Sperm Storage in the Yellow Dung Fly Scathophaga stercoraria: Identifying the Sperm of Competing Males in Separate Female Spermathecae

Abstract

I. 1997: Sperm storage in the yellow dung fly Scatbophaga stercorana: identifying the sperm of competing males in separate female spermathecae. Ethology 103, 844-854. AbstractWe examined the effects of male and female behaviour and morpholohy on the process of sperm storage in the yellow dung fly Scatbophaga (.Tcut@agu) stercoranu. The larger of two males was more successful in transferring sperm to females' spermathecae the greater the difference in weight to his smaller competitor, as expected from previous studies by other authors. Sperm length, which is not correlated to body size, affected sperm access to the spermathecae, the female storage organs; longer sperm were more likely to be found in the spermathecae. A female typically had a singlet spermatheca and two spermathecae arranged as a pair, a doublet. However, there was variation from this pattern, which influenced the pattern of sperm storage. We measured the proportion of sperm from two competing males in females' singlet and doublet spermathecae. When the larger male's sperm were longer than his competitor's, they were more often in a female's singlet when he was her first mate and equally likely to be in the singlet or doublet when he was her second mate. When the larger male's sperm were shorter than his competitor's, the pattern was more complicated; principally because his sperm were not as successful at entering the female's doublet when he was her second mate. Counts of sperm, made using the same experimental procedure, showed that these effects were due to greater numbers of sperm entering the females' doublets when the larger male mated second. Sperm length was thus the factor with the largest single influence on the pattern of sperm storage. However, our most important result is that it was the interactions between male and female characters that were significant. Males mostly determine the early, especdly precopulatory, events and females strongly influence the later ones.

Response to selection and realized heritability of sperm length in the yellow dung fly (Scathophaga stercoraria)

“…What is less clear is that if sperm length has no effect on sperm competitiveness, what are the cause and consequences of the apparent sperm length effect on sperm storage in S. stercoraria (Otronen et al, 1997), especially as sperm number is so important in competitive siring success. This previous finding is difficult to interpret, however, because copulations were experimentally interrupted and males did not, therefore, complete a full copulation.…”

“…Although some of this variation may be due to environmental temperature differences during development (Blanckenhorn and Hellriegel, 2002), there is no indication that the variation is due to inherent developmental instability . In addition, although there is some evidence that sperm length influences sperm storage (Otronen et al, 1997), when fly populations were forced to evolve with and without sperm competition, sperm size did not evolve, even though other reproductive characters did . Furthermore, sperm length is heritable in yellow dung flies, and there is a significant autosomal contribution to this (Ward 2000).…”

Sexual Selection: The Logical Imperative

“…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. 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.…”

An Analytical Framework for Estimating Fertilization Bias and the Fertilization Set from Multiple Sperm-Storage Organs

“…Here, we specifically use a binomial mixture model with binomial parameters x, y, and z. We treat the paired spermathecae as a single unit, since, unlike with the yellow dung fly Scathophaga stercoraria (Ward 1993;Otronen et al 1997;Hellriegel and Bernasconi 2000), we have no evidence to suggest that they store or use sperm differently in Drosophila. Parameters x, y, and z are all bounded by 0 and 1; when z p , sperm are equally likely to be selected from either 0.5 storage organ X or Y, and x or represent equal y p 0.5 probabilities for first-or second-male sperm to be used for fertilization (i.e., no fertilization bias, meaning sperm Female Fertilization Bias 557 from competing males used in proportion to their representation).…”

“…Indeed, the closer we get to fertilization, the less we understand about the mechanisms of cryptic female choice. Thus, we know more about female-mediated processes influencing intromission (e.g., Brennan et al 2007), sperm transfer (e.g., Sakaluk and Eggert 1996;Pilastro et al 2004;Bussière et al 2006;Hall et al 2010), and retention of sperm (e.g., Bishop et al 1996;Pizzari and Birkhead 2000;Dean et al 2011;Lüpold et al 2012) than we do about sperm storage and use for fertilization (e.g., Ward 1993;Otronen et al 1997;Có rdoba-Aguilar 1999;Fedina and Lewis 2004;Pattarini et al 2006).…”