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 high MF 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.

中文翻译：

---

法尼酸甲酯对水蚤（Daphnia pulex）无性系的传代影响：雄性和ephippia的产生以及涉及性别确定的基因的表达

1. 环境性别决定意味着性别是由影响母体生物和/或卵母细胞的环境因素定义的。它提供了人口性别比，这将使人们在原始环境下的可持续性达到最大。决定性别的环境因素包括温度，光周期，食物供应和人口密度。这些提示刺激决定新生儿性别的化学信号分子（即激素）的释放。
2. 我们已经分析了从不同自然种群中分离出的三个水蚤（Daphnia pulex）克隆（I，K和S）对法呢酸甲酯（MF）的存活率，生殖力，雄性和ephippia的产生以及基因表达的反应参与性别决定（dsx1）和荷尔蒙调节（JHAMT，Met，RXR）。克隆个体暴露于：（1）高MF浓度（0.8 µM）加上压力条件；（2）低的MF浓度（0.15 µM），并在随后的两个世代中结合了人口密度和食物供应的最佳条件。
3. D. pulex的遗传上不同的克隆在雄性和ephippia产生的倾向以及对导致从孤雌生殖转换为有性生殖的刺激的敏感性方面有所不同。
4. 当暴露于MF时，在所有三个克隆中均观察到雄性产生的显着诱导。在模拟拥挤的条件下，即使无MF，也只有克隆S产生雄性后代，并表现出较高的JHAMT和dsx1表达水平基因。该反应表明克隆S具有产生雄性的高可能性，这可能是由于受体对激活MF的内源性生物合成的环境刺激具有高度敏感性。但是，暴露于高MF浓度的克隆S产生的雄性比克隆I和K少，后者通常不易产生雄性。高MF浓度加上模拟拥挤的条件，可能通过对参与合成以及MF降解的基因的差异调节来激活克隆S中的反馈机制。
5. 连续两代暴露于低MF浓度导致雄性子代产量从第一代（G1）减少到第二代（G2）。在MF处理的两个世代中，所有分析的基因均上调，但在G2中的表达较低。dsx1基因上调的减少与雄性产量的降低相匹配。基于我们的结果和文献数据，我们假设母体dsx1 mRNA可能在卵母细胞发育的早期发挥了某些作用，并导致接受dsx1信号的卵发育成雄性。
6. 暴露于MF的两代人显着增加了ephippia的产生。这些结果表明，雄性和ephippia的诱导可能在MF途径中有共同的根源。MF对生活史特征（成熟期和生育力下降的年龄延迟），性别比和ephippia产生的影响表明，少年激素对人口增长水平有直接影响。因此，由于从孤雌生殖向有性生殖的转变，MF影响水蚤的种群指标，并以此方式可能在可变的环境条件下使种群的可持续性最大化。