How biases in sperm storage relate to sperm use during oviposition in female yellow dung flies

• Published in: Behavioral ecology 2021-07, Vol.32 (4), p.756-768
• Main Authors: Demont, Marco, Ward, Paul I, Blanckenhorn, Wolf U, Lüpold, Stefan, Martin, Oliver Y, Bussière, Luc F
• Format: Article
• Language: English
• Subjects: cryptic female choice; Ecology; Ekologi; Evolutionary Biology; Evolutionsbiologi; paternity success; postcopulatory sexual selection; Scathophaga; sperm competition
• ISSN: 1045-2249; 1465-7279; 1465-7279
• DOI: 10.1093/beheco/arab026

Abstract Precise mechanisms underlying sperm storage and utilization are largely unknown, and data directly linking stored sperm to paternity remain scarce. We used competitive microsatellite PCR to study the effects of female morphology, copula duration and oviposition on the proportion of stored sperm provided by the second of two copulating males (S2) in Scathophaga stercoraria (Diptera: Scathophagidae), the classic model for sperm competition studies. We genotyped all offspring from potentially mixed-paternity clutches to establish the relationship between a second male’s stored sperm (S2) and paternity success (P2). We found consistent skew in sperm storage across the three female spermathecae, with relatively more second-male sperm stored in the singlet spermatheca than in the doublet spermathecae. S2 generally decreased with increasing spermathecal size, consistent with either heightened first-male storage in larger spermathecae, or less efficient sperm displacement in them. Additionally, copula duration and several two-way interactions influenced S2, highlighting the complexity of postcopulatory processes and sperm storage. Importantly, S2 and P2 were strongly correlated. Manipulation of the timing of oviposition strongly influenced observed sperm-storage patterns, with higher S2 when females laid no eggs before being sacrificed than when they oviposited between copulations, an observation consistent with adaptive plasticity in insemination. Our results identified multiple factors influencing sperm storage, nevertheless suggesting that the proportion of stored sperm is strongly linked to paternity (i.e., a fair raffle). Even more detailed data in this vein are needed to evaluate the general importance of sperm competition relative to cryptic female choice in postcopulatory sexual selection. Using molecular markers when studying sperm storage in female yellow dung flies Scathophaga stercoraria, we found consistent skew across their three sperm storing organs, with relatively more sperm of the second male stored in the singlet spermatheca. The timing of egg laying (none vs. between vs. after the two matings) strongly influenced the observed sperm-storage patterns, revealing male- as well as female-mediated effects on sperm storage and presumably also the insemination patterns.