Interactions between maternal provisioning and natural selection on seed mass fluctuate across heat waves

Changing climates are leading to more frequent and severe heat waves, potentially threatening plant populations. Both acclimation to stress and selection for heat-escape or heat-resistance phenotypes occur during heat waves. However, plastic responses and selection do not necessarily interact cohesively-even producing trait responses in opposite directions that could cause maladaptation.

We examined patterns of maternal provisioning and natural selection on seed mass and germination characteristics after two natural heat waves across 12 annual populations of Mimulus guttatus. We then recreated heat-wave events occurring early and late within a growing season using a manipulative experiment to determine the reproducibility of our results.

There was selection for greater seed mass during the year of the early season heat wave, and seed mass remained high through the late-season heat wave. Maternal provisioning also impacted seed mass and germination during the late-season heat wave. Field observations of seed mass only reflect selection for increased seed mass after the early season heat wave, and plants that experienced a late season heat wave had small seeds that germinated later. This pattern during the late season heat wave reflects decreased maternal provisioning coinciding with selection for larger seeds.

Our results demonstrate that the relationship between selection and maternal provisioning fluctuates between years, facilitating adaptive evolution during an early-season heat wave and causing conflict during a late-season heat wave. The consequences for evolutionary responses are uncertain and depend on the seed bank, but could result in high mortality in heat waves occurring during successive years.