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

Abstract

Molecular techniques have substantially improved our knowledge of postcopulatory sexual selection. Nevertheless, studies examining sperm utilization in natural populations of nonsocial insects are rare, support for sperm selection (biased use of stored sperm, e.g. to match offspring genotypes to prevailing environmental conditions) is elusive, and its relevance within natural populations unknown. We performed an oviposition site choice experiment in the field where female yellow dung flies Scathophaga stercoraria could deposit eggs into three different microenvironments on a dung pat (the east-west ridge, north- or south-exposed side), and genotyped the offspring and sperm remaining in storage after oviposition. Females exhibited plasticity in the number of eggs deposited according to pat age. Additionally, temperature strongly influenced egg placement: the warmer the temperature, the higher the proportion of eggs laid into the north-exposed side of dung. The number of ejaculates in storage differed amongst spermathecae, and females stored sperm from more males than fathered their offspring (2.11 sires vs. 2.84 males within sperm stores). Mean last male paternity was 83.4%, roughly matching previous laboratory estimates. Importantly, we found no evidence that females selectively lay eggs of different genotypes, by biasing paternity towards certain males, depending on offspring's microclimate. Thus, while we show female choice over number of eggs and where these are deposited, there was no evidence for sperm selection. We further revealed positive effects of multiple mating on total number of offspring and proportion of offspring emerging from the dung. We argue that the integration of field studies and laboratory experiments is essential to promote our understanding of polyandry and cryptic female choice

Geographic clines in wing morphology relate to colonization history in New World but not Old World populations of yellow dung flies

“…(), and Demont et al. (). Table additionally provides information on the sample sizes, the number of alleles, and the mean observed ( H O ) and expected ( H E ) heterozygosity for each locus across North America, Europe, and Japan.…”

Sexual Selection: The Logical Imperative

“…They concluded that although pgm activity differences were apparent, and that pgm genotype did differentially affect development time, eggs laid on the north versus south slopes showed no biases in pgm composition as indicated from the previous work, removing the basis for cryptic female choice of sperm with different pgm genotypes. Further, Demont et al (2012) performed field experiments in which females could choose to lay eggs in three different dropping microenvironments (south slope, ridge, and north slope), and genotyped both (i) the resulting offspring, and (ii) the sperm remaining in the female sperm stores after oviposition. Although (as expected) females showed a greater preference to oviposit on north slopes as ambient temperature increased, they found no evidence that females biased paternity towards certain male genotypes depending on the offspring's microclimate.…”

The developmental plasticity and functional significance of an additional sperm storage compartment in female yellow dung flies

“…Differential sperm storage and usage has been repeatedly demonstrated in S. stercoraria using phenotypic or molecular markers (Otronen, Reguera & Ward ; Hellriegel & Bernasconi ; Bussière et al . ; Demont, Martin & Bussière ). Since artificial selection for expressing 4s induced fecundity costs in female S. stercoraria , potential mate choice benefits could trade‐off against costs that females incur from producing an additional sperm storage compartment (Ward ; Ward, Wilson & Reim ).…”