Phenotypic Engineering Unveils the Function of Genital Morphology

Abstract

The rapidly evolving and often extraordinarily complex appearance of male genital morphology of internally fertilizing animals has been recognized for centuries. Postcopulatory sexual selection is regarded as the likely evolutionary engine of this diversity, but direct support for this hypothesis is limited. We used two complementary approaches, evolution through artificial selection and microscale laser surgery, to experimentally manipulate genital morphology in an insect model system. We then assessed the competitive fertilization success of these phenotypically manipulated males and studied the fate of their ejaculate in females using high-resolution radioisotopic labeling of ejaculates. Males with longer genital spines were more successful in gaining fertilizations, providing experimental evidence that male genital morphology influences success in postcopulatory reproductive competition. Furthermore, a larger proportion of the ejaculate moved from the reproductive tract into the female body following mating with males with longer spines, suggesting that genital spines increase the rate at which seminal fluid passes into the female hemolymph. Our results show that genital morphology affects male competitive fertilization success and imply that sexual selection on genital morphology may be mediated in part through seminal fluid.

Sexual conflict and the function of genitalic claws in guppies (Poecilia reticulata)

“…Our study joins a very few 'phenotypic engineering' studies of genitalia [6,7,[10][11][12][13][14][15]. These studies have alternately supported a pre-and/or post-copulatory function of the manipulated structures.…”

“…Although most of the evidence we have for the role of sexual selection in genital evolution tends to be correlative [3], a few recent studies provide more direct evidence from the experimental evolution of genitalia [8][9][10] or phenotypic manipulations of genital features [6,7,[10][11][12][13][14][15]. These latter 'phenotypic engineering' studies are particularly informative, because they focus directly on function and offer the promise of finely distinguishing among alternative mechanisms of selection, unimpeded by unmeasured correlated traits.…”

Limnephilid taxa revised by speciation traits: Rhadicoleptus, Isogamus, Melampophylax genera, Chaetopteryx rugulosa, Psilopteryx psorosa species groups, Drusus bolivari, Annitella kosciuszkii species complexes (Trichoptera: Limnephilidae)

“…Recently the competitive fertilization success of male's ejaculate, whose aedeagal spine length was manipulated by micro-scale laser surgery, was assessed by using high-resolution radioisotopic labelling. Male with longer species-specific sclerotized genital spines on aedeagus were more successful in gaining fertilization (Hotzy et al 2012). The importance of male driven copulatory mechanics and strategies has been revealed in the rapid diversification of genitalia of Drosophila flies (Jagadeeshan & Singh 2006).…”

Mechanisms and Evidence of Genital Coevolution: The Roles of Natural Selection, Mate Choice, and Sexual Conflict

“…Creating males and females with different genital genotypes using experimental evolution can reveal coevolutionary processes underlying genital traits. A recent experiment in seed beetles Callosobruchus maculatus generated phenotypic variation via both artificial selection and experimental ablation of spines and elegantly showed that longer spines, are beneficial to male fitness (Hotzy et al 2012). Similarly, a recent study in Drosophila generated variation in male genitalia by creating introgressed lines between D. mauritiana and D. sechelia.…”