Phenotypic plasticity in host-plant specialisation in Aphis fabae

1. The study of phenotypic plasticity in host utilisation is crucial for predicting evolutionary patterns of insect-plant interactions. The presence of sufficient variation in plasticity may facilitate host race formation and sympatric speciation. 2. Aphis fabae genotypes showed high levels of phenotypic plasticity in the intrinsic rate of natural increase (r(m)), relative growth rate (RGR), birth weight (BW), adult weight (AW), fecundity (F), and development time (1/d). 3. Thirteen A. fabae genotypes reared both on broad bean and nasturtium exhibited statistically significant genotypic variability in phenotypic plasticity. 4. Some genotypes displayed fitness improvement on novel host plants. 5. Differences in genotypic correlation among fitness components between the two hosts and increased variance on nasturtium indicated different genomic expression on nasturtium. 6. The results indicated that phenotypic plasticity in a novel environment may be a major determinant of the evolutionary trajectory of a parasitic species and might support the idea that speciation starts with phenotypic plasticity.