Mechanisms and consequences of water stress-induced parental effects in an invasive annual grass.
Premise of research. Tests of the adaptive value of parental effects have generally focused on offspring fitness. However, the evolution of parental effects depends on their consequences for both offspring and parental fitness. Whether parental effects are adaptive can also depend on the mechanism of these effects. Parental effects caused by differences in the quality rather than quantity of resources provisioned to offspring may be more likely to be adaptive because they can persist through the life cycle. Methodology. We estimated parental effects in response to water stress in the invasive annual Avena barbata. To test whether these effects were adaptive, we reciprocally transplanted offspring of wet- and dry-grown parents into wet and dry environments. We also tested whether seed size and nitrogen content, which represent the quantity and quality of parental investment, were mechanisms of parental effects in A. barbata. Pivotal results. We found evidence of parental effects in response to water stress in A. barbata; dry-grown parents produced offspring with significantly higher germination, longer radicles, and earlier emergence than wet-grown parents. The offspring of dry-grown parents had higher biomass and seed production than the offspring of wet-grown parents. However, when cumulative fitness was calculated across parental and offspring generations, dry-grown parents had significantly lower fitness than wet-grown parents because of trade-offs between seed size and number. Although dry-grown parents provisioned their offspring with more nitrogen than wet-grown parents, offspring performance was primarily explained by variation in seed mass. Conclusions. Water stress-induced parental effects were adaptive from the offspring but not the parental perspective, suggesting that the evolution of these effects may be constrained. In addition, water stress-induced parental effects were primarily caused by differences in seed mass, suggesting that the quantity of resources provisioned to offspring is a more important mechanism of parental effects than resource quality.