Estimation of the propensity for sexual selection in a cyclical parthenogen

Abstract

18Cyclical parthenogenesis is a widespread reproductive strategy in which organisms go through one or 19 multiple rounds of clonal reproduction before sexual reproduction. In populations of the planktonic 20 cladoceran Daphnia magna sexual reproduction is typically less common than parthenogenesis and 21 therefore hardly studied. We studied the sexual process and its relation to sexual selection in Daphnia 22

Sex differences in the strength of selection under facultative sex

“…Kiørboe 2007;Rhainds 2019) or because of time, energy or predation costs of mate-searching. Another category assumes that the selection coefficients sa, sf and sm themselves become a function of q, and in our Supplementary material section 2b we analyze a case where the strength of selection on males depends on q because females mate predominantly monandrously when males are scarce, and polyandrously when they are common (Duneau et al 2020); therefore, selection on males to succeed in sperm competition becomes stronger with male frequency. These additional analyses add nuance to our baseline model: it is possible to find causal routes where q can alter 𝑠̅ , but this requires making additional and system-specific assumptions, and so these do not act to restore any simple expectation that 𝑠̅ is proportional to the frequency of expressers.…”

“…Assume that females mate with either 1 or 2 males (this range captures the overwhelming majority of matings in sexual generations of the cyclical parthenogens Daphnia magna, Duneau et al 2020). We assume that polyandrous matings occur with a probability f(q), a function that increases with q.…”

Evolution of sperm morphology in a crustacean genus with fertilization inside an open brood pouch

“…However, in Daphnia magna , there is a large heterogeneity among populations. In unstable environments, males can be periodically abundant, and several males can be found copulating at the same time with a female, suggesting that sexual selection can be strong, maybe more than in most other species (Duneau et al ., bioRxiv 2020). Noticeably, D. magna is the only species here that have exaggerated genital papilla and the only species for which matings with multiple males have been reported (Duneau et al ., 2020).…”

“…In unstable environments, males can be periodically abundant, and several males can be found copulating at the same time with a female, suggesting that sexual selection can be strong, maybe more than in most other species (Duneau et al ., bioRxiv 2020). Noticeably, D. magna is the only species here that have exaggerated genital papilla and the only species for which matings with multiple males have been reported (Duneau et al ., 2020). One could wonder if the evolution of exaggerated genital papilla could have evolved in D. magna due to selection to improve sperm deposition close to the oviduct and reduce the thinning effect in a species under strong sexual selection.…”

“…These resting stages contribute to genetically diverse egg-banks from which future populations can be established. There is evidence for males competing for fertilization in Daphnia magna (Duneau et al ., bioRxiv 2020), but the extend of sperm competition in this receptacle is unknown. It is not entirely clear where fertilization exactly occurs, whether in the brood pouch or inside the oviduct exiting in the brood pouch.…”