Heteropopulation males have a fertilization advantage during sperm competition in the yellow dung fly (Scathophaga stercoraria)

Abstract

Sexual conflict occurs whenever there is not strict genetic monogamy. The sexually antagonistic coevolution that potentially occurs because of this conflict involves adaptation by one sex followed by the counter-adaptation by the other, and may be thought of as an evolutionary arms-race. As a result of these cycles of antagonistic coevolution, females from one population may be less resistant to heteropopulation males, at least after short periods of allopatry, as they will not have evolved any resistance to them. We tested this prediction in yellow dung fly (Scathophaga stercoraria) populations from the UK and Switzerland. Males from each population mated as first and second males to females from each population, and the mean numbers of offspring sired by the last male to mate in each situation were compared. We also compared the fertility and fecundity of single females mated to males from both populations, as well as the fertility and fecundity of the F 1 crosses. Both crosses produced viable and fertile offspring and the offspring sex ratios were not skewed. However, the fecundity of F 1 -cross females was greater than that of the parentals. In the sperm-competition experiment, there was a significant interaction between male and female origin influencing the proportion of offspring sired by the second male to mate, with heteropopulation males always outcompeting conpopulation males. This effect was independent of copula duration and the delay between copulations. In a separate experiment, we tested to see whether this was due to female preference for genetically dissimilar males but found no evidence for paternity biasing based on genetic similarity. Our results therefore seem to be best explained by sexually antagonistic coevolution as females appear less resistant to males with which they have not coevolved.

Detecting sexually antagonistic coevolution with population crosses

“…This prediction has been tested in several species but with mixed results (Andres & Arnqvist 2001;Brown & Eady 2001;Knowles & Markow 2001;Hosken et al 2002;Nilsson et al 2002). In crosses of our model strains this prediction was not upheld: females may or may not be least responsive to those males with which they have coevolved (figure 4).…”

“…Specifically, it has been argued that the rank effectiveness of the signal from any one male strain would be independent of female strain when the signal-receiver system involves only one signal and one receptor (the female preference; Andres & Arnqvist 2001). Alternatively, if the rank effectiveness of male signals depends upon the female strain with which they are crossed, then a complex multi-signal and receiver system is indicated (see also Clark et al 1999;Hosken et al 2002;Nilsson et al 2002).…”

“…Several crossing experiments relevant to these ideas have already been conducted (Clark et al 1999;Andres & Arnqvist 2001;Brown & Eady 2001;Knowles & Markow 2001;Hosken et al 2002;Nilsson et al 2002), and further experiments are ongoing. Both the results and interpretation of these experiments are mixed (Chapman et al 2003).…”

“…Therefore, without direct knowledge that a given trait is (or has been) shaped by sexual conflict, this sort of evidence does not indicate a primary role for sexually antagonistic coevolution. To address this issue, investigators have begun to build and test a verbal theory that similarly assumes rapid and diversifying trait evolution, but claims to have the power to distinguish sexually antagonistic coevolution from other forms of sexual selection (Andres & Arnqvist 2001;Brown & Eady 2001;Knowles & Markow 2001;Hosken et al 2002;Nilsson et al 2002).…”

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

“…Females were, at least, 14 days old and males, at least, 6 days old, to ensure they were sexually mature (Foster 1967;Hosken et al, 2002). In each line, 35 pairs were provided with the opportunity to copulate.…”