal-based metamorphosis. Expression of the “adult-specifier” E93 factor has been shown to be required for proper metamorphosis in holometabolous insects. Here, we investigated the involvement of E93 in female neoteny by cloning XvE93. Interestingly, while we detected a clear up-regulation of XvE93 expression in pupal and adult stages of males, persistent low levels of XvE93 were detected in X. vesparum females. However, a specific up-regulation of XvE93 was observed in the cephalothorax of late 4^th female instar larva, which correlates with the occurrence of neotenic-specific features in the anterior part of the female body. Moreover, the same expression dynamic in the cephalothorax and abdomen was also observed for other two critical metamorphic regulators, the anti-metamorphic XvKr-h1 and the pupal specifier XvBr-C. The specific up-regulation of XvE93 and XvBr-C in the female cephalothorax seems to be the result of an increase in 20-hydroxyecdysone (20E) signaling in this region for we detected higher expression levels of the 20E-dependent nuclear receptors XvHR3 and XvE75 in the cephalothorax. Overall, our results detect a sex-specific expression pattern of critical metamorphic genes in X. vesparum, suggesting that neoteny in Strepsiptera results from the modification of the normal expression of E93, Br-C and Kr-h1 genes.

Traumatic insemination and female counter-adaptation in Strepsiptera (Insecta)

In a few insect groups, males pierce the female's integument with their penis during copulation to transfer sperm. This so-called traumatic insemination was previously confirmed for Strepsiptera but only in species with free-living females. The more derived endoparasitic groups (Stylopidia) were suggested to exhibit brood canal mating. Further, it was assumed that females mate once and that pheromone production ceases immediately thereafter. Here we examined Stylops ovinae to provide details of the mating behaviour within Stylopidia. By using μCT imaging of Stylops in copula, we observed traumatic insemination and not, as previously suggested, brood canal mating. The penis is inserted in an invagination of the female cephalothorax and perforates its cuticle. Further we show that female Stylops are polyandrous and that males detect the mating status of the females. Compared to other strepsipterans the copulation is distinctly prolonged. This may reduce the competition between sperm of the first mating male with sperm from others. We describe a novel paragenital organ of Stylops females, the cephalothoracic invagination, which we suggest to reduce the cost of injuries. In contrast to previous interpretations we postulate that the original mode of traumatic insemination was maintained after the transition from free-living to endoparasitic strepsipteran females.