Biocontrol exerts natural selection against fecundity traits in Cytisus scoparius (L.)

Abstract

Biological invasions represent a useful experimental system with which evolutionary processes can be investigated in a contemporary timespan. One process that can be studied is which traits change when evolution is unconstrained by natural enemies. While some traits may have evolved when divorced from natural enemies, they are predicted to rapidly evolve in the opposite direction when these antagonists are reintroduced. How the evolution of increased competitive abilities will affect susceptibility to attack by antagonists when they are introduced is an under-researched question and an essential facet of biocontrol. The logical outcome would be the elimination of more susceptible genotypes that had allocated resources towards increased competitive ability but had become more vulnerable to attack by antagonists. In the invasive legume Cytisus scoparius (Scotch broom), larger seeds are found in invaded ranges compared to native, presumably due to evolution for increased competitive ability. Larger seeds are believed to produce faster growing and larger seedlings; however, the re-introduction of antagonists in the form of seed-eating biocontrols is predicted to exert natural selection for smaller seeds. In our study, we investigated correlations between seed mass and other fecundity traits (fruit length and seed number), as well as the selective pressure biocontrol exerts on fecundity traits across multiple sites over 3 years. We found a lack of trade-offs between fecundity traits, as well as natural selection against seed size, even though seed size is a trait that emerges after biocontrol selection occurs. Although the selection gradients against longer fruits were negative, they were not significantly so. Thus, we conclude that although seed size is under negative phenotypic selection, it is a pleiotropic effect of selection on fruit length.