母体效应在生物的适应性进化过程中具有重要意义。对于营周期性孤雌繁殖的单巢纲轮虫，其生活史中孤雌和有性繁殖的选择体现了其在短期适合度和长期适合度之间的权衡；而适应性表型可塑性是轮虫响应环境快速变化的有效策略。本研究以淡水水体中广泛分布的萼花臂尾轮虫（Brachionus calyciflorus）为模式生物，探究了母体效应对单巢纲轮虫繁殖对策和表型可塑性的影响，主要研究结果有：

  1. 现有研究表明，除了外界环境因子（例如种群密度）外，年龄因素也会影响萼花臂尾轮虫个体所产后代中混交雌体的比率，即母体年龄效应。而母体效应本身具有环境依赖性。但母体年龄（内因）和所经历环境（外因）对后代影响的综合作用，研究尚缺乏。本研究探究了母体年龄效应是否受母体经历的环境因素（种群密度）的影响。结果显示当母体（F₀）经历高的种群密度信号时，年轻母体所产后代（F₁）中混交雌体的比率（有性繁殖率）显著高于高龄母体所产后代（F₁）中混交雌体的比率；并且年轻母体所产的非混交雌体F₁代较高龄母体所产的非混交雌体F₁代在种群密度信号诱导下产出的混交后代（F₂）的比率也明显较高。当母体经历较低的种群密度信号时，其生活史早期和后期所产后代中混交雌体的比率以及后代对种群密度信号的响应则无显著差异。本研究结果表明，母体所经历的种群密度环境会改变母体年龄效应对萼花臂尾轮虫有性繁殖策略的影响，在高的种群密度下母体倾向于在繁殖期早期优先投资有性繁殖。

  2. 在自然水体中萼花臂尾轮虫常常遭遇捕食者卜氏晶囊轮虫的捕食。有研究表明捕食者的出现会对萼花臂尾轮虫的有性繁殖投入（后代中混交雌体的比率）产生影响。捕食风险是否会影响母体对不同出生顺序后代的有性繁殖投入，即影响母体的年龄效应，有待研究。本研究以在自然水体中经历捕食者背景不同的两品系萼花臂尾轮虫（H1和D1）为模式生物，通过对比无捕食者和捕食风险下母体繁殖期早期和后期所产后代中混交雌体的比率的差异来探究母体年龄效应对萼花臂尾轮虫有性繁殖策略的影响是否受环境中捕食风险的影响。实验结果发现，在自然水体中常处于高捕食风险背景的品系H1在无捕食和捕食风险下所产后代中混交雌比率无显著差异，在自然水体中常经历捕食者季节性波动的品系D1也具有相同结果。不过，H1母体在捕食风险下倾向于在繁殖期的早期投入更多给有性繁殖，而D1母体没有这一倾向。本研究结果进一步证明了母体年龄效应对萼花臂尾轮虫有性繁殖的影响具有环境依赖性，并且这种影响具有品系间特异性。

  3. 种群中个体生活史性状的改变会对种群未来动态产生影响，而母体经历的环境会影响后代生活史性状的表达，因此母体效应可能是一种潜在的介导种群调节的机制。在周期性孤雌繁殖的萼花臂尾轮虫中，密度依赖的有性繁殖是种群调节的一种机制。不过，随着种群动态改变，种群性比也会发生变化。并且Fisher’s种群性比理论同样适用于萼花臂尾轮虫（产生雄性的混交雌体与产生休眠卵的混交雌体比例为1:1）。本研究探究了母体经历的种群性比对后代有性繁殖的影响，以及种群性比介导的母体效应对后代有性繁殖投入的改变是否会对未来种群动态产生影响。研究结果发现随着母体（F₀）经历的种群性比的增加，所产后代（F₁）中混交雌体的比率显著增加；并且在不同的种群密度梯度下，来自经历低种群性比母体（F₀）的F₁代产混交雌体F₂代的比率均显著低于母体（F₀）经历了高种群性比的F₁代。不同种群性比下的种群动态存在显著差异，高种群性比下的种群相较于对照组和低种群性比组具有显著小的平衡种群大小，而低种群性比下的种群具有最大的平衡种群大小。研究表明种群性比诱导的母体效应是介导萼花臂尾轮虫种群调节的一个潜在机制。

  4. 萼花臂尾轮虫有性繁殖产生的休眠卵可以通过较长时间的滞育有效躲避不良的环境条件，因此有性繁殖是萼花臂尾轮虫应对环境不确定性的有效方式。休眠卵再萌发的环境可能会与母体产生休眠卵时的环境不一致。本研究探究了母体产生休眠卵与休眠卵萌发时的水体温度的一致性对休眠卵萌发率的影响，萌发出的干雌体所建立的单克隆种群的有性繁殖策略以及种群增长的影响。研究发现20℃下所产休眠卵在20℃和30℃萌发温度下的萌发率均显著高于30℃下所产休眠卵。当母体产生休眠卵时的温度与休眠卵萌发时的温度不一致时，萌发的干雌体所建立的单克隆种群开始有性繁殖时的种群密度阈值较低，种群中平均混交率较高，种群平均增长率较低。实验结果表明环境异质性会影响有性繁殖的投入，而在异质性的环境中增加有性繁殖投入可能是一种适应性“两头下注”策略。

