Heterosis indices: What do they really measure?

Heterosis (hybrid vigour) is a universal phenomenon of crucial agro-economic and evolutionary importance. We show that the most common heterosis indices do not properly measure deviation from additivity because they include both a component accounting for "real" heterosis and a term that is not related to heterosis since it is derived solely from parental values. Therefore, these indices are inadequate when the aim of the study is to compare heterosis levels between different traits, environments, genetic backgrounds or developmental stages, as these factors may affect not only heterosis but also parental values. The only relevant index for such comparisons is the so-called "potence ratio". These observations argue for the careful choice of heterosis indices depending on the purpose of the work. HighlightUnlike dominance indices, heterosis indices, with one exception, do not properly measure the level of deviation from additivity, thus making them unsuitable for comparative analyses.