Sperm competition and female remating rate in the scorpionfly <i>Panorpa germanica</i>, L. (Mecoptera, Panorpidae)

Abstract

Sperm competition can be a powerful selective force in the evolution of reproductive strategies and mating systems. In the last decades many insect species have been studied in terms of sperm competition (for reviews see Gwynne 1984; Birkhead & Hunter 1990; Danielsson 1998; Simmons & Siva-Jothy 1998). Patterns of sperm use are often inferred from the mean species value of P2, defined as the mean proportion of offspring sired by the second male in double-mating trials (Boorman & Parker 1976). The within-species variance in P2, however, has mostly been ignored, although explaining this intraspecific variance may be crucial for understanding the underlying mechanisms of sperm competition (Lewis & Austad 1990; Simmons & Siva-Jothy 1998; Simmons & Achmann 2000).<br /> Aim of the present study was to elucidate patterns of sperm use and possible evolutionary causes and consequences in the scorpionfly Panorpa germanica (Mecoptera, Panorpidae). Paternity analysis among the offspring of doubly mated females of P. germanica revealed a very high intraspecific variance in P2. A correlation between proportional copulation duration and paternity was found if females had two copulations of disproportionate duration. These results suggest some mixing of sperm from both males inside the female's spermatheca. Yet, this mixing of sperm appears to be somewhat incomplete, as the second male on average sired a higher percentage of the female's offspring than its proportional copulation duration would predict. Testing the actual lifespan of sperm it can be concluded that this partial last male sperm precedence is not caused by death of first male sperm. Estimating sperm transfer rates of both mates of a female and controlling for mating durations by interrupting all copulations after a given period of time it can furthermore be concluded that the high intraspecific variance in P2 that was found in all experiments cannot be explained by variances in sperm transfer rates among P. germanica males. Other factors possibly causing the observed patterns of sperm use are discussed.<br /> As the intensity of sperm competition largely depends on the remating frequency of females I also aimed to estimate female remating rates in natural populations in the field. In mutual elucidation with previous investigations (Gerhards 1999; and Sauer, unpublished) it turned out that females of P. germanica usually mate only once, sometimes twice during their entire life. I therefore assume the selective force of sperm competition to act rather weakly in this species. Consequently, there is hardly any selection pressure favouring any complex mechanism of sperm competition.<br /> Discussing the observed patterns it becomes evident that in P. germanica females can maximise their reproductive success largely by choosing high quality males prior to copulation rather than by discriminating between sperm from different males. Males on the other hand can maximise their reproductive output by mating with as many females as possible, always aiming to achieve rather long copulation durations, thereby reducing the risk of sperm competition.