Sperm competition dynamics: ejaculate fertilising efficiency changes differentially with time.

BACKGROUND: A fundamental challenge in evolutionary biology is to resolve the mechanisms that maintain paternity a hypervariable fitness component. Because females are often sexually promiscuous, this challenge hinges on establishing the mechanisms through which the ejaculates of different males compete for fertilisation (sperm competition). The competitive quality of an ejaculate is mediated by the relative number of live sperm and their motile performance. The differential rate at which rival ejaculates lose their fertilising efficiency over time is therefore expected to influence the outcome of sperm competition. RESULTS: Here, we artificially inseminated into sets of replicate domestic hens, Gallus gallus domesticus, experimentally engineered heterospermic ejaculates containing a large number of low-quality sperm from one male, and a lower number of high-quality sperm from another male. Large, low-quality ejaculates fertilised the first eggs produced after insemination, but small, high-quality ejaculates prevailed in the long run despite their numerical disadvantage. CONCLUSION: Together, these results provide the first experimental demonstration that the relative competitive value of an ejaculate changes drastically over the time during which competing ejaculates are stored within the reproductive tract of a female, resulting in a marked temporal pattern of variation in paternity. A high level of replication makes these results robust. However, our study was restricted to few males of a well characterised study population, and future work should explore the generality of these results.

Sperm motility in birds: insights from fowl sperm.

This review outlines how computer-assisted sperm motion analysis helped explain variation in fertility among roosters, investigate sperm cell energetics, and predict sperm allocation within the oviduct. The context for these experiments was the analysis of sperm mobility, a quantitative trait discovered in the mid-1990s. Sperm mobility denotes the movement of a sperm cell population against resistance at body temperature. Whereas mobile sperm must be motile, not all motile sperm are mobile. For example, straight-line velocity (VSL) must be > 30 microm/s for sperm to penetrate an Accudenz solution within a cuvette from an overlaid sperm suspension. Absorbance of the Accudenz solution is proportional to the number of mobile sperm within the semen sample. Phenotypic variation among roosters was explicable in terms of VSL distributions, which in turn reflected varying proportions of sperm containing dysfunctional mitochondria. In the case of mobile sperm, motility was enabled by calcium cycling, which drives phospholipase A2 activity and thereby controls the release of endogenous fatty acids for beta-oxidation within mitochondria. In view of sperm storage tubule ultrastructure and sperm rheotaxis, experimental outcomes led to the realization that sperm residence within the hen's sperm storage tubules was most readily explained in terms of progressive motility and that decreased VSL may account for sperm egress. It is noteworthy that male fertility can be altered by genetic selection using in vitro sperm mobility as the selection criterion. Sperm immobility may arise from formation of the mitochondrial permeability transition pore. Ongoing work addresses underlying mechanisms and related genes.

Social competitiveness associated with rapid fluctuations in sperm quality in male fowl.

When females copulate with multiple males, paternity is determined by the competitive ability of a male to access females and by the ability of its ejaculates to out-compete those of other males over fertilization. The relationship between the social competitiveness of a male and the fertilizing quality of its sperm has therefore crucial implications for the evolution of male reproductive strategies in response to sexual selection. Here, we present a longitudinal experimental study of the relationship between social status and sperm quality. We monitored sperm quality in socially naive male domestic fowl, Gallus gallus domesticus, before and after exposure to a social challenge which comprised two stages. In the first stage, social dominance was established in male pairs divergent in sperm quality, and in the second, social status was experimentally manipulated by re-shuffling males across pairs. We show that sperm quality fluctuates within males both before and after a social challenge. Importantly, such fluctuations followed consistently different patterns in males that displayed different levels of social competitiveness in the social challenge. In particular, following the social challenge, sperm quality dropped in males that won both contests while the sperm quality of males that lost both contests remained constant. Together, these results indicate that males of different social competitiveness are predisposed to specific patterns of fluctuations in sperm quality. These rapid within-male fluctuations may help explain the recent findings of trade-offs between male social and gametic competitive abilities and may help maintain phenotypic variability in these traits.

Sperm mobility: mechanisms of fertilizing efficiency, genetic variation and phenotypic relationship with male status in the domestic fowl, Gallus gallus domesticus.

When females are sexually promiscuous, sexual selection continues after insemination through sperm competition and cryptic female choice, and male traits conveying an advantage in competitive fertilization are selected for. Although individual male and ejaculate traits are known to influence paternity in a competitive scenario, multiple mechanisms co-occur and interact to determine paternity. The way in which different traits interact with each other and the mechanisms through which their heritability is maintained despite selection remain unresolved. In the promiscuous fowl, paternity is determined by the number of sperm inseminated into a female, which is mediated by male social dominance, and by the quality of the sperm inseminated, measured as sperm mobility. Here we show that: (i) the number of sperm inseminated determines how many sperm reach the female sperm-storage sites, and that sperm mobility mediates the fertilizing efficiency of inseminated sperm, mainly by determining the rate at which sperm are released from the female storage sites, (ii) like social status, sperm mobility is heritable, and (iii) subdominant males are significantly more likely to have higher sperm mobility than dominant males. This study indicates that although the functions of social status and sperm mobility are highly interdependent, the lack of phenotypic integration of these traits may maintain the variability of male fitness and heritability of fertilizing efficiency.