Natural levels of polyandry: differential sperm storage and temporal changes in sperm competition intensity in wild yellow dung flies

Abstract

Summary1. Polyandry is common in insects. Nevertheless, the evolutionary causes and consequences of this phenomenon remain contentious, in part because of a lack of information about natural mating rates and the fact that most post-copulatory processes are hidden from view within female reproductive tracts. 2. We captured wild female yellow dung flies (Scathophaga stercoraria) over the whole spring season and genotyped the sperm from their spermathecae to obtain information on sperm transfer, sperm storage and natural levels of polyandry for this model species of post-copulatory sexual selection research. 3. On average, females stored sperm from a minimum of 2AE47 males (based on the most conservative estimate). Incorporating knowledge of population allele frequencies yielded a slightly higher estimate of 3AE33 mates per female. 4. Sperm storage and therefore sperm competition intensity showed high temporal variation. The proportion of multiply mated females (i.e. females with sperm from ‡2 males within their sperm stores) and the absolute number of ejaculates detected within females increased strongly over the spring season before sharply decreasing as midsummer approached. 5. Interestingly, we detected a positive relationship between the number of stored ejaculates and females' wing injuries, suggesting that mating not only causes measurable cumulative damage to wild females but also provides a potential mechanism by which males may be able to assess the intensity of sperm competition within a female. 6. Our study found no evidence for intraejaculate sperm sorting, but importantly, the number of ejaculates in storage differed amongst the three sperm storage organs (spermathecae) of female yellow dung flies. Different sperm mixtures across the spermathecae could enable females to bias paternity towards certain males if females can selectively use sperm from a certain spermatheca at the time of fertilization.

Sexual Selection: The Logical Imperative

“…Bussière et al (2010) used molecular techniques to demonstrate that although the mean proportion of sperm stored in the spermathecae match the published mean average paternity for the last male (the P 2 value), sperm from different males are not stored randomly across the female's sperm stores (see also Otronen et al 1997;Hellriegel and Bernasconi 2000). The mean number of ejaculates stored also differs across spermathecae (Demont et al 2011. Thus while hints are present, and the capacity for it certainly exists, clear evidence for cryptic female choice in dung flies has so far proven elusive.…”

“…This pattern poses the question of why females are polyandrous (the average number of ejaculates stored is around 3 per female; Demont et al 2011). Tregenza et al (2003) found no simple benefits or costs of double versus single mating for females, but Hosken et al (2002) found that females mated once survived longer than those mated three times, suggesting that longevity costs are associated with multiple mating.…”

“…Tregenza et al (2003) found no simple benefits or costs of double versus single mating for females, but Hosken et al (2002) found that females mated once survived longer than those mated three times, suggesting that longevity costs are associated with multiple mating. There are physical costs of mating to females (Hammer 1941;Parker 1978a;Demont et al 2011). There are also obvious time costs of supernumerary matings (Parker 1970b).…”

Wild yellow dung fly females may not select sperm based on dung pat microclimate but could nevertheless benefit from polyandry

“…Sperm were extracted from the spermathecae using a method described by Tripet et al (2001) and applied in yellow dung flies before Demont et al 2011). We separated the abdomens of the dung fly females from the rest of the body and stored them for 48 h in 70% ethanol.…”

“…Measuring the degree of polyandry by tracking females and directly observing matings can be difficult for insects, but sperm storage in female insects is almost ubiquitous, and genotyping sperm stores can provide useful information on female mating frequency in natural populations (Demont et al 2011). Several studies have investigated how sperm storage patterns translate into paternity and the number of fathers contributing to a clutch Rodriguez-Muñoz et al 2010;Simmons and Beveridge 2010).…”