The effect of seminal fluid gene expression on paternity

Abstract

When females mate with more than one male, competition between rival ejaculates is expected to favor adaptations that promote fertilization success. There is now compelling evidence that sperm competition selects for increased production and allocation of sperm. However, sperm comes packaged in ejaculates that also contain protein-rich seminal fluids. Predicting how males should allocate individual seminal fluid proteins in response to sperm competition is hampered by our limited knowledge of their precise function. We use gene expression studies and interference RNA to ask how seminal fluid proteins in the ejaculate of a cricket, Teleogryllus oceanicus, affect a male's paternity share when in competition for fertilizations. We find that the relative expression of one seminal fluid gene, gagein, positively affects the paternity share of competing males and that knockdown of this and two other seminal fluid protein genes renders males mating in the offensive role of sperm competition incapable of fathering living offspring. Despite having a negative effect on offspring viability these seminal fluid genes have been found to be up regulated in response to rival males, consistent with a role in promoting competitive fertilization success. Our data contribute to a growing body of evidence that, like sperm, seminal fluid gene expression is subject to post-mating sexual selection via sperm competition.

Keywords: RNA interference; Teleogryllus oceanicus; fertilization success; seminal fluid proteins; sperm competition.

Figure 1.

Boxplots of the expression (2^−ΔCT) of seven seminal fluid protein genes in first (teal) and second (wheat) males in unmanipulated sperm competition trials. The median is indicated by the black horizontal bar, the box represents the upper and lower quartiles, whiskers are 1.5× interquartile range, and circles are outliers.

Figure 2.

The proportion of offspring sired by the second of two males to mate in relation to the relative expressions of gagein. The regression line is presented with its 95% confidence intervals.

Figure 3.

Boxplots of paternity (proportion of offspring sired) achieved by second males to mate when one of their seminal fluid protein genes had been knocked down using interference RNA. Shown also is the distribution of second male paternity for unmanipulated males that had none of their sfp genes knocked down. The median is indicated by the black horizontal bar, the box represents the upper and lower quartiles, whiskers are 1.5× interquartile range, and circles are outliers.