Evidence for host race formation in the leaf beetle Galerucella lineola

We examined preference and performance of four Finnish Galerucella lineola F. populations on alder and willow. In standardized two-choice laboratory feeding trials with alder and willow, only two naturally alder-associated G. lineola populations accepted alder. Two conspecific willow-associated populations preferred willow. These preferences seem to be unstable, however, because they can be modified by the beetles' experience. Thus, there probably is not a complete host preference-based isolation of alder- and willow-associated G. lineola beetles in nature. In performance experiments, larvae of all four populations survived better on willow than on alder. This may indicate that willows are the ancestral hosts for G. lineola. Nevertheless, larvae of the two alder- associated G. lineola populations survived better on alder than larvae of the two willow-associated populations. On the other hand, larvae of the two willow-associated populations survived better on willow than larvae of the two alder- associated populations. This performance trade-off suggests that G. lineola encounters different selective pressures on alders and willows. On both of them, selection probably disfavours those G. lineola genotypes that are the most successful and abundant on alternative hosts. This may reduce the effects of gene flow that is likely to occur as a consequence of incomplete host preference-based isolation of alder- and willow-associated G. lineola populations. Data from pupal weights support the idea that alder- and willow-associated G. lineola populations may be genetically differentiated. Pupae of the two alder- associated populations were heavier than those of the willow-associated populations irrespective of whether larvae had fed on alder or on willow. Overall, our results indicate host race formation in G. lineola. This process may be enforced by the variable abundance of alders and willows in local communities.