Microsurgical manipulation reveals pre-copulatory function of key genital sclerites

Abstract

The copulatory organs of male insects are generally complex, species-specific arrangements of hardened sclerotized plates bound together by flexible, less sclerotized cuticle. Their extensive morphological diversification is a recurrent pattern in the evolutionary radiation of animals, yet a clear consensus as to what selection pressures drive this divergence is still to emerge. In part, this stems from the fact that the function of individual sclerites that integrate to form the aedeagus are poorly understood. In insects, the male copulatory organ is often bounded by two lateral parameres tipped with setae. In a number of species, these have been observed to brush against the terminal abdominal sclerites of the female, suggesting a role in pre- and/or post-copulatory female choice. However, in the absence of experimental manipulation, their function remains elusive. Here, we used microsurgery to reduce paramere length and show that males with one or both paramere tips removed were less likely to achieve genital coupling than sham-operated male control groups. Where treatment males did achieve copulation, surgical removal of the paramere tip(s) had no detectable effect on copulation duration or on the outcome of sperm competition. Surgical manipulation of the end-plate, a genital sclerite that covers the ostium of the median lobe (the non-intromittent section of the aedeagus), resulted in near-complete failure of males to achieve copulation. Our experimental manipulations show that the parameres and end-plate function during pre-copulatory sexual interactions and thus most likely evolved in response to sexual selection occurring prior to insemination.

Individual and synergistic effects of male external genital traits in sexual selection

“…This possibility is consistent with the fact that surgical manipulation of either trait significantly prolonged copulation duration, indicating that the spines likely interact with the female sensory apparatus, which is a foundation stone of the cryptic female choice hypothesis for genital evolution (Eberhard, ). The idea is that the ablation resulted in impaired female stimulation, delaying both normal female response and the sensory feedback male would receive to trigger the dismount (and see Cocks & Eady, ; Eady & Brown, ). Copulation duration in Drosophila is arguably under male control (MacBean & Parsons, ; Jagadeeshan & Singh, ; Crickmore & Vosshall, , but see Mazzi, Kesäniemi, Hoikkala, & Klappert, ; Edward, Poissant, Wilson, & Chapman, ), as a function of sensory cues males receive in the form of behavioural and/or neuromuscular activity produced by the female at an appropriate time post‐insemination.…”