Sexual selection and conflicts
In the interest of better understanding the differences between sexes, we have studied sexual selection and conflict. We did so in two ways:
I- Parthenogenetic species produce mainly daughters without males. We study selection during the rare events of sexual reproduction in Daphnia.
Cyclic parthenogenesis, a reproductive strategy whereby organisms alternate between several cycles of clonal reproduction and occasional sexual events, is widespread across the tree of life. Although often overlooked, sexual reproduction plays an important role in the evolution of cyclically parthenogenetic organisms. Daphnia is one such organism, in which sexual reproduction is rare in most natural populations. We studied the sexual process in Daphnianatural populations and found clear evidence that sexual selection operates in this predominantly parthenogenetic species, likely through a combination of female choice and male competition. (Duneau et al. BioRxiv)
We further characterised sperm morphology across several Daphnia species and found that they possess some of the smallest sperm cells ever recorded. Using a recent phylogeny, we showed that larger sperm appears to have evolved independently at least twice, eventually leading in some lineages to the loss of sperm encapsulation — a structure that otherwise allows for cell condensation in the spermiduct. (Duneau et al., Peer Community Journal 2022)
II- Seminal fluid proteins are generally considered male tools to manipulate females. We investigated the growing hypothesis that seminal fluid proteins are important signals for females to adjust their egg-laying and mating decisions, and thus help reduce sexual conflict.
Using mathematical models of reproductive physiology informed by the biology of Drosophila melanogaster, we investigated how seminal fluid proteins (Sfps) can serve as honest signals for females rather than tools of male manipulation.
In a first study (Michalak et al., The American Naturalist 2026), we showed that without any regulation, females face an unavoidable trade-off: the strategy that maximises total offspring production also results in slow reproduction and large quantities of wasted unfertilized eggs. We demonstrated that sex peptide (SP), a seminal protein gradually released from sperm stored in the female reproductive tract, can act as a reliable signal of remaining sperm availability. By using this signal to down-regulate egg laying as sperm depletes, females can simultaneously maximise offspring number, reproduce quickly, and minimise egg wastage. The depletion of SP from sperm — rather than sperm depletion itself — is the key mechanism enabling this coordination.
In a second study (Michalak et al.), we extended the model to include multiple matings and examined the consequences for sexual conflict. While males always benefit from delaying female remating as long as possible, females have an intermediate optimal remating interval. We found, however, that SP regulation substantially reduces the intensity of this conflict: by reducing the wastage of resources between matings, SP-regulated females maintain near-optimal reproductive output across a wide range of remating intervals. This flexibility buffers females against the cost of delayed remating, weakening selection on males to manipulate female remating timing, and potentially allowing females to be more selective about mate quality.
