Female reproductive fluids 'rescue' sperm from phenotypic ageing in an external fertilizer

Per capita sperm metabolism is density dependent

From bacteria to metazoans, higher density populations have lower per capita metabolic rates than lower density populations. The negative covariance between population density and metabolic rate is thought to represent a form of adaptive metabolic plasticity. A relationship between density and metabolism was actually first noted 100 years ago, and was focused on spermatozoa; even then, it was postulated that adaptive plasticity drove this pattern. Since then, contemporary studies of sperm metabolism specifically assume that sperm concentration has no effect on metabolism and that sperm metabolic rates show no adaptive plasticity. We did a systematic review to estimate the relationship between sperm aerobic metabolism and sperm concentration, for 198 estimates spanning 49 species, from protostomes to humans from 88 studies. We found strong evidence that per capita metabolic rates are concentration dependent: both within and among species, sperm have lower metabolisms in dense ejaculates, but increase their metabolism when diluted. On average, a 10-fold decrease in sperm concentration increased per capita metabolic rate by 35%. Metabolic plasticity in sperm appears to be an adaptive response, whereby sperm maximize their chances of encountering eggs.

Paternal environment effects are driven by female reproductive fluid but not sperm age in an external fertilizer

Sperm ageing after ejaculation can generate paternal environment effects that impact offspring fitness. In many species, female reproductive fluids (FRFs), i.e. ancillary fluids released by eggs or within the female reproductive tract, may protect sperm from ageing and can additionally interact with sperm to influence offspring viability. This raises the intriguing prospect that FRFs may alleviate paternal effects associated with sperm ageing. Here, we test this novel hypothesis using the broadcast spawning mussel, Mytilus galloprovincialis. We show that incubating sperm in FRF prior to fertilization increases offspring viability, and that these effects occur independently of sperm age. Our results provide novel evidence that FRFs allow females to selectively bias fertilization toward higher quality sperm within an ejaculate, which in turn yields more viable offspring. We consider this FRF-mediated paternal effect in the context of female physiological control over fertilization and the transgenerational effects of female-regulated haploid selection.