Adaptive responses to environmental variability in rotifers: Integrating plasticity and bet hedging in dormancy exit

Environmental variability imposes strong selective pressures that may favor adaptive responses such as phenotypic plasticity and bet hedging. Plasticity is generally favored under predictable conditions, whereas bet hedging is advantageous against environmental uncertainty. Plastic responses rely on reliable cues to adjust phenotypes to future conditions, whereas bet hedging spreads risks when cues are unreliable. However, because environments often consist of both predictable and unpredictable components, combined responses may be selected. Here, plasticity and bet hedging were examined in diapause-exit traits of Brachionus plicatilis clones sampled along a natural gradient of environmental predictability regarding hydroperiod length. Diapausing egg hatching was experimentally assessed under six salinity treatments, with salinity considered as a potential cue for diapause termination in predictable habitats. Because salinity integrates water volume and evaporation, it can act as an informative cue of the expected hydroperiod duration. For each clone, hatching fraction and timing of hatching were quantified across salinities, from which three key traits were derived: (1) a plasticity index of hatching fraction, capturing sensitivity to salinity; (2) the magnitude of hatching fraction across the studied salinity range, reflecting risk spreading among growing seasons through incomplete hatching (i.e., among-season bet hedging); and (3) the variation in hatching time, spreading risk within a growing season through asynchronous hatching (i.e., within-season bet hedging). Across clones, hatching fractions were higher and hatching occurred faster at low salinities, whereas hatching was delayed or even inhibited at higher salinities. Such a peaked response suggests that salinity is a reliable cue of favorable future conditions. Moreover, clones showed intermediate hatching fractions, indicative of bet hedging. Clones with higher hatching fractions also showed more asynchronous hatching, indicating potential functional redundancy between among-season and within-season bet hedging. Additionally, clones exhibiting stronger plastic responses showed higher hatching fractions, supporting the interpretation of phenotypic plasticity and bet hedging as alternative yet potentially co-occurring adaptive responses. Finally, plasticity was greater in clones originating from predictable habitats, whereas bet-hedging traits were stronger in clones from less predictable environments. Overall, these findings highlight how the reliability of environmental cues can shape the evolution of combined adaptive responses in fluctuating environments.