As climate change threatens marine ecosystems, sessile species such as kelps are heavily impacted by rapidly changing environmental conditions. In this context, phenotypic plasticity (genotype x environment interaction) is a key acclimation mechanism to improve resilience to high temperatures. Here, we investigated the effects of long-term thermal stress on intra- and transgenerational plasticity of the warm-temperate kelp Laminaria pallida. Our results highlight the effects of warm-temperature seasonality and heat stress in promoting gametophyte reproduction. Effects of parental thermal history on juvenile F1 sporophytes are more complex, with high thermal history sporophytes showing increased photosynthetic efficiency, but no definitive transgenerational effects on growth and survival. Significant genetic variation depending on female/maternal genetic lineage was observed for both intra- and transgenerational plasticity. Effects of gametophyte thermal history on reproduction and juvenile sporophyte growth differed based on female/maternal effects. Irrespective of thermal history, sporophyte fitness-related traits were optimal under mild to warm temperatures (14- 20ºC) but declined sharply when exposed to 23ºC. Laminaria pallida therefore benefits from high temperature exposure during the haploid (gametophyte) life stage, but milder temperatures promote juvenile sporophyte growth and health.
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