Early life trade-offs in corals selected for adult heat tolerance are absent under heat stress

Technical Summary: Early Life Trade-offs in Corals Selected for Adult Heat Tolerance

Problem Statement
Marine heatwaves are driving mass coral mortality, threatening global reef persistence. While "assisted evolution" strategies, specifically selective breeding for adult heat tolerance, are being proposed to enhance population resilience, the fitness costs of this selection across different life stages remain poorly understood. Traits are rarely inherited independently; selection on one trait (adult heat tolerance) may produce correlated responses in others due to genetic covariances (e.g., pleiotropy). Crucially, trade-offs may exist across life stages, where genetic variants improving adult survival could negatively impact early life history stages (gametes, embryos, larvae). Given that early coral stages are pelagic, structurally fragile, and highly sensitive to thermal stress, it is critical to determine if selecting for adult tolerance inadvertently compromises offspring performance under benign or stressful conditions.

Methodology
The study utilized a selective breeding design with the reef-building coral Acropora aff. digitifera at the Palau International Coral Reef Center (PICRC).

1. Parental Selection: 66 adult colonies were subjected to a long-term heat stress assay (simulating a marine heatwave) to quantify their heat tolerance. Tolerance was measured using a Bleaching and Mortality Index (BMI) and converted to an effective dosage metric ($BMIED_{50}$), representing the cumulative heat stress required to reach 50% mortality.
2. Selective Crosses: Eight colonies with contrasting heat tolerance levels (high vs. low $BMIED_{50}$) were selected for breeding. These parents were crossed to produce 22 distinct genetic crosses.
3. Embryo Stress Exposure: Embryos from these crosses were exposed to two conditions:
   • Ambient: ~28.9°C.
   • Acute Heat Stress: Temperature gradually increased to 33.5°C over 27 hours (averaging 32.2°C).
4. Performance Metrics: The study quantified three key early-life traits:
   • Larval Survivorship: Assessed 36 hours post-exposure.
   • Settlement Success: Assessed 7 days post-settlement on crustose coralline algae (CCA) substrates.
   • Juvenile Growth: Radial growth rates measured 64 and 98 days post-settlement.
5. Statistical Analysis: Generalized linear mixed-effects models (GLMMs) were used to evaluate the interactive effects of maternal heat tolerance ($dam\ BMIED_{50}$) and temperature treatment on survivorship and settlement. Linear mixed-effects models were used for growth. Cross-environment correlations were calculated to assess consistency of performance across conditions.

Key Results
The study found that the relationship between parental heat tolerance and offspring performance was highly context-dependent, varying significantly between ambient and heat stress conditions:

• Under Ambient Conditions (Trade-offs): Offspring from parents with higher heat tolerance exhibited reduced larval survivorship and settlement success compared to offspring from low-tolerance parents. This indicates a transgenerational, cross-stage trade-off: the genetic factors conferring adult heat tolerance appear to carry a fitness cost during early development in benign environments.
• Under Heat Stress Conditions (Benefits): The pattern reversed. Offspring from high-tolerance parents showed improved survivorship and settlement success compared to those from low-tolerance parents when embryos were exposed to acute heat stress. This suggests a transgenerational benefit where parental tolerance confers resilience to offspring under stress.
• Growth: No detectable effect of parental heat tolerance or embryo heat exposure was found on the radial growth of 3-month-old juveniles.
• Cross-Environment Correlations: Survivorship showed a moderate positive correlation between ambient and stress conditions (crosses performing well in one tended to perform well in the other). However, settlement success showed no significant correlation between environments, indicating that settlement performance under stress cannot be predicted by performance under ambient conditions.

Significance and Claims
The authors claim that selection for adult heatwave tolerance influences offspring performance in complex, context-dependent ways. The primary significance of the work lies in demonstrating that:

1. Trade-offs are Stage-Specific: Selecting for adult heat tolerance may incur fitness costs (reduced survivorship/settlement) in early life stages under benign conditions, which could explain the low frequency of high-tolerance phenotypes in wild populations.
2. Context-Dependence: These trade-offs disappear or reverse under heat stress, where the same genetic background provides a survival advantage.
3. Implications for Assisted Evolution: The findings underscore the necessity of evaluating cross-stage and cross-environment trade-offs when designing selective breeding programs. The authors caution that simple "heat culling" (selecting only survivors from mass crosses under stress) may be insufficient to identify the most tolerant lineages because absolute performance differences are subtle. Instead, they suggest that family-specific paired exposures (comparing stress vs. control performance within a family) are required to robustly identify tolerant lineages.

The paper concludes that while selective breeding can accelerate adaptation, successful implementation requires a holistic understanding of the full coral life cycle, from gametes to adults, to avoid unintended negative consequences on early life history stages.