  5. 同一基因型在不同环境下能够表达出不同的表型，这种适应性表型可塑性能够使生物快速响应环境变化。不同环境选择压力下的种群在同一性状的表型可塑性会发生差异，探究这种差异形成的机制对于预测生物适应环境变化的潜能具有重要意义。本研究以适应捕食者卜氏晶囊轮虫不同捕食风险的两个萼花臂尾轮虫品系（H1和D1）为模式生物，探究母体效应在表型可塑性分化过程中的作用。研究发现适应不同捕食风险环境的两个萼花臂尾轮虫品系在捕食者诱导表型可塑性上存在差异。对于经历高捕食风险的品系H1，母体经历捕食风险后所产后代在捕食风险下能够产出具有最大防御形态的后代，即，母体经历过的捕食风险与后代经历的捕食风险具有累积效应；而对于适应捕食者季节性波动的品系D1， 在捕食风险下，来自母体经历过捕食风险的后代的防御形态与来自对照组母体所产后代无显著差异。研究结果表明母体效应在一定程度上促进了两个种群的表型可塑性的分化。

Maternal effects play an important role in adapting to environmental changes and evolutionary adaptation of animals. In cyclically parthenogenetic monogonont rotifers, investment to asexual reproduction or sexual resproduction reflects a trade-off between short- and long-term fitness. Adaptive phenotypic plasticity is an important life history strategy for rotifers in adaption to the changing environment. The present work investigates how maternal effects affect reproductive strategy and phenotypic plasticity in monogonont rotifer using a widely-distributed freshwater rotifer Brachionus calyciflorus as a model organism. The following are our main results:

1. At present, studies examining the interaction between maternal environment effects and maternal age (birth order) effects are scarce. We designed an experiment to reveal whether environmentally induced maternal manipulation works equally on offspring across birth orders in the rotifer B. calyciflorus. In the experiment, newborn amictic mothers (F₀) were cultured parallelly in low (LD) and high (HD) population density. Offspring (F₁) of young (YF₀) and old (OF₀) mothers in both LD and HD groups were cultured under a crowded condition and the mixis ratios of both F₁ and F₂ were examined. Results showed that F₁ from HD-YF₀ had a significantly higher mixis ratio and those amictic ones produce daughters (F₂) also with higher mixis ratio in response to crowding than those from HD-OF₀. In contrast, no such differences among F₁ siblings were found in the LD-F₀ group. Therefore, maternal age effect on sexual propensity of the offspring can be affected independently by the maternal population density.

2. Mortality caused by extrinsic factors such as predator had been shown to influence the maternal age effects. In the filed, predator rotifer Asplanchna brightwellii usually accompanies with the rotifer B. calyciflorus. We tested whether predator cues in the maternal environment could influence the maternal age effects on the offspring’s sexual reproduction with two strains of rotifer B. calyciflorus that had different predator contexts (H1 and D1). Our results showed that in both strain H1 and D1, there were no significant differences in proportion of mictic offspring between mothers that cultured with and without predators. But, in strain H1 that experienced a constant high predator context, when mothers were cultured with predator cues, they produced more mictic offspring in their early reproductive period than those from the control group. While in strain D1 that had a variable predator context, no such differences were found. Our results confirm that maternal age effect on sexual reproduction in the rotifer B. calyciflorus is environment-dependent.

3. Maternal effects transmit environmental cues between generations by influence on the offspring’s life-history traits, thereby, can be one potential source of population regulation. In the present study we manipulated sex ratio (male-producing mictic females vs. resting-egg producing mictic females) of the maternal population to test whether sex ratio can affect future population dynamics by regulation of the offspring’s sexual reproduction via maternal effects in the rotifer B. calyciflorus population. Comparing with females in higher sex ratio environment, mothers from lower sex ratio population produced fewer mictic offspring. Moreover, their amictic offspring produced lower proportion of mictic females in response to population density as well. Consequently, the changes in mictic offspring production resulted in variation of population dynamics. Populations that were manipulated with high initial sex ratio showed significantly smaller equilibrium population sizes compared with those with low initial sex ratio. Our results indicate that mothers can regulate sexual reproduction of their offspring in response to the current population sex ratio, thus, modulate future population dynamics besides of population density.

4. Sexual reproduction of the rotifer B. calyciflorus produces resting eggs, which allows survival in harsh environments. Usually, the hatching environment of resting eggs might be different from that of resting eggs when they are produced. Here we investigated if the consistency of the producing and hatching temperatures of resting eggs would affect the hatching rate, threshold population density for sexual reproduction initiation, population sexual reproduction rate, and population growth rates of clonal population found by resting eggs at two environmental temperatures. The results showed that at hatching temperature 20℃ and 30℃, resting-eggs from 20℃ had an average higher hatching rate than those from 30℃. When the hatching temperature did not match the producing temperature, the populations showed a lower threshold population density for sexual reproduction, produced more mictic females, consequently had a lower population size than those that had a matched condition. Our results indicate a “bet-hedging” strategy for rotifer to cope with environmental unpredictability.

5. Existing evidences show that maternal effect can affect the magnitude and direction of reaction norm of their offspring, but less clear is its role in mediating divergence of reaction norms between populations. In this study, we tested how maternal effects affected offspring reaction norms of inducible anti-predator defenses with two strains of rotifer B. calyciflorus. Previous field studies on those two populations revealed that one population (H1) experienced relatively constant, strong predator pressure, whereas the other population (D1) experienced seasonal variations in predator pressure. We found that H1 exhibited a clear transgenerational effect, and it worked independently, but additively with the offspring environment on offspring reaction norms to predator. While, D1 exhibited transgenerational effects only when mothers were constantly exposed to predation cues, however, when mothers experienced a short exposure, no transgenerational effect was shown. Our results indicate that patterns of maternal effects on the offspring reaction norms can be divergent between different populations of rotifer B. calyciflorus. Maternal effect might facilitate the adaptive divergence in anti-predator trait between the two populations.