Rotifers hedge their bets against environmental unpredictabilitySubmitted by editor on 9 May 2017.Get the paper!
Humans are not the only species to hedge their bets against uncertainty. No habitat is completely predictable and organisms –from microorganisms to vertebrates– living in fluctuating environments may exhibit bet-hedging strategies, which are believed to have evolved to deal with unpredictably. However, despite extensive theoretical development for bet-hedging, the scientific empirical evidence for it is still lacking.
Selection due to environmental unpredictability can act on multiple traits of the life cycle of an organism to reduce the impact of environmental variance. In our research we focus in how diapause-related life-history traits evolve in fluctuating environments. Our model organisms are monogonont rotifers –small aquatic organisms that are capable of both sexual and asexual reproduction (Fig.1).
Figure 1. Brachionus plicatilis female carrying an asexual egg.
Rotifers are a good choice because their short generation times allow for developing rapid evolutionary patterns. As in other facultative sexual organisms there is a linkage between sex and production of resistant, diapausing eggs (Fig. 2) –these being the only way to survive unsuitable periods.
Figure. 2. Diapausing eggs are the only way to survive from a growing planktonic period to the next in temporary habitats.
We used an experimental evolution approach to study short-term evolutionary responses under controlled conditions for two traits: the timing of sex (i.e. initiation of diapausing egg production) and diapause duration in the rotifer Brachionus plicatilis. In our experiment, B. plicatilis laboratory populations were subjected to two contrasting regimes (predictable vs unpredictable) through seven cycles of selection (Fig. 3 & 4).
Figure 3. Schematic of the experimental evolution design. Growing cycles of the active population in the ‘water column’ and diapause in the ‘egg bank’ are displayed. Arrows represent bioassays for estimation of the life history traits.
Figure 4. Experimental evolution of rotifer populations in the chemostat device required intensive lab work. E. Tarazona working with the stereomicroscope. Chemostat picture by C. Gabaldón
Our results showed that rotifer populations can rapidly adapt to unpredictable environments. Populations under the unpredictable regime initiated earlier sexual reproduction and diapausing-egg production and had a lower hatching fraction of diapausing eggs (longer diapause) than populations under the predictable regime (Fig. 5). These findings demonstrate empirically the existence of bet-hedging strategies in B. plicatilis regarding both traits, consistent with theoretical predictions of bet-hedging evolution under conditions of unpredictable environmental variance.
Figure 5. Evolutionary progression of (1) timing of sex and (2) hatching fraction of diapausing eggs, in two selective regimes (predictable vs unpredictable).
With this study, we highlight the usefulness of laboratory selection experiments to demonstrate the relevance of unpredictability as a selective factor and how they can contribute to our understanding of bet-hedging evolution.
The authors through Eva Tarazona