Timing of reproduction modifies transgenerational effects of chronic exposure to stressors in an annual vertebrate.

Stressful environmental conditions can shape both an individual's phenotype and that of its offspring. However, little is known about transgenerational effects of chronic (as opposed to acute) stressors, nor whether these vary across the breeding lifespan of the parent. We exposed adult female (F0 generation) three-spined sticklebacks ( Gasterosteus aculeatus ) to chronic environmental stressors and compared their reproductive allocation with that of non-exposed controls across early, middle and late clutches produced within the single breeding season of this annual population. There was a seasonal trend (but no treatment difference) in F0 reproductive allocation, with increases in egg mass and fry size in late clutches. We then tested for transgenerational effects in the non-exposed F1 and F2 generations. Exposure of F0 females to stressors resulted in phenotypic change in their offspring and grandoffspring that were produced late in their breeding lifespan: F1 offspring produced from the late-season clutches of stressor-exposed F0 females had higher early life survival, and subsequently produced heavier eggs and F2 fry that were larger at hatching. Changed maternal allocation due to a combination of seasonal factors and environmental stressors can thus have a transgenerational effect by influencing the reproductive allocation of daughters, especially those born late in life.