Are sex ratio distorting endosymbionts responsible for mating system variation among dance flies (Diptera: Empidinae)?

Abstract

Maternally inherited bacterial endosymbionts are common in many arthropod species. Some endosymbionts cause female-biased sex ratio distortion in their hosts that can result in profound changes to a host’s mating behaviour and reproductive biology. Dance flies (Diptera: Empidinae) are well known for their unusual reproductive biology, including species with female-specific ornamentation and female-biased lek-like swarming behaviour. The cause of the repeated evolution of female ornaments in these flies remains unknown, but is probably associated with female-biased sex ratios in individual species. In this study we assessed whether dance flies harbour sex ratio distorting endosymbionts that might have driven these mating system evolutionary changes. We measured the incidence and prevalence of infection by three endosymbionts that are known to cause female-biased sex ratios in other insect hosts (Wolbachia, Rickettsia and Spiroplasma) across 20 species of dance flies. We found evidence of widespread infection by all three symbionts and variation in sex-specific prevalence across the taxa sampled. However, there was no relationship between infection prevalence and adult sex ratio measures and no evidence that female ornaments are associated with high prevalences of sex-biased symbiont infections. We conclude that the current distribution of endosymbiont infections is unlikely to explain the diversity in mating systems among dance fly species.

The role of functional constraints in nonrandom mating patterns for a dance fly with female ornaments

“…Second, swarms of Hilara sp. are male‐biased (Marden, ), whereas those of R. longicauda are consistently female‐biased (Bussière et al., ; Funk & Tallamy, ; Gwynne, ; Murray et al., ). Given the variation in ornamentation and contrasting operational sex ratios, sexual selection and mate choice in the two species are expected to be quite different (Clutton‐Brock, ; Emlen & Oring, ; Hare & Simmons, ; Herridge et al., ; Kokko, Klug, & Jennions, ).…”

“…We collected R. longicauda samples on the banks of the Credit River in June 2012 near Glen Williams (Halton Co., Ontario, Canada: 43°41′117″N, 79°55′34″W). This site has been used for several previous studies on R. longicauda (Bussière et al., ; Gwynne & Bussière, ; Gwynne, Bussière, & Ivy, ; Murray, Herridge, Ness, & Bussiere, ; Murray et al., ; Wheeler et al., ). At each swarming event, we collected mated pairs from nuptial flight using an entomological sweep net.…”

Competition for access to mates predicts female‐specific ornamentation and male investment in relative testis size

“…However, female-specific ornament evolution remains uncommon, even among taxa in which females experience relatively strong sexual selection (Amundsen 2000). In the rare cases when female-specific ornaments do evolve (Funk and Tallamy 2000;Charlat et al 2007;Tobias et al 2012;Liker et al 2013;Murray et al 2017), they are hypothesized to improve attractiveness to mates or as signals in intrasexual competition, just as they do in males (Amundsen 2000). However, there are several theoretical reasons to expect that selection on females need not necessarily directly mirror the situation for males with a male-biased OSR (Gwynne and Simmons 1990;Forsgren et al 2004;Silva et al 2010).…”

Sexual selection on multiple female ornaments in dance flies

“…Females of the long-tailed dance fly, R. longicauda, possess two extravagant ornaments: pinnate scales over the length of all femora and tibia, and abdominal pleural sacs that are inflated just prior to swarming. Relatively few empidine dance fly species have abdominal ornaments, whereas pinnate leg scales are reasonably common throughout the group [23,24,27]. Both ornaments appear to exaggerate a female's apparent size and to improve female attractiveness [8] in the highly competitive context of R. longicauda mating swarms (which are usually heavily female-biased [8,22,28]).…”