Post‐ejaculatory modifications to sperm (PEMS)

Abstract

Mammalian sperm must spend a minimum period of time within a female reproductive tract to achieve the capacity to fertilize oocytes. This phenomenon, termed sperm ‘capacitation’, was discovered nearly seven decades ago and opened a window into the complexities of sperm–female interaction. Capacitation is most commonly used to refer to a specific combination of processes that are believed to be widespread in mammals and includes modifications to the sperm plasma membrane, elevation of intracellular cyclic AMP levels, induction of protein tyrosine phosphorylation, increased intracellular Ca2+ levels, hyperactivation of motility, and, eventually, the acrosome reaction. Capacitation is only one example of post‐ejaculatory modifications to sperm (PEMS) that are widespread throughout the animal kingdom. Although PEMS are less well studied in non‐mammalian taxa, they likely represent the rule rather than the exception in species with internal fertilization. These PEMS are diverse in form and collectively represent the outcome of selection fashioning complex maturational trajectories of sperm that include multiple, sequential phenotypes that are specialized for stage‐specific functionality within the female. In many cases, PEMS are critical for sperm to migrate successfully through the female reproductive tract, survive a protracted period of storage, reach the site of fertilization and/or achieve the capacity to fertilize eggs. We predict that PEMS will exhibit widespread phenotypic plasticity mediated by sperm–female interactions. The successful execution of PEMS thus has important implications for variation in fitness and the operation of post‐copulatory sexual selection. Furthermore, it may provide a widespread mechanism of reproductive isolation and the maintenance of species boundaries. Despite their possible ubiquity and importance, the investigation of PEMS has been largely descriptive, lacking any phylogenetic consideration with regard to divergence, and there have been no theoretical or empirical investigations of their evolutionary significance. Here, we (i) clarify PEMS‐related nomenclature; (ii) address the evolutionary origin, maintenance and divergence in PEMS in the context of the protracted life history of sperm and the complex, selective environment of the female reproductive tract; (iii) describe taxonomically widespread types of PEMS: sperm activation, chemotaxis and the dissociation of sperm conjugates; (iv) review the occurence of PEMS throughout the animal kingdom; (v) consider alternative hypotheses for the adaptive value of PEMS; (vi) speculate on the evolutionary implications of PEMS for genomic architecture, sexual selection, and reproductive isolation; and (vii) suggest fruitful directions for future functional and evolutionary analyses of PEMS.

The role of female reproductive fluid in sperm competition

“…The search for underlying mechanisms of the FRF's effect on postmating sexual selection has so far focused on the composition of these fluids. The chemical characteristics of FRF, such as osmolality, pH and ionic concentration (Na + , K + , Mg 2+ and Ca 2+ ) are likely to be important in both external [37] and internal fertilizers [6]. However, biochemical components of FRF, such as nutrients (glucose, pyruvate, lactate and fructose), free amino acids, hormones (e.g.…”

Novelty and emergent patterns in sperm: Morphological diversity and evolution of spermatozoa and sperm conjugation in ground beetles (Coleoptera: Carabidae)

“…We also studied 11 specimens preserved in 10% neutral-buffered formalin, a common sperm fixative (Schmoll, Sanciprian, & Klevne, 2016), from Germany and Guatemala (Supplemental online material, Table S3). Our sampling (Table S2) is biased toward male beetles (96 of 177 species are based on only males) because aspects of our sampling were largely opportunistic, the probability of collecting mature sperm is high in males whereas in females it requires their having been inseminated, and we found consistent evidence that sperm, particularly sperm conjugates, undergo changes in the female reproductive tract, posing challenges for documenting sperm form prior to their exposure to the environment of the female reproductive tract (Pitnick, Wolfner, & Dorus, 2020).…”

Conceptual developments in sperm competition: a very brief synopsis

“…John Sivinski [15] was possibly the first to ask how sperm competition has shaped sperm phenotype. This topic has since attracted considerable attention, with increasing evidence that sperm morphology [56,61] and functional postejaculatory sperm modifications such as sperm capacitation [62] are affected by sperm competition.…”