Experimental reduction of intromittent organ length reduces male reproductive success in a bug

Abstract

It is now clear in many species that male and female genital evolution has been shaped by sexual selection. However, it has historically been difficult to confirm correlations between morphology and fitness, as genital traits are complex and manipulation tends to impair function significantly. In this study, we investigate the functional morphology of the elongate male intromittent organ (or processus) of the seed bug Lygaeus simulans, in two ways. We first use micro-computed tomography (micro-CT) and flash-freezing to reconstruct in high resolution the interaction between the male intromittent organ and the female internal reproductive anatomy during mating. We successfully trace the path of the male processus inside the female reproductive tract. We then confirm that male processus length influences sperm transfer by experimental ablation and show that males with shortened processi have significantly reduced post-copulatory reproductive success. Importantly, male insemination function is not affected by this manipulation per se. We thus present rare, direct experimental evidence that an internal genital trait functions to increase reproductive success and show that, with appropriate staining, micro-CT is an excellent tool for investigating the functional morphology of insect genitalia during copulation.

Natural breakage of the very long intromittent organ of the seed bug Lygaeus simulans (Heteroptera: Lygaeidae)

“…The processus gonopori ends in a characteristic cuplike structure which can be seen easily (see Fig. 2 in Dougherty et al, 2015). A male was said to exhibit a genital breakage if this tip was missing ( Fig.…”

“…This structure is tightly coiled inside the male genital capsule prior to mating (Ludwig, 1926). During mating the entire intromittent organ is everted into the female bursa copulatrix, and the processus gonopori is then threaded along the female spermathecal duct (Gschwentner & Tadler, 2000;Dougherty et al, 2015). The lumen of the spermathecal duct is narrower than the width of the processus gonopori (Gschwentner & Tadler, 2000), so that any broken fragments of processus gonopori in the spermathecal duct from a previous mating will likely prevent subsequent males from inseminating the female.…”

“…In the seed bug Lygaeus simulans Deckert (Heteroptera: Lygaeoidea: Lygaeidae), the male intromittent organ ends in a coiled, sclerotized tube known as the processus gono pori (Ludwig 1926;Deckert, 1990), which is approximately 6.8 mm long (which is around 60% of the average male body length: Tadler, 1999; Dougherty et al, 2015). This structure is tightly coiled inside the male genital capsule prior to mating (Ludwig, 1926).…”

“…The slides were then imaged using an Olympus SZX10 stereo microscope and attached camera, and measured using the image analysis program Cell^D (Soft Imaging System, Olympus Corp). The processus gonopori was measured from the "turning point" (where the fleshy base of the intromittent organ ends) to the tip (Tadler, 1999;Dougherty et al, 2015). The processus gonopori ends in a characteristic cuplike structure which can be seen easily (see Fig.…”

“…This will be especially important if breakage significantly impairs a male's insemination ability, either because the intromittent organ is damaged so as to prevent sperm release, or because the length of the organ is reduced so that it is unable to release sperm in the correct place (e.g. Dougherty et al, 2015). Alternatively, such breakages could be adaptive if they increase a male's reproductive success.…”