The natural history of model organisms is often overlooked despite its importance to correctly interpret the outcome of laboratory studies. Ageing is particularly understudied in natural populations. To address this gap, we present lifetime demographic data from wild populations of an annual species, the turquoise killifish, Nothobranchius furzeri, a model species in ageing research, and two other species of coexisting annual killifishes. Annual killifish hatch synchronously, have non-overlapping generations, and reproduce daily after reaching sexual maturity. Data from 13 isolated savanna pools in southern Mozambique demonstrate that the pools supporting killifish populations desiccated 1–4 months after their filling, though some pools persisted longer. Declines in population size over the season were stronger than predicted, because they exceeded the effect of steady habitat shrinking on population density that, contrary to the prediction, decreased. Populations of N. furzeri also became more female-biased with progressing season suggesting that males had lower survival. Nothobranchius community composition did not significantly vary across the season. Our data clearly demonstrate that natural populations of N. furzeri and its congeners suffer strong mortality throughout their lives, with apparent selective disappearance (condition-dependent mortality) at the individual level. This represents selective force that can shape the evolution of lifespan, and its variation across populations, beyond the effects of the gradient in habitat persistence.
Ephemeral habitats can impose challenging conditions for population persistence. Survival strategies in these environments can range from high dispersal capacity to the evolution of dormant stages able to tolerate a harsh environment outside the temporal window of favourable conditions [1]. Annual killifish have evolved to live in seasonal pools on the African savannah and display a range of adaptations to cope with an unpredictable environment [2,3]. For most of the year, killifish populations survive as diapausing embryos buried in dry sediment. When savannah depressions fill with rainwater, the fish hatch, grow rapidly and, after attaining sexual maturity, reproduce daily [2,4]. Nothobranchius furzeri, a model species in ageing research [2,3], is distributed in a region where the climate is particularly dry and rains are unpredictable [5]. Here, we demonstrate that the fast juvenile growth and rapid sexual maturation shown by N. furzeri in captivity is actually an underestimate of their natural developmental rate. We estimated the age of N. furzeri in natural populations by counting daily-deposited increments in the otoliths and performing histological analysis of gonads. We found that N. furzeri are capable of reaching sexual maturity within 14 days after hatching, which to our knowledge is the fastest rate of sexual maturation recorded for a vertebrate. We also demonstrate that N. furzeri can grow from an initial length of 5 mm up to 54 mm over the course of a two-week period. Such rapid juvenile development is likely to be adaptive since some pools were entirely desiccated 3-5 weeks after filling, but retained a viable killifish population that reproduced before the adults succumbed to the disappearance of their pool.
Embryos often passively depend on their environment and can be extremely sensitive to environmental shifts. A subtle environmental insult during a key developmental window can have a major impact on embryo viability, and embryonic and postembryonic performance (e.g., Wilson, 1973). The embryonic phase is a critical component of ontogeny, and most vertebrates attempt to protect or shield their embryonic stages from environmental fluctuations through a variety of physiological and behavioral mechanisms such as internal incubation, careful oviposition choice, and parental care (e.g., Slater & Milinski, 1996).In variable or highly seasonal environments, embryos often tolerate periods of unfavorable conditions in a state of dormancy.Embryonic dormancy has evolved in many plant, invertebrate, and vertebrate taxa (e.g., Cáceres, 1997;Childs et al., 2010;Hand et al., 2016). Diapause is a specific form of this dormancy which is under the control of an endogenous program. Its initiation precedes
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