1. Environmental sex determination implies that sex is defined by environmental factors influencing the maternal organism and/or oocytes. It provides population sex ratios that will maximise the sustainability of the population under incipient environmental conditions. Environmental factors responsible for sex determination include temperature, photoperiod, food availability, and population density. These cues stimulate the release of chemical signalling molecules (i.e. hormones) that determine the sex of newborns. 2. We have analysed the responses of three Daphnia pulex clones (I, K, and S), isolated from different natural populations, to juvenile hormone methyl farnesoate (MF) in terms of survivorship, fecundity, production of males and ephippia, and expression of genes involved in sex determination (dsx1) and hormonal regulation (JHAMT, Met, RXR). Clonal individuals were exposed to: (1) high MF concentration (0.8 µM) combined with stressful conditions; (2) low MF concentration (0.15 µM) combined with optimal conditions of population density and food availability in two subsequent generations. 3. The genetically different clones of D. pulex differed in the propensity for male and ephippia production and in the sensitivity to the stimuli that lead to the switch from parthenogenetic to sexual reproduction. 4. A significant induction of male production was observed in all three clones when exposed to MF. In conditions mimicking crowding, only clone S produced male offspring even in absence of MF and showed higher expression levels of JHAMT and dsx1 genes. This response suggests that clone S has a high propensity to produce males probably due to a high sensitivity of receptors to environmental stimuli activating the endogenous biosynthesis of MF. However, clone S exposed to highMF concentration produced fewer males than clones I and K, which generally have a low natural propensity to produce males. High MF concentration coupled with conditions mimicking crowding, probably activated a feedback mechanism in clone S via the differential modulation of genes involved in the synthesis as well in the degradation of MF. 5. Exposure to low MF concentration for two subsequent generations resulted in a reduction of male progeny production from the first (G1) to the second (G2) generation. All analysed genes were up-regulated in both generations treated with MF, but with lower expression in G2. The reduced up-regulation of the dsx1 gene matched with the lower male production. Based on our results and literature data, we hypothesised that the maternal dsx1 mRNA probably plays some role early in oocyte development, and causes the eggs that receive the dsx1 signal to develop into males. 6. Exposure to MF for two generations significantly increased ephippia production. These results suggest that the induction of male and ephippia might have a common root in the MF pathway. The effect of MF on life-history traits (delay in the age at maturity and fecundity reduction), sex ratio, and ephippia production suggests a direct impact of the juvenile hormone on the level of population growth. Thus, as a consequence of the switch from parthenogenetic to sexual reproduction, MF influences Daphnia population metrics and in this way probably maximises the sustainability of the population under variable environmental conditions.
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