Seminal fluid-mediated fitness effects in the simultaneously hermaphroditic flatworm Macrostomum lignano

Timeless or tainted? The effects of male ageing on seminal fluid

Reproductive ageing can occur due to the deterioration of both the soma and germline. In males, it has mostly been studied with respect to age-related changes in sperm. However, the somatic component of the ejaculate, seminal fluid, is also essential for maintaining reproductive function. Whilst we know that seminal fluid proteins (SFPs) are required for male reproductive success across diverse taxa, age-related changes in SFP quantity and composition are little understood. Additionally, only few studies have explored the reproductive ageing of the tissues that produce SFPs, and the resulting reproductive outcomes. Here we provide a systematic review of studies addressing how advancing male age affects the production and properties of seminal fluid, in particular SFPs and oxidative stress, highlighting many open questions and generating new hypotheses for further research. We additionally discuss how declines in function of different components of seminal fluid, such as SFPs and antioxidants, could contribute to age-related loss of reproductive ability. Overall, we find evidence that ageing results in increased oxidative stress in seminal fluid and a decrease in the abundance of various SFPs. These results suggest that seminal fluid contributes towards important age-related changes influencing male reproduction. Thus, it is essential to study this mostly ignored component of the ejaculate to understand male reproductive ageing, and its consequences for sexual selection and paternal age effects on offspring.

The repeatable opportunity for selection differs between pre- and postcopulatory fitness components

In species with multiple mating, intense sexual selection may occur both before and after copulation. However, comparing the strength of pre- and postcopulatory selection is challenging, because (i) postcopulatory processes are generally difficult to observe and (ii) the often-used opportunity for selection (I) metric contains both deterministic and stochastic components. Here, we quantified pre- and postcopulatory male fitness components of the simultaneously hermaphroditic flatworm, Macrostomum lignano. We did this by tracking fluorescent sperm-using transgenics-through the transparent body of sperm recipients, enabling to observe postcopulatory processes in vivo. Moreover, we sequentially exposed focal worms to three independent mating groups, and in each assessed their mating success, sperm-transfer efficiency, sperm fertilizing efficiency, and partner fecundity. Based on these multiple measures, we could, for each fitness component, combine the variance (I) with the repeatability (R) in individual success to assess the amount of repeatable variance in individual success-a measure we call the repeatable opportunity for selection (IR ). We found higher repeatable opportunity for selection in sperm-transfer efficiency and sperm fertilizing efficiency compared to mating success, which clearly suggests that postcopulatory selection is stronger than precopulatory selection. Our study demonstrates that the opportunity for selection contains a repeatable deterministic component, which can be assessed and disentangled from the often large stochastic component, to provide a better estimate of the strength of selection.

Seminal fluid and accessory male investment in sperm competition

Sperm production and allocation strategies have been a central concern of sperm competition research for the past 50 years. But during the 'sexual cascade' there may be strong selection for alternative routes to maximizing male fitness. Especially with the evolution of internal fertilization, a common and by now well-studied example is the accessory ejaculate investment represented by seminal fluid, the complex mixture of proteins, peptides and other components transferred to females together with sperm. How seminal fluid investment should covary with sperm investment probably depends on the mechanism of seminal fluid action. If seminal fluid components boost male paternity success by directly enhancing sperm function or use, we might often expect a positive correlation between the two forms of male investment, whereas trade-offs seem more likely if seminal fluid acts independently of sperm. This is largely borne out by a broad taxonomic survey to establish the prevailing patterns of seminal fluid production and allocation during animal evolution, in light of which I discuss the gaps that remain in our understanding of this key ejaculate component and its relationship to sperm investment, before outlining promising approaches for examining seminal fluid-mediated sperm competitiveness in the post-genomic era. This article is part of the theme issue 'Fifty years of sperm competition'.

Effects of two seminal fluid transcripts on post-mating behaviour in the simultaneously hermaphroditic flatworm Macrostomum lignano

The seminal fluid proteins (SFPs) transferred to mating partners along with sperm often play crucial roles in mediating post-mating sexual selection. One way in which sperm donors can maximize their own reproductive success is by modifying the partner's (sperm recipient's) post-copulatory behaviour to prevent or delay re-mating, thereby decreasing the likelihood or intensity of sperm competition. Here, we adopted a quantitative genetic approach combining gene expression and behavioural data to identify candidates that could mediate such a response in the simultaneously hermaphroditic flatworm Macrostomum lignano. We identified two putative SFPs-Mlig-pro46 and Mlig-pro63-linked to both mating frequency and 'suck' frequency, a distinctive behaviour, in which, upon ejaculate receipt, the worm places its pharynx over its female genital opening and apparently attempts to remove the received ejaculate. We, therefore, performed a manipulative experiment using RNA interference-induced knockdown to ask how the loss of Mlig-pro46 and Mlig-pro63 expression, singly and in combination, affects mating frequency, partner suck propensity and sperm competitive ability. None of the knockdown treatments impacted strongly on the mating frequency or sperm competitive ability, but knockdown of Mlig-pro63 resulted in a significantly decreased suck propensity of mating partners. This suggests that Mlig-pro63 may normally act as a cue in the ejaculate to trigger recipient suck behaviour and-given that other proteins in the ejaculate have the opposite effect-could be one component of an ongoing arms race between donors and recipients over the control of ejaculate fate. However, the adaptive significance of Mlig-pro46 and Mlig-pro63 from a donor perspective remains enigmatic.