Parental environment modulates offspring thermal tolerance in a foundational intertidal seaweed

Thermotolerance acquisition is an important ecophysiological trait under global warming scenarios because it can allow organisms and populations to adapt, particularly during the most sensitive early stages of a life cycle. Here we used seasonality as a natural scenario to explore whether parental thermal histories can modulate thermotolerance of recruits of a canopy-forming intertidal seaweed (<i>Fucus guiryi</i>) across an ecologically relevant thermal gradient (15–28°C). For this purpose, we harvested embryos from parents after the periods of maximum and minimum accumulated heat exposure (late summer and late winter), and at the onset of summer. During early ontogeny we followed initial embryo size, internal nutrient content, survival, growth and developmental stages as performance metrics to address whether parental acclimation modulates thermal tolerance via provisioning or parental effects. Late winter recruits of <i>F. guiryi</i> exhibited the greatest thermotolerance, showing a broader range of optimal temperatures and higher upper thermal limits for growth and survival, probably associated with better provisioning from parental thalli. Physiological fitness of recruits decreased above 25°C, showing arrested growth, impaired development and dropping survival rates, but functional loss was more abrupt in early summer. Late summer responses confirmed that heat hardening occurs in natural populations, but at the seasonal scale the adaptive significance of this increased thermotolerance is much lower than that induced by winter parental provisioning. Heat-induced thermotolerance occurred from early to late summer due to parental exposure to warming. However, winter provisioning promoted greater thermotolerance acquisition. Exposure to moderate thermal stress at the onset of summer without prior seasonal acclimation resulted in minimum levels of thermal tolerance and loss of offspring fitness. While warmer winters might be neutral or benefit early development, increasing temperatures and poor nutritional conditions at the onset of the summer season may reduce survival and hamper population recruitment. Thermotolerance of <i>Fucus guiryi</i> increased towards the late cold season.Early summer recruits had the lowest survival under warming.Seasonal performance of recruits might be driven by parental environment. Thermotolerance of <i>Fucus guiryi</i> increased towards the late cold season. Early summer recruits had the lowest survival under warming. Seasonal performance of recruits might be driven by parental environment.

在全球变暖背景下，耐热性获得是一项关键的生态生理性状，可帮助生物及其种群实现适应性演化，尤其在生命周期最为敏感的早期阶段至关重要。本研究以季节节律作为天然实验场景，旨在探究亲本热暴露历史是否会调控冠层型潮间带藻类——吉氏墨角藻（*Fucus guiryi*）的幼体在15–28℃这一生态相关温度梯度下的耐热性。
为此，我们分别在累积热暴露量最高（夏末）、最低（冬末）的时期，以及夏季伊始，从亲本藻体上采集其胚胎。在个体发育早期，我们以初始胚胎大小、体内营养储备、存活率、生长速率及发育阶段作为性能指标，探究亲本驯化是否通过营养供给或亲本效应调控幼体的耐热性。
冬末采集的吉氏墨角藻幼体展现出最强的耐热性：其生长与存活的适宜温度范围更广，高温耐受上限更高，这可能与亲本藻体为其提供了更充足的营养储备有关。当温度超过25℃时，幼体的生理适合度开始下降，表现为生长停滞、发育受损及存活率降低；但在夏季伊始，这种生理功能丧失的进程更为急剧。夏末的实验结果证实，自然种群中存在热硬化效应，但从季节尺度来看，这种热诱导获得的耐热性的适应性意义远低于冬末亲本营养供给所带来的耐热性提升。
由于亲本在夏季受到热暴露，热诱导的耐热性在夏初至夏末逐渐形成；但冬末的亲本营养供给能带来更显著的耐热性获得。若在未经历前期季节驯化的情况下于夏季伊始遭遇中等热胁迫，幼体的耐热性将降至最低，且后代适合度会出现损失。尽管暖冬可能对幼体早期发育无负面影响甚至有益，但夏季伊始的温度升高与营养条件匮乏，可能会降低幼体存活率并阻碍种群补充。
吉氏墨角藻（*Fucus guiryi*）的耐热性在晚冷季逐渐提升。夏季伊始的幼体在升温条件下存活率最低。幼体的季节性能表现可能受亲本所处环境的调控。
吉氏墨角藻（*Fucus guiryi*）的耐热性在晚冷季逐渐提升。夏季伊始的幼体在升温条件下存活率最低。幼体的季节性能表现可能受亲本所处环境的调控。