Transgenerational plasticity reflects local adaptation, but is not always advantageous for fitness

Transgenerational plasticity (TGP) could contribute to phenotypic differences as much as allelic variation and within-generation plasticity. Here we use northern and southern populations of Mimulus cardinalis collected prior to and following a severe drought and grew plants in a multi-factorial experiment across three generations to tease apart allelic, TGP, parental effects, and plasticity. Where grand-offspring are likely to experience similar stressors as grandparents, TGP is often considered advantageous; our results show no such advantage. Instead, we find a TGP pattern analogous to local adaptation. Regardless of current environment, grand-offspring exhibit increased fitness when grandparents were exposed to environments that matched historical conditions and reduced fitness when exposed to foreign environments. We also find the evolutionary loss of TGP; prior to drought, northern flowering time and southern pollen viability were significantly affected by grandparent treatment but following four years of drought, grandparent treatment no longer affected these reproductive traits. Lastly, we find that TGP can have a greater effect on fitness than allelic change, parental environment, and within-generation plasticity. Our results indicate an additional way that phenotypes arise, sheds light on the potential for how rapid evolution might occur, and has significant implications for how we design evolutionary ecology experiments.