Heterosis in crosses between remnant populations of a rare prairie forb: implications for restoration genetics

Calls to adopt seed sourcing strategies in biological restoration that preserve local adaptation while maximizing genetic variation (e.g., regional admixture provenancing) are increasingly common. Heterosis, the increased fitness of progeny from between- relative to within-population crosses, could provide an added benefit under such strategies, but several open questions remain. We quantified heterosis in crosses between three small remnant populations of a rare prairie forb. We measured early fitness components (seed number per fruit, germination, and juvenile survival) in a greenhouse. Adult fitness components (survival and reproduction) were quantified over two flowering seasons in two different environments: a field common garden simulating the initial stages of a restoration, and a greenhouse. Our approach is unique in estimating heterosis in an environment most relevant to the early stages of a restoration and in comparing heterosis under field and controlled conditions. The consequences of between-population crosses for cumulative fitness in the field were strongly positive in two of the populations (281% and 50% heterosis) and nearly neutral in a third. Heterosis was generally stronger when measured under field conditions, and stage specific patterns of heterosis varied among populations. Regional admixture provenancing in restorations should be beneficial for this species. We advocate for more research on heterosis in rare species, particularly in restorations. We also provide a cautionary note on how to calculate and report results for heterosis studies.