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

Environmental variability imposes major selective pressures that may drive the evolution of adaptive responses such as phenotypic plasticity and bet hedging. Plasticity is generally favoured under predictable conditions, whereas bet hedging is advantageous when unpredictability prevails. However, because environments often consist of both predictable and unpredictable components, combined responses may be selected. In this study, the co-expression of plasticity and bet hedging was examined in diapause exit traits of Brachionus plicatilis clones sampled along a natural gradient of environmental predictability. Diapausing egg hatching dynamics were experimentally assessed under six salinity treatments, with salinity considered as a potential cue for diapause termination. The hatching patterns of each clone in the salinity range were characterized based on the fraction and time of hatching, and from these, three traits of interest were derived: (1) a plasticity index of the hatching fraction, (2) the maximum hatching fraction (i.e., that observed at optimal salinity), and (3) the variation in hatching time, with the latter two treated as bet-hedging-related traits. In general, across clones, regardless of their origin, hatching fractions were higher and hatching times shorter at lower salinities, whereas hatching was delayed or even inhibited at higher salinities. Such a marked optimum in the hatching response to salinity suggests that it is a reliable cue of favourable future conditions for survival. A positive correlation was found between the maximum hatching fraction and its temporal variability, evidencing a trade-off between these two bet-hedging strategies. A positive correlation was also found between the plasticity index and maximum hatching fraction, supporting the interpretation of phenotypic plasticity and bet hedging as alternative, yet potentially co-occurring adaptive responses. Furthermore, the plasticity index was significantly associated with environmental predictability, whereas bet-hedging-related traits were more prevalent in unpredictable environments. Overall, these findings underscore the need for integrating the multicomponent nature of environmental variability into ecological and evolutionary studies, and highlight that combined adaptive responses can evolve based on the reliability of cues that predict environmental changes